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Susitna‐Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
Site-specific seismic hazard study plan Section 16.6, Study Completion
Report. [ Main report and Attachment 1]
Formal title of Attachment 1: Susitna-Watana Hydroelectric Project site-
specific seismic hazard study summary report
SuWa 289
Author(s) – Personal:
Author(s) – Corporate:
MWH
AEA‐identified category, if specified:
November 2015; Study Completion and 2014/2015 Implementation Reports
AEA‐identified series, if specified:
Attachment 1: Report ; 15-03-REP
Series (ARLIS‐assigned report number):
Susitna-Watana Hydroelectric Project document number 289
Existing numbers on document:
Attachment 1 numbers:
AEA11-022
16-1503-REP-102915
Published by:
[Anchorage : Alaska Energy Authority, 2015]
Date published:
October 2015
Published for:
Alaska Energy Authority
Date or date range of report:
Volume and/or Part numbers:
Study plan Section 16.6
Final or Draft status, as indicated:
Document type:
Pagination:
iii, 25 pages (main report)
x, 4, 141 pages (Attachment 1)
Related works(s):
Pages added/changed by ARLIS:
Notes:
Contents of this PDF:
[Main report] -- Attachment 1. Site-specific seismic hazard study.
All other parts (Attachments 2-11) are in separate electronic files due to large file sizes.
All reports in the Susitna‐Watana Hydroelectric Project Document series include an ARLIS‐
produced cover page and an ARLIS‐assigned number for uniformity and citability. All reports
are posted online at http://www.arlis.org/resources/susitna‐watana/
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Site-Specific Seismic Hazard Study Plan Section 16.6
Study Completion Report
Prepared for
Alaska Energy Authority
Prepared by
MWH
October 2015
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i October 2015
TABLE OF CONTENTS
1. Introduction ....................................................................................................................... 1
2. Study Objectives................................................................................................................ 3
3. Study Area ......................................................................................................................... 3
4. Methods and variances ..................................................................................................... 4
4.1. Board of Consultants Review ................................................................................. 4
4.1.1. Variances ......................................................................................... 5
4.2. Review of Project Documentation .......................................................................... 5
4.2.1. Variances ......................................................................................... 5
4.3. Seismic Hazard Analysis ........................................................................................ 5
4.3.1. Variances ......................................................................................... 7
4.4. Long-Term Earthquake Monitoring System ........................................................... 7
4.4.1. Variances ......................................................................................... 8
4.5. Preliminary Reservoir Triggered Seismicity .......................................................... 8
4.5.1. Variances ......................................................................................... 8
5. Results ................................................................................................................................ 8
5.1. Crustal Seismic Source Evaluation ......................................................................... 8
5.2. Long-Term Seismic Monitoring ........................................................................... 10
5.3. Seismic Hazard Analysis ...................................................................................... 11
5.3.1. Preliminary Probabilistic Seismic Hazard Analysis ..................... 11
5.3.2. PSHA Sensitivity Analysis - Intraslab .......................................... 11
5.3.3. DSHA for Intraslab Events ........................................................... 11
5.3.4. Seismic Shear Wave Velocity ....................................................... 12
5.4. Seismic Design Criteria ........................................................................................ 12
6. Discussion......................................................................................................................... 13
7. Conclusion ....................................................................................................................... 15
8. Literature Cited .............................................................................................................. 15
9. Tables ............................................................................................................................... 19
10. Figures .............................................................................................................................. 21
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
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FERC Project No. 14241 Page ii October 2015
LIST OF TABLES
Table 5-1. Peak Ground Acceleration Values for the MCE, MDE and OBE .............................. 20
Table 5-2. Peak Ground Acceleration and Percentile for Deterministic Response Spectra ........ 20
Table 5-3. Selected Time Histories for Feasibility Analysis– Intraslab and Crustal ................... 20
Table 5-4. Selected Time Histories for Feasibility Analysis – Interface ..................................... 20
LIST OF FIGURES
Figure 3.1-1. Regional Faults........................................................................................................22
Figure 5.1-1. Dam Site Area Tectonic Features ...........................................................................23
Figure 5.2-1. Crustal Stress Orientations and Strain Ellipses .......................................................24
Figure 5.4-1. Design Response Spectra ........................................................................................25
ATTACHMENTS
Attachment 1: Site-Specific Seismic Hazard Study
Attachment 2: Seismic Hazard Characterization and Ground Motion Analyses
Attachment 3: Lineament Mapping and Analysis
Attachment 4: Watana Seismic Network Station Vs30 Measurements
Attachment 5: Crustal Seismic Source Evaluation
Attachment 6: Revised Intraslab Model and PSHA Sensitivity Results
Attachment 7: Discussion of MCE and OBE
Attachment 8: Seismic Network 2013 Annual Seismicity Report
Attachment 9: Seismic Network 2014 Annual Seismicity Report
Attachment 10: Susitna-Watana Seismic Monitoring Project: January – June 2015 Report
Attachment 11: Preliminary Reservoir Triggered Seismicity
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
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FERC Project No. 14241 Page iii October 2015
List of Acronyms, Abbreviations, and Definitions
Abbreviation Definition
AEA Alaska Energy Authority
AEC Alaska Earthquake Center
BOC Board of Consultants
CFR Code of Federal Regulations
COSMOS Consortium of Strong Motion Observations Systems
DSHA Deterministic Seismic Hazard Assessment
FERC Federal Energy Regulatory Commission
ft feet
GMPE Ground Motion Prediction Equations
INSAR Interferometric Synthetic Aperture Radar
ILP Integrated Licensing Process
IMASW Interferometric Multichannel Analysis of Surface Waves
ISR Initial Study Report
km kilometer
K-NET Kyoshin Network
LiDAR Light Detection and Ranging
M Magnitude
Mmax Maximum Magnitude
ML Local Magnitude
m meter
mi miles
MCE Maximum Credible Earthquake
MDE Maximum Design Earthquake
NGA Next Generation Attenuation
OBE Operating Basis Earthquake
PEER Pacific Earthquake Engineering Research Center
PGA Peak Ground Acceleration
PHA Peak Horizontal Acceleration
PSHA Probabilistic Seismic Hazard Assessment
RSP Revised Study Plan
RTS Reservoir Triggered Seismicity
SCR Study Plan Completion Report
SPD Study Plan Determination
SSSHA Site Specific Seismic Hazard Assessment
USGS United States Geological Survey
Vs30 Shear Wave Velocity
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 1 October 2015
1. INTRODUCTION
The Site Specific Seismic Hazard Study, Section 16.6 of the Revised Study Plan (RSP) approved
by the Federal Energy Regulatory Commission (FERC) for the Susitna-Watana Hydroelectric
Project, FERC Project No. 14241, focuses on conducting deterministic and probabilistic seismic
hazard evaluations to estimate earthquake ground motion parameters at the Project site, assessing
the risk at the site and the loads that the Project facilities would be subject to during and
following seismic events, and proposing design criteria for Project facilities and structures
considering the risk level.
A summary of the development of this study, together with the Alaska Energy Authority’s
(AEA) implementation of it through the 2013 study season, appears in Part A, Section 1 of the
Initial Study Report (ISR) filed with FERC in June 2014. As required under FERC’s regulations
for the Integrated Licensing Process (ILP), the ISR describes AEA’s “overall progress in
implementing the study plan and schedule and the data collected, including an explanation of any
variance from the study plan and schedule.” (18 CFR 5.15(c)(1)).
Since filing the ISR in June 2014, AEA has continued to implement the FERC-approved plan for
the Site-Specific Seismic Hazard Study, Section 16.6. For example:
Evaluate crustal seismic sources through review of newly acquired LiDAR imagery and
based on geomorphic characteristics in the field and geologic relationships around the
lineament feature in the Project area with an emphasis in the dam site area,
Perform Interferometric Multichannel Analysis of Surface Wave measurements to
estimate shear wave velocity in the Project area,
Evaluate the surface fault rupture hazard in the dam site area, focusing in the possibility
of displacement along existing planes of weakness in the bedrock,
Perform field geologic transects to access geologic relationships and styles and patterns
of structural deformation
Review of site-specific surface and subsurface investigations at the dam site
Establish a geologic datum for evaluating tectonic (fault) activity during the late
Quaternary; develop understanding of Quaternary geologic history
Monitor earthquake activity in the Project area to refine the intraslab model, check the
source characterization of background sources, define focal mechanisms for larger
detected earthquakes and document the background level of seismicity to monitor
reservoir triggered seismicity,
Update ground motion prediction equations and utilize updated source models for seismic
hazard analysis, and
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
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FERC Project No. 14241 Page 2 October 2015
Development of preliminary seismic design criteria for the finite element analysis for the
dam.
In furtherance of the next round of ISR meetings and FERC’s Study Plan Determination (SPD)
expected in 2016, this report contains a comprehensive discussion of results of the Site Specific
Seismic Hazard Study from the beginning of the AEA’s study program in 2012, through the June
of 2015. It describes the methods of the SSSHS, and explains how all Study Objectives set forth
in the Commission-approved Study Plan have been met. Accordingly, with this report, AEA has
now completed all field work, data collection, data analysis, and reporting for this study.
Various technical memoranda and reports prepared for the Site Specific Seismic Hazard Study,
attached to this report, include:
Attachment Reference
1 MWH, (2015a), Site-Specific Seismic Hazard Study for the Susitna-Watana Dam, prepared for Alaska
Energy Authority, October 2015, 153 p.
2
Fugro Consultants, Inc., (2012), Seismic Hazard Characterization and Ground Motion Analyses for the
Susitna-Watana Dam Site Area, prepared for Alaska Energy Authority, Technical Memorandum No. 4,
February 24, 2012, 146 p and appendices.
3 Fugro Consultants, Inc., (2013), Lineament Mapping and Analysis for the Susitna-Watana Dam Site,
prepared for Alaska Energy Authority, Technical Memorandum No. 8, March 27, 2013, 61p. and appendix.
4
Fugro Consultants, Inc. (2014a), Watana Seismic Network Station Vs30 Measurements for the Susitna-
Watana Dam Site, prepared for Alaska Energy Authority, Technical Memorandum No. 14-12-TM, March 20,
2014, 51p. and appendix.
5 Fugro Consultants, Inc., (2015a), Crustal Seismic Source Evaluation for the Susitna-Watana Dam Site,
prepared for Alaska Energy Authority, Report No. 14-33-REP, May 2015, 141p and appendices.
6
Fugro Consultants, Inc., (2014b), Revised Intraslab Model and PSHA Sensitivity Results for the Susitna-
Watana Dam Site Area., prepared for Alaska Energy Authority, Technical Memorandum No. 14-11 -TM, April
2014, 31 p.
7 MWH, (2014a), Briefing Document – Discussion of MCE and OBE, prepared for Alaska Energy Authority,
Technical Memorandum No. 14-13-BD, April 30, 2014, 6 p.
8 Fugro Consultants, Inc., (2014c), Seismic Network 2013 Annual Seismicity Report for the Susitna-Watana
Dam Site Area prepared for Alaska Energy Authority, Report No. 14-06-REP, March 2015, 40p.
9 Fugro Consultants, Inc., (2015b), Seismic Network 2014 Annual Seismicity Report for the Susitna-Watana
Dam Site Area prepared for Alaska Energy Authority, Report No. 14-32-REP, March 2015, 55p.
10 Alaska Earthquake Information Center, (AEC), (2015). Susitna-Watana Seismic Monitoring Project: January
–June 2015 Report. Prepared for the Alaska Energy Authority, September 2015.
11 MWH, (2013), Preliminary Reservoir Triggered Seismicity, prepared for Alaska Energy Authority, Technical
Memorandum No. 10 v3.0, March 29, 2013, 95 p.
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FERC Project No. 14241 Page 3 October 2015
The details of the site-specific seismic hazard study have been summarized and are presented in
the report titled Site-Specific Seismic Hazard Study Summary Report (MWH 2015a;
Attachment 1). In addition, the details of the engineering aspects of the project including seismic
design as presented in the Engineering Feasibility Report (MWH 2014b).
2. STUDY OBJECTIVES
The goals of this study are to conduct deterministic and probabilistic seismic hazard evaluations
to estimate earthquake ground motion parameters at the Project site, assess the risk at the site and
the loads that the Project facilities would be subject to during and following seismic events, and
propose design criteria for Project facilities and structures considering the risk level. The intent
of the study is to fulfill specific objectives including, but not limited to the following:
1. Identify the seismic sources along which future earthquakes are likely to occur, including
the potential for reservoir-triggered seismicity;
2. Characterization of the degree of activity, style of faulting, maximum magnitudes, and
recurrence information of each seismic source;
3. Develop maps and tables depicting the spatial and geometric relations of the faults and
seismic source zones together with specific distance parameters to evaluate ground
motion parameters from each source;
4. Assemble available historical and instrumental seismicity data for the region, including
maximum and minimum depth of events;
5. Determine the distance and orientation of each fault with respect to the site;
6. Estimate the earthquake ground motions at the proposed dam site, updating previous
studies to include changes in practice and methodology since the 1980s;
7. Propose the seismic design criteria for the site;
8. Prepare supporting design report that includes the seismic criteria and results of dam
stability analysis under seismic loading (this will be addressed as part of the dam
analysis, not as part of the initial seismic characterization); and,
9. Use of Board of Consultants for independent technical review and guidance during
development of site-specific studies.
3. STUDY AREA
The study area for the seismic hazard evaluation was necessarily large in order to include
potentially significant seismic sources throughout the region. The study area encompassed
subduction-related sources (i.e. plate interface between the North American and Pacific Plates,
which was the source of the 1964 earthquake, the epicenter of which is a significant distance
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FERC Project No. 14241 Page 4 October 2015
south of the Project, and intraslab sources within the down-going Pacific Plate) and all applicable
Quaternary crustal seismic sources within about 62 miles (100 kilometers) of the site (Figure 3.1-
1). Crustal seismic sources beyond these distances are not expected to provide significant
ground motion contributions at the dam site relative to nearby sources. A more focused study
area included the dam site and reservoir areas. The study area thus included much of the
Talkeetna block and surrounding fault zones such as the Denali; Castle Mountain; Northern
Foothills fold and thrust fault zone; inferred Talkeetna fault; and Broad Pass Fault.
4. METHODS AND VARIANCES
The study methods proceeded in accordance with Chapter 13 of the FERC Engineering
Guidelines for the Evaluation of Hydropower Projects (FERC 2011). The site-specific seismic
hazard evaluation for assessing the seismic risks and developing the seismic design criteria in
support of licensing and detailed design included the following tasks:
Update the understanding of geologic conditions and seismo-tectonic setting for the dam
site area;
Identify and characterize the seismic sources, including detailed geologic studies and
lineament analyses;
Identify whether a fault may be encountered beneath or adjacent to the dam and assess
the activity of the feature and, if active, the likelihood for potential fault displacement or
ground offset;
Perform a deterministic and probabilistic seismic hazard assessment in order to define
earthquake ground motions for structural analyses;
Evaluate the potential for Reservoir Triggered Seismicity (RTS);
Assess risks to Project structures and operation associated with seismic loading
conditions; and
Propose appropriate seismic design criteria.
These tasks were completed per methods described in the ISR Study 16.6 Section 4 in assessing
the seismic risks for the Project.
4.1. Board of Consultants Review
As requested by FERC (FERC 2012), the Board of Consultants (BOC) has been engaged for
technical review of the dam analysis and engineering feasibility. AEA convened a Board of
Consultants comprising Joseph Ehasz; Alfred J. Hendron; Yusof Ghanaat; and Brian Forbes,
George Taylor and Ellen Faulkner, supplemented by William Lettis for some meetings
monitoring progress on seismic hazard study and analysis. The Board of Consultants reviewed
the interim SSSHS report and the sections of the Engineering Feasibility Report for the Susitna-
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
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FERC Project No. 14241 Page 5 October 2015
Watana Hydroelectric Project relating to the dam analysis and derivation of the ground motions.
Their comments were taken into account in the finalization of the document. Norm
Abrahamson, an independent technical reviewer to the engineering consultant also has reviewed
the Site Specific Seismic Hazard Assessment (SSSHA) Summary Report and his comments have
been addressed in the document. Technical review has included review of the site specific
seismic hazard studies, seismic design criteria, dam finite element analysis and the engineering
feasibility report.
4.1.1. Variances
There were no variances to this study component of the Study Plan.
4.2. Review of Project Documentation
The review was conducted of the existing documentation and included additional Light
Detection and Ranging (LiDAR) imagery data set coverage for the upper Watana Creek and Fog
Lakes / Stephan Lake areas, earthquake event data from the Project seismic monitoring network,
and recently published scientific results by others within the Project area. The available data and
reporting on studies were used to characterize the geologic, seismic and tectonic framework
within the Project area.
4.2.1. Variances
There were no variances to this study component of the Study Plan.
4.3. Seismic Hazard Analysis
Preliminary deterministic and probabilistic seismic hazard evaluations were undertaken
previously to update the seismic hazard studies from the 1980s that included an update of the
site-specific seismic source model as presented in the technical memorandum Seismic Hazard
Characterization and Ground Motion Analyses for the Susitna-Watana Dam Site Area (Fugro,
2012; Attachment 2). The methods follow the recommendations of Chapter 13 of the Federal
Energy Regulatory Commission’s Engineering Guidelines (FERC 2011). Subtasks for the
seismic hazard analysis included:
Update evaluations of geologic, seismologic, and seismotectonic literature for the Project
study area to identify data gaps and uncertainties that may require further evaluations.
Update the seismicity catalogue for evaluation of seismicity rates, depths, magnitudes,
and focal mechanisms.
Develop a seismotectonic model that identifies and characterizes seismic sources of
engineering significance to the Project.
Conduct geologic studies using newly acquired LiDAR and Interferometric Synthetic
Aperture Radar (INSAR) datasets to aid in the identification and evaluation of potential
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seismic sources and geohazards. Criteria were established for determining significant
crustal seismic source potential (e.g., rupture length and earthquake magnitude, length –
distance screening criteria).
Collect field geologic data for characterization of potential crustal seismic sources and
surface displacement hazards.
Perform fault displacement hazard analysis to evaluate the significance (likelihood and
amount) of potential ground surface displacement from faulting in the area of the Project,
including beneath the dam.
Perform sensitivity studies on selected surface tectonic features, faults and lineaments,
identified and being considered as potential crustal seismic sources of engineering
significance on the design of the Project.
Determined preliminary ground motion parameters by conducting a Probabilistic Seismic
Hazard Analysis (PSHA) and a Deterministic Seismic Hazard Analysis (DSHA) for the
Project area.
Detect and monitor local earthquakes to understand the seismic hazards in the Project
area.
The methods employed to complete the crustal seismic source evaluation since the ISR included
geologic reconnaissance and mapping, evaluation of seismic sources, shear wave velocity
measurements at multiple locations in the Project area, collection of soil samples for age dating,
and a dam site area fault rupture evaluation. The detailed results are presented in technical
memoranda and a report titled Lineament Mapping and Analysis for the Susitna-Watana Dam
Site (Fugro 2013; Attachment 3), Watana Seismic Network Station Vs30 Measurements for the
Susitna-Watana Dam Site (Fugro 2014a, Attachment 4), and Crustal Seismic Source Evaluation
for the Susitna-Watana Dam Site (Fugro 2015a; Attachment 5).
Furthermore the data collected during these studies, various components of the initial PSHA
relative to the controlling source at the Watana site, the intraslab source, were updated and
revised. A PSHA sensitivity study was performed to better understand the refinements to the slab
position, variance of maximum magnitude (Mmax), and the correlation distance or the width of
the spatial smoothing kernel used for smoothed seismicity. The detailed results of this study
component are presented in the technical memorandum titled Revised Intraslab Model and PSHA
Sensitivity Results for the Susitna-Watana Dam Site Area (Fugro, 2014b; Attachment 6).
The seismic design criteria were developed following FERC guidelines using a DSHA, while the
maximum design earthquake (MDE) was defined based on the 5,000-year return frequency
ground motions from a PSHA. The operating basis earthquake (OBE) was selected to be the 500
year return period from the PSHA. It should be noted that the maximum credible earthquake
(MCE) is represented by four different response spectra from three different sources: the
subduction zone events – interface and intraslab and crustal events. The intraslab is represented
by a M7.5 and M8.0. The results of this study component are presented in the briefing document
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FERC Project No. 14241 Page 7 October 2015
titled Discussion of MCE and OBE (MWH 2014a; Attachment 7) and the report filed in
November 2015 titled Engineering Feasibility Report (MWH 2014b).
The seismic hazard at the dam site encompasses contributions from three different sources: the
subduction zone events – interface and intraslab (also referred to as the slab), and crustal events.
Response Spectra and time histories were developed for each type of event to evaluate the
difference in frequency content. The time histories were selected from the COSMOS, PEER, K-
NET (Japanese Earthquake Database), and a database run by the University of Chile and the
Chile Ministry of the Interior and Public Safety for ground motions that had magnitude, distance
and record properties similar to the controlling events. Once a time history record was selected it
was synthetically modified to match the target spectra. The time history details are presented in
the Engineering Feasibility Report Appendix B6 (MWH 2014b).
4.3.1. Variances
There were no variances to this study component of the Study Plan.
4.4. Long-Term Earthquake Monitoring System
The long-term earthquake monitoring system established in 2012 to monitor and document
earthquake events in the Project area continued to be operated through June 2015. Following
expansion of the initial monitoring system in 2013, the network consisted of seven instrumented
locations (WAT-1 through WAT-7) within about 30 mi (48 km) of the dam site. The network
was comprised of four 6 component strong motion and broadband seismograph station, and three
3-component broadband seismograph stations. At seismic station WAT-1, located at the dam
site, a high resolution GPS station was co-located to track crustal motion relative to the North
American Plate during August 2013. The monitoring system is linked and integrated into the
Alaska Seismographic Network operated by the Alaska Earthquake Information Center (AEC)
for real-time data acquisition, processing, and analysis.
During the 2014 field season, maintenance was performed on a number of the monitoring
stations. The seismic monitoring network continued to detect and provide event data on the
earthquakes in the Project area through June 2015. In June 2015, the number of seismograph
stations comprising the monitoring network was reduced with equipment removal and restoration
of the sites at WAT2, WAT3, WAT4, and WAT5 (AEC 2015, Attachment 10). A summary of
the seismic activity recorded by the Susitna-Watana Seismic Network are presented in the
reports titled Susitna-Watana Hydroelectric Project Seismic Network 2013 Annual Seismicity
Report (Fugro 2014c; Attachment 8) and Susitna-Watana Hydroelectric Project Seismic Network
2014 Annual Seismicity Report (Fugro 2015b; Attachment 9). The seismic event data obtained
during the first six months of 2015 are presented in the report titled Susitna-Watana Seismic
Monitoring Project: January – June 2015 Report (AEC 2015, Attachment 10).
This study component is complete.
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4.4.1. Variances
There were no variances to this study component of the Study Plan.
4.5. Preliminary Reservoir Triggered Seismicity
The preliminary reservoir triggered seismicity (RTS) study component methods were discussed
in the ISR, Section 16.6.4.6 (AEA 2014). No additional effort was undertaken since this task
was reported out in the ISR. The results of the preliminary study are presented in the technical
memorandum titled Preliminary Reservoir Triggered Seismicity (MWH 2013; Attachment 11).
This study component is complete.
4.5.1. Variances
There were no variances to this study component of the Study Plan.
5. RESULTS
5.1. Crustal Seismic Source Evaluation
As discussed in the ISR (AEA, 2014), an initial assessment was made of existing and additional
crustal seismic sources utilizing two high-resolution elevation datasets, a coarser resolution
INSAR dataset and a high resolution LiDAR dataset recently made available for the Project site
area. These datasets were used to identify lineaments and faults in the Project area, within 62 mi
(100 km) from the Watana dam site.
A total of 22 lineament groups and 3 boarder lineaments were identified for ev aluation through
field reconnaissance and on the ground inspection (Fugro 2013; Attachment 3). These lineaments
groups were evaluated for signs of recent activity such as deflected streams, offsets in
Quaternary deposits, faceted ridges, uphill facing scarps, etc. many of the lineament groups
investigated are judged to be dominantly erosional in origin, or to a lesser extent, related to rock
bedding or jointing, are not associated with tectonic faults.
Of the 25 lineament groups or lineaments, two groups, associated with the Sonona Creek and
Castle Mountain faults, were found to have defensible justification for consideration or inclusion
as a crustal seismic source. However since the Sonona Creek fault was included in the
preliminary PSHA and did not result in significant contributions to the seismic hazard at Watana
due to slip rate and distance (42 mi, 70 km), no additional field investigation is required. For the
lineaments associated with a northeast extension of the Castle Mountain, while Quaternary
deposits are lacking, there is a sharpness of the geomorphic expression in bedrock, it would be
prudent to include this Holocene active fault system in alternatives considered for an updated
crustal seismic source model.
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Previous mapping conducted in the Watana dam site area depicted or inferred several nearby
potential fault structures of crustal scale (Figure 5.1-1). The Talkeetna fault was recognized as a
major northeast-southwest trending tectonic feature near the dam site (Kachadoorian and Moore
1979; WCC 1982). Based on lineament mapping, field reconnaissance and the paleoseismic
trenching of a scarp, geomorphic evidence supports that interpretation that the scarp is likely an
ice-marginal feature that was not formed by surface fault rupture. Furthermore, recent
geophysical surveys and mapping by Twelker et al. (2014) concluded that the Talkeetna fault is
not expressed in bedrock geology as a single, continuous fault but is interpreted to be a series of
complex, high angle, northeast-trending fault strands, which strands appear to be cross-cut and
truncated by north-northwest trending bedrock faults. This evidence suggests that the Talkeetna
fault is not active in the contemporary stress regime.
In addition several topographic lineaments (e.g., Susitna feature, Watana lineament) were
observed using Landsat imagery (Gedney and Shapiro 1975). The Susitna feature is described as
a northeast-southwest structural grain in the Talkeetna Mountains that begins at Tsusena Creek at
the Susitna River that extends to the northeast to the Susitna Glacier in the Alaska Range,
approximately 70-mi long (110 km). WCC (1982) evaluated geomorphic features along the
lineament including excavation of a prominent scarp (S-1) but concluded the scarp is not related
to faulting but rather is of glacial origin. In this study, it was observed that there are a number of
scarps of similar morphology in the vicinity of the paleoseismic trench suggesting that it is not an
anomalous feature on the landscape. Moreover, a number of lineament groups were mapped as
cross-cutting the feature (e.g., LG-16, LG-21b, LG-22); the lineaments mapped trend sub-
perpendicular to the Susitna feature, and are not displaced where they overlie the projection.
Thus, based on the lineament mapping and analysis, coupled with field investigations and in light
of previous investigation findings, the data do not support evidence for the existence of a Susitna
fault or feature and therefore it is discarded as a tectonic lineament or fault-related feature near
the dam site.
It had been postulated that a significant geologic feature, known as the Watana lineament,
corresponding to a series of east-west trending, relatively linear segments of the Susitna River,
could potentially be present below the Susitna River at the location of the dam. If present, such a
feature would present a significant engineering challenge to the project. A number of lineaments
are located both on the north and south sides of the Susitna River; however, these lineaments and
their orientations are attributed to glacial ice flow or erosion and do not line up with the proposed
Watana lineament. In addition, two angled borings, DH14-9b and DH14-10, were drilled across
the width of the river in opposing directions with the intent of intersecting any significant
geologic structure, if present. In general, the rock encountered slightly weathered, strong to very
strong diorite with occasional zones of alteration. While small shears with one to two inches of
clay gouge were encountered, no significant geologic structures were observed in either hole.
This provides subsurface evidence in support of interpretations made from geologic mapping that
the existence of a through-going fault in the thalweg of the river channel at the dam site is
improbable.
Recent dam site mapping as part of the dam site fault rupture evaluation was performed and
considered regional geologic history, sub-regional deformation patterns, observed in Mesozoic
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and Cenozoic rocks around the site, emplacement of intrusions, volcanics, crustal stress
orientations from earthquake focal mechanisms, known active faulting, plate motions and GPS
data, geomorphic landform evaluations, and current understanding of geologic features at the
dam site. Based on geologic mapping and drilling, bedrock at the dam site is pervasively
fractured with thin shear zones present as high-angle features. Furthermore, these investigations
imply that there is less continuity of dam foundation geologic structures (i.e., geologic features)
than has previously been depicted, therefore reducing the potential significance of potential
surface fault rupture. The lines of evidence that support this conclusion are:
The apparent lack of continuity and small scale of structural geologic features (shear
zones) at the dam site;
The dominant northwest-southeast trend of geologic features is unfavorably oriented with
respect to the contemporary tectonic stress regime;
The absence of nearby crustal scale fault structures and any neotectonic or paleoseismic
evidence of Quaternary faulting; and
The absence of Quaternary faults mapped within about 15 mi (25 km) of the site.
Additional discussion of the geologic conditions at the Watana dam site is presented in the study
implementation report for Geology and Soils Resource Characterization (AEA 2015).
5.2. Long-Term Seismic Monitoring
Since initiation of the Susitna-Watana Seismic Network on November 16, 2012 through
December 2014 a total of 2,523 earthquakes have been recorded. In 2014 a daily average of 3.8
events was recorded (1.8 crustal events per day and 2.0 intraslab events per day). The largest
event in 2014, ML 4.6, occurred on November 29, 2014 at a depth of 37.9 mi (62.1 km), with an
epicenter 24.5 mi (40 km) southeast of the proposed Watana dam site (Fugro 2014c, 2015b;
Attachments 8 and 9).
The spatial pattern of both crustal and intraslab seismicity is variable over the Project Area, but
much more dense in the western half of the Project Area. There is a notable lack of seismicity to
the east and southeast of the dam site from both crustal and intraslab sources.
Focal mechanisms calculated by the AEC based on the larger earthquake event data detected by
the Susitna-Watana Seismic Monitoring Network indicate that the shallow crust within the
Talkeetna Block, a region the includes the Watana dam site is undergoing north-northwest south-
southeast compression, consistent with the relative Pacific-North America plate motion, with the
maximum horizontal stress rotating progressively in a counterclockwise direction from east to
west (Figure 5.2-1). For the intraslab events, the majority of focal mechanisms indicate strike-
parallel horizontal compression within the down-going Pacific Plate (which also show the same
counterclockwise rotation of horizontal maximum stress axes seen in the crustal mechanisms),
and three focal mechanisms indicate normal faulting with inconsistently oriented stress axes.
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Based on recurrence calculations for the period of December 1, 2012 through December 31,
2014 (from combined crustal and intraslab events), about two ML 4 events per year would be
anticipated within the Project Area, a ML 5 event would be anticipated about every three years,
and a ML 6 event about every 20 years. Due to the low fraction of crustal earthquakes compared
to the total, and low upper magnitudes of the crustal seismicity, the intraslab seismicity forms the
preponderance of events contributing to these recurrence statistics and thus drives the results.
5.3. Seismic Hazard Analysis
5.3.1. Preliminary Probabilistic Seismic Hazard Analysis
Based on the review of the literature and the previous studies, an updated site-specific seismic
source model was developed and preliminary ground motion parameters (based on FERC
guidelines) were developed for the Project for use in initial dam analyses. As part of the hazard
update, a new seismic source characterization model of the dam region and site was developed.
At the time of the SSSHA report preparation current ground motion prediction equations
(GMPEs) including next generation attenuation (NGA) relationships for shallow crustal sources,
and a recently developed GMPE for the Cascadia subduction zone, were used in the PSHA and
DSHA for the proposed Watana Dam.
5.3.2. PSHA Sensitivity Analysis - Intraslab
The intraslab earthquake source of the Alaska Subduction Zone lies directly beneath the
proposed Watana dam site, and was found to be the dominant contributor to the seismic hazard to
the Project. The existing United States Geological Survey (USGS) model was updated using data
from newly obtained seismic event monitoring. This data enabled the development of a 3D
intraslab model (dipping planes) defined by seismicity clouds within the down going slab. The
data was also used to develop 2D earthquake occurrence rate grids (Fugro 2014b; Attachment 6).
A PSHA sensitivity evaluation was conducted to determine the effect of varying key parameters
of the new intraslab model. The sensitivity of smoothing (correlation distance), Mmax, distance
to the slab, and averaged seismic shear wave velocity (Vs30) were evaluated. The results of this
study indicated that the PSHA was relatively insensitive to correlation distance and Vs30;
however, significant variations were observed with changes to Mmax (Fugro 2014b;
Attachment 6).
5.3.3. DSHA for Intraslab Events
A DSHA was conducted to evaluate seismic events associated with intraslab rupture. This
analysis was conducted to aid in the selection of design criteria at the Watana Dam. Values for
distance, magnitude, and epsilon (a statistical value) were evaluated and selected based on a
review of available data. The parameters were then used in a GMPE to determine preliminary
seismic design parameters (MWH 2014b).
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The following determinations were made base on the findings of this DSHA:
GMPEs for the DSHA should be selected based on hypocentral distance and hypocentral
depth rather than distance to the rupture plane.
A postulated Mmax 8.0 was recommended for the PSHA. Previous models had
considered a maximum magnitude of 7.5 (Fugro 2012; Attachment 2).
An epsilon value of 0.5 (corresponding to the 69th percentile value) was recommended
for the site if a large (magnitude 8) event is used for the MCE.
An epsilon value corresponding to the 84th percentile should be used if a smaller
(magnitude 7.5) is used for the MCE.
Peak ground acceleration (PGA) of 0.81g was calculated using the recommended epsilon
value of 0.5.
5.3.4. Seismic Shear Wave Velocity
The average seismic shear wave velocity (Vs30) of the upper 100 ft. (30 meters) was determined
at the location of each of the recently installed seismic monitoring stations. The Vs30 values
were determined using IMASW (interferometric multichannel analysis of surface waves) and
resulted in velocities ranging from 2415 ft/s to 10345 ft/s (737 to 3154 m/s) predominantly in
rock (Fugro 2014a; Attachment 4, Table 3). High on the north abutment above the Watana dam
site, a VS30 value was calculated to be 3,556 ft/s (1,084 m/s). The calculated shear wave
velocities were all determined to be higher than the value used in the initial PSHA (2,635 ft/s;
800 m/s) (Fugro 2012; Attachment 2 Subsection 5.2).
5.4. Seismic Design Criteria
Table 5-1 summarizes the PGA resulting from the MCE, MDE and OBE for Watana Dam. It
should be noted that the MCE is represented by four different response spectra from three
different sources: the subduction zone events – interface and intraslab and crustal events. The
intraslab is represented by a M7.5 and M8.0. Additional details regarding the PGA and
deterministic percentile selected for these events are presented in Table 5-2. The results are
summarized in the report titled Site-Specific Seismic Hazard Study (16.6) Summary Report
(MWH 2015a; Attachment 1).
Based on the initial PSHA for the Watana Dam site area, (Fugro, 2012; Attachment 2), the
seismic hazard at the dam site encompasses contributions from three different sources: the
subduction zone events – interface and intraslab (also referred to as the slab), and crustal events.
Response Spectra and time histories were developed for each type of event to evaluate the
difference in frequency content.
Prior to the completion of the crustal seismic source evaluation, the crustal event was selected to
be a M 7.0 event on the Fog Lake graben located at a distance of 4.4 mi (7 km). In the Crustal
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Seismic Source Evaluation (Fugro, 2015a; Attachment 5) the evaluation of potential crustal
seismic sources has not identified any specific features with evidence of late Quaternary faulting
within at least 25 mi (40 km) of the Watana dam site; however, this event is a conservative
representation of the background crustal event from the PSHA.
Guidance furnished by FERC, Evaluation of Earthquake Ground Motions, was followed and a
deterministic spectrum was used (Idriss & Archuleta, 2007). Figure 5.4-1 illustrates the response
spectrum. The 2,500, 5,000, and 10,000 year return period uniform hazard spectra are also
included on Figure 5.4-1; this data is from the seismic hazard analysis report (MWH 2015a,
Attachment 1, Section 11.1). It should be noted that the VS30 used in the initial probabilistic
seismic hazard assessment was 2,625 ft. (800 m/s).
The 84th percentile or above was used for all of the events, except the M 8.0 event for the slab,
where the 69th percentile is used. The interface event was scaled up at the fundamental period of
the dam (0.55 seconds) to match the 5,000 year return period, resulting in the 88th percentile, see
Figure 5.4-1.
In total, four sets of time histories containing three records each have been developed for the
slab, interface and crustal events using spectral matching techniques. All of the ground motions
are based on the deterministic analyses using a VS30 of 1,100 m/s. The intraslab event utilized
two different earthquake records, one was from the El Salvador Mw7.6 and the other was from
the Japan Mw7.0.
For the MCE, the time histories used are shown in Table 5-3.
Based on review comments on the initial design response spectra from the Board of Consultants
the design response spectra was increased for the interface event .The Chilean event, Table 5-4,
was also used for the dam analysis (MWH 2014b).
6. DISCUSSION
Three critical seismic sources are identified: (1) crustal faults, (2) subduction interface, and (3)
intraslab. For the crustal seismic sources, the principle sources considered in this study were the
Denali fault and Fog Lake graben. Deterministic evaluations found that the intraslab source
produces the largest PGA at the site. The deterministic evaluation indicates that the largest
values of ground motions at the site are associated with the subduction interface and intraslab
sources, because of their large magnitude, relatively short distance, and GMPEs used for these
sources. The deterministic results for the crustal sources (e.g. Denali fault, Fog Lakes graben,
Castle Mountain fault, and 10,000-year crustal seismicity) indicate that these sources are
relatively less significant, as compared to subduction megathrust and intraslab seismic sources.
The results of the field investigations of potential crustal seismic sources did not identify any
specific features with evidence of late Quaternary faulting within at least 25 mi (40 km) of the
Watana dam site (Fugro 2015a; Attachment 5). For most of this area, the time and detection
limits of the imagery and field investigations imply post-glacial limits of about 12,000 to 15,000
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years and detection of surface offsets of more than three feet (about 1m) over several miles. At
the Watana dam site where detailed LiDAR data is available, potential detection limits of surface
fault displacements are much lower, about 18 inches (0.5m) over several hundreds of feet. The
findings are consistent with the observations that the reservoir area is structurally coherent with a
lack of pervasive penetrative deformation.
Moreover, geomorphic and geological data similarly suggest that potential “blind” sources,
located at shallow crustal depths and capable of producing significant rates of long-term vertical
deformation are also absent from the dam site area. Thus given the absence of primary
seismogenic structures with appreciable rates of surface deformation in the immediate dam site
area, it is inferred that the potential for secondary faulting on structures at the dam site is also
absent or negligible.
Shallow crustal deformation in the nearby region of Watana dam site appears to be characterized
by near-horizontal maximum compressive stresses oriented northwest-southeast, parallel to the
geologic features at the site. Strain ellipse deformation concepts suggest that the likelihood of
reactivating northwest-oriented features under the existing conditions is low because of their near
parallelism with compressive stress.
With the installation of the Susitna-Watana Seismic Network, the seismic station density in the
region was increased. This led to greater magnitude detection capabilities, a decrease in
magnitude of completeness, and greater location accuracy. Focal mechanisms produced by the
AEC in the Susitna-Watana project area indicate that the crust around the proposed dam site is
undergoing north-northwest south-southeast horizontal compression, consistent with the relative
Pacific – North America plate motion, with the maximum horizontal stress rotating in a
counterclockwise direction from east to west in the network area (Fugro 2014c, 2015b).
Source deaggregation plots were developed, one for each of the four spectral response periods
(PHA, 0.5 sec, 1.0 sec, and 3.0 sec). Only sources contributing 5% or more at any ground
motion level are plotted on the deaggregations. The peak horizontal acceleration (PHA) hazard
is dominated by the Alaskan subduction zone intraslab source at all return periods.
Sensitivity studies indicate hazard variations due to Mmax choices of 7.8 and 8.1 are significant.
For the two response periods, at a 10,000 year return period, ground motion increases of about
25% and 50% are indicated, respectively. However, due to the paucity of ground motion records
for magnitudes above 7.5, magnitude scaling of ground motions in the GMPEs above Mw 7.5 is
highly uncertain, and warrants further investigation during final design.
Preliminary investigations into historical occurrence of the largest earthquake magnitudes for
worldwide subduction zones indicate that an upper bound value for future slab Mmax
distributions used in the final PSHA analysis is likely to lie above 7.5.
For the proposed Watana Dam, MCE ground motions were estimated following FERC
guidelines using DSHA, while the MDE was defined based on the 5,000-year return period
ground motions from a PSHA. The OBE was selected to be the 500 year return period from the
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PSHA. For analyses performed for engineering feasibility, a deterministic approach was
followed and design response spectra were recommended as follows:
MCE
Interface 88th percentile, M9.2 at a rupture distance of 78 km, PGA=0.58g
Intraslab 84th percentile, M7.5 at a hypocentral distance of 50km, PGA=0.69g
Intraslab 69th percentile M8.0 at a hypocentral distance of 50km, PGA=0.81g
Crustal 84th percentile M7.0 at a rupture distance of 3.5km, PGA=0.49g
OBE
The development of the OBE followed a probabilistic approach and was assigned a return period
of 500 years, which has resulted in a projected PGA of 0.27g.
7. CONCLUSION
The overall objectives of the Site Specific Seismic Hazard Assessment included conducting
deterministic and probabilistic seismic hazard evaluations to estimate earthquake ground motion
parameters, assess seismic risks, determine seismic loads that the Project facilities would be
subject to during and following seismic events, and propose seismic design criteria. These
objectives have been met and the efforts undertaken for this study provides the basic
requirements necessary for the license application. This study plan is complete.
The tasks conducted to-date demonstrates that the proposed Susitna-Watana Dam is feasible
from a seismic risk standpoint. These studies are intended to meet the requirements of filing for
a FERC license. Further evaluation will be required for detailed design and should include and
update to the site specific seismic hazard assessment and update and/or confirmation of the
seismic design criteria for final design.
8. LITERATURE CITED
Alaska Earthquake Information Center (AEC). 2015. Susitna-Watana Seismic Monitoring
Project: January–June 2015. Report. Prepared for the Alaska Energy Authority.
Anchorage, Alaska. Included as Attachment 10.
Alaska Energy Authority (AEA). 2014. Initial Study Report: Susitna-Watana Hydroelectric
Project FERC Project No. 14241. June 2014. Prepared for the Federal Energy
Regulatory Commission by the Alaska Energy Authority, Anchorage,
Alaska. http://www.susitna-watanahydro.org/type/documents/.
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
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FERC Project No. 14241 Page 16 October 2015
Alaska Energy Authority (AEA). 2015. 2014-2015 Study Implementation Report: Geology and
Soils Characterization Study 4.5: Susitna Hydroelectric Project. Prepared for the Federal
Energy Regulatory Commission by the Alaska Energy Authority. Anchorage, Alaska.
Csejtey, B., Nelson, W., Jones, D., Silberling, N., Dean, R., Morris, M., et al. 1978.
Reconnaissance geologic map and geochronology, Talkeetna Mountains quadrangle,
northern part of Anchorage quadrangle, and southwest corner of Healy Quadrangle,
Alaska. U.S. Geological Survey Open File Report 78-558-A. 62p. Plate.
FERC. 2012. Approval of Board of Consultants for the Susitna Watana Hydroelectric Project.
Letter to the Alaska Energy Authority from the Federal Energy Regulatory Commission.
October 23, 2012. P-14241-AK. 3p.
FERC. 2011. Engineering guidelines for the evaluation of hydropower projects. Retrieved
October 6, 2011. from Federal Energy Regulatory Commission:
http://www.ferc.gov/industries/hydropower/safety/guidelines/eng-guide/chap13-draft.asp
Fugro Consultants, Inc. (Fugro). 2012. Seismic Hazard Characterization and Ground Motion
Analyses for the Susitna-Watana Dam Site Area. Report prepared for Alaska Energy
Authority, Technical Memorandum No. 4, February 24, 2012, 146p. and appendices.
Included as Attachment 2.
Fugro Consultants, Inc. (Fugro). 2013. Lineament Mapping and Analysis for the Susitna-Watana
Dam Site. Report to the Alaska Energy Authority, Technical Memorandum No. 8, Dated
March 27, 2013, 61p. plus figures, plates, and appendix. Included as Attachment 3.
Fugro Consultants, Inc. (Fugro). 2014a. Watana Seismic Network Station Vs30 Measurements
for the Susitna-Watana Dam Site. Report prepared for Alaska Energy Authority.
Anchorage, Alaska. Technical Memorandum No. 14-12-TM. March 20, 2014. 51p. and
appendix. Included as Attachment 4.
Fugro Consultants, Inc., (Fugro). 2014b. Revised Intraslab Model and PSHA Sensitivity Results
for the Susitna-Watana Dam Site Area. Report prepared for Alaska Energy Authority.
Technical Memorandum No. 14-11-TM. April 25, 2014. 31p. Included as Attachment 6.
Fugro Consultants, Inc., (Fugro). 2014c. Seismic Network 2013 Annual Seismicity Report for
the Susitna-Watana Dam Site Area prepared for Alaska Energy Authority, Report No. 14-
06-REP, March 2015, 40p. Included as Attachment 8.
Fugro Consultants, Inc., (Fugro). 2015a. Crustal Seismic Source Evaluation for the Susitna-
Watana Dam Site. Report prepared for Alaska Energy Authority. Anchorage, Alaska.
Report No. 14-33-REP, May 2015, 141p. and appendices. Included as Attachment 5.
Fugro Consultants, Inc., (Fugro). 2015b. Seismic Network 2014 Annual Seismicity Report for
the Susitna-Watana Dam Site Area. Report prepared for Alaska Energy Authority.
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 17 October 2015
Anchorage, Alaska. Report No. 14-32-REP. March 31, 2015. 55 p. Included as
Attachment 9.
Gedney, L., & Shapiro, L. 1975. Structural Lineaments, Seismicity and Geology of the Talkeetna
Mountains Area, Alaska: Prepared for the U.S. Army Corps of Engineers, Alaska
Division. 18 pgs.
Idriss, I., & Archuleta, R. (2007). Evaluation of earthquake ground motions, draft manuscript for
FERC Chapter 13, draft 06.5. Retrieved October 6, 2011, from
http://www.ferc.gov/industries/hydropower/safety/guidelines/eng-guide/chap13-draft.pdf
Kachadoorian, R., & Moore, H. 1979. Preliminary Report of the Recent Geology of the Proposed
Devils Canyon and Watana Dam Sites, Susitna River, Alaska. in Southcentral Railbelt
Area, Alaska Upper Susitna River Basin Supplemental Feasibility Report. U.S. Army
Corps of Engineers, Alaska District. Appendix D
MWH. 2013. Preliminary Reservoir Triggered Seismicity. Prepared for Alaska Energy
Authority. Anchorage, Alaska. Te chnical Memorandum No. 10. March 29, 2013. 95 p.
Included as Attachment 11.
MWH. 2014a. Briefing Document – Discussion of MCE and OBE. Prepared for Alaska Energy
Authority. No. 14-13-BD. April 30, 2014. 6p. Included as Attachment 7.
MWH. 2014b. Engineering Feasibility Report for the Susitna-Watana Dam. Report prepared for
Alaska Energy Authority. Anchorage, Alaska. December 2014.
MWH. 2015a. Site-Specific Seismic Hazard Study Summary Report. Report prepared for Alaska
Energy Authority. Anchorage, Alaska. October 2015. 156 p. Included as Attachment 1.
MWH. 2015b. Dam Site Geology. Prepared for Alaska Energy Authority. Anchorage, Alaska.
Report No. 14-34-REP. February 2015. 62 p and appendices.
Plafker, G., Gilpin, L., & Lahr, J. 1994. Neotectonic map of Alaska. (a. H. edited by G. Plafker,
Ed.) The Geology of Alaska, The Geology of North America, Vol. G-f, Plate 12.
Twelker, E., Wypych, A., Sicard, K., Newberry, R., Freeman, L., Reioux, D., et al. 2014.
Preliminary results from 2014 geologic mapping in the Talkeetna Mountains
(presentation): Alaska Miners Association Annual Convention, Anchorage, Alaska,
November 3-9, 2014. Alaska Division of Geological & Geophysical Surveys file
po2014_007.pdf
Williams, J., & Galloway, J. 1986. Map of western Copper River basin, Alaska, showing lake
sediments and shorelines, glacial moraines, and location of stratigraphic sections and
radiocarbon-dated samples. U.S. Geological Survey. Open File Report 86-390. 30 p. 1
sheet, scale 1:250,000.
STUDY COMPLETION REPORT SITE-SPECIFIC SEISMIC HAZARD STUDY (16.6)
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Woodward Clyde Consultants (WCC). 1982. Final Report on Seismic Studies for Susitna
Hydroelectric Project. Prepared for Acres for the Alaska Power Authority.
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9. TABLES
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Table 5-1. Peak Ground Acceleration Values for the MCE, MDE and OBE
CASE DESIGN EVENT PGA
MCE Deterministic 0.81g
MDE 5,000-yr Return Period 0.66g
OBE 500-yr Return Period 0.27g
Table 5-2. Peak Ground Acceleration and Percentile for Deterministic Response Spectra
DESIGN EVENT CASE
Crustal Interface Intraslab
Fog Lake Alaskan Subduction Zone
MCE - DSHA
Magnitude 7.0 9.2 7.5 8.0
PGA(g) [percentile] 0.49 [84th] 0.58 [88th] 0.69 [84th] 0.81 [69th]
Table 5-3. Selected Time Histories for Feasibility Analysis– Intraslab and Crustal
EVENT RECORDING STATION TIME HISTORIES SEISMIC SOURCE
El Salvador (M 7.6) STTEC Mw 7.5 – 84th percentile Intraslab
Japan 2011 (M 7.0) MYG 009 Mw 8.0 – 69th percentile Intraslab
Loma Prieta, California (M 6.93) GIL Mw 7.0 – 84th percentile Crustal
Table 5-4. Selected Time Histories for Feasibility Analysis – Interface
EVENT RECORDING STATION TIME HISTORY SEISMIC SOURCE
Chile 2010 CURI Mw 9.2 – 88th percentile Interface
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10. FIGURES
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Figure 3.1-1. Regional Faults (Csejtey et. al. 1978; Williams and Galloway 1986; Plafker et. al. 1994)
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Figure 5.1-1. Dam Site Area Tectonic Features (Fugro 2015a)
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Figure 5.2-1. Crustal Stress Orientations and Strain Ellipses (Fugro 2015a)
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Figure 5.4-1. Design Response Spectra
0
0.5
1
1.5
2
2.5
3
0.01 0.1 1Spectral Acceleration (g)Period (s)
10,000 Year Return Period -Vs30=800m/s
5,000 Year Return Period -Vs30=800m/s
2,500 Year Return Period - Vs30=800m/s
Intraslab M8, D50km Vs30=1100m/s - 69th
Intraslab M7.5, D50km Vs30=1100m/s - 84th
Interface M9.2, D78km Vs30=1100m/s - 88th
Crustal M7.0, D7km Vs30=1100m/s - 84th
Fundamental Period
Range 0.55-0.62 s
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ATTACHMENT 1: SITE-SPECIFIC SEISMIC HAZARD STUDY
Report
15-03-REP
v0.0
Susitna-Watana Hydroelectric Project
Site-Specific Seismic Hazard Study Summary Report
(Attachment 1 to Study Completion Report 16.6)
AEA11-022
Prepared for: Prepared by:
Alaska Energy Authority MWH
813 West Northern Lights Blvd. 1835 South Bragaw St., Suite 350
Anchorage, AK 99503 Anchorage, AK 99508
October 2015
16-1503-REP-102915
[This page intentional blank]
ALASKA ENERGY AUTHORITY
AEA11-022
16-1503-REP-102915
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i October 2015
TABLE OF CONTENTS
Executive Summary ................................................................................................................ ES-1
1. Introduction ....................................................................................................................... 1
1.1. Background ............................................................................................................. 1
1.2. Regulators ............................................................................................................... 2
1.3. Objective ................................................................................................................. 3
1.4. Study Area .............................................................................................................. 4
1.5. Limitations .............................................................................................................. 5
2. Geologic Setting, Seismotectonic Setting and Seismicity ............................................... 6
2.1. Regional Geologic Setting ...................................................................................... 6
2.2. Regional Structure ................................................................................................ 12
2.3. Seismotectonics..................................................................................................... 15
2.4. Historical Seismicity ............................................................................................. 21
2.4.1. 2002 Denali Fault Earthquake ...................................................... 22
2.4.2. 1964 Great Alaskan Earthquake ................................................... 23
2.4.3. 1912 Delta River Earthquake ........................................................ 24
2.5. Susitna-Watana Seismic Network......................................................................... 24
3. Preliminary Seismic Hazard Analysis Approach ........................................................ 27
4. Crustal Seismic Source Evaluation Summary ............................................................. 29
4.1. General .................................................................................................................. 29
4.2. Methods................................................................................................................. 29
4.3. Results ................................................................................................................... 31
5. Dam Site Area Fault Rupture Evaluation .................................................................... 34
5.1.1. General .......................................................................................... 34
5.1.2. Methodology ................................................................................. 34
5.1.3. Regional Evidence ........................................................................ 36
5.1.4. Sub-Regional Geologic Transects................................................. 37
5.1.5. Dam Foundation Geologic Features ............................................. 41
5.1.6. Summary of Dam Foundation Fault Rupture Evaluation ............. 41
6. Seismic Source Characterization ................................................................................... 46
6.1. Subduction Zone ................................................................................................... 46
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6.1.1. Interface ........................................................................................ 48
6.1.2. Intraslab......................................................................................... 50
6.2. Crustal Faults ........................................................................................................ 50
6.2.1. Denali Fault ................................................................................... 53
6.2.2. Castle Mountain Fault ................................................................... 54
6.2.3. Pass Creek – Dutch Creek Fault .................................................... 55
6.2.4. Sonona Creek Fault ....................................................................... 56
6.3. Zones of Distributed Deformation ........................................................................ 56
6.3.1. Northern Foothills Fold and Thrust Belt Zone .............................. 56
6.4. Talkeetna Block Structures ................................................................................... 57
6.4.1. Talkeetna Thrust Fault / Talkeetna Suture .................................... 57
6.4.2. Susitna Lineament ......................................................................... 59
6.4.3. Shorter Structures Proximal to the Dam Site ................................ 59
6.5. Crustal Seismicity ................................................................................................. 60
6.5.1. Earthquake Catalog ....................................................................... 60
6.5.2. Crustal Source Zones .................................................................... 63
6.6. Earthquake Recurrence from Seismicity ............................................................... 63
7. Ground Motion Prediction Equations .......................................................................... 65
7.1. Subduction Zone ................................................................................................... 65
7.2. Crustal ................................................................................................................... 66
7.2.1. Shear Wave Velocity .................................................................... 66
8. Probabilistic Seismic Hazard Analysis.......................................................................... 67
8.1. Methodology ......................................................................................................... 67
8.2. Inputs..................................................................................................................... 67
8.2.1. Subduction Zone ........................................................................... 67
8.2.2. Crustal Sources ............................................................................. 68
8.3. Results ................................................................................................................... 74
8.3.1. Hazard Curves ............................................................................... 74
8.3.2. UHS............................................................................................... 78
8.3.3. Deaggregations ............................................................................. 80
9. Deterministic Seismic Hazard Analysis ........................................................................ 86
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9.1. Methodology ......................................................................................................... 86
9.2. Inputs..................................................................................................................... 86
9.3. Results ................................................................................................................... 87
10. Comparison of Probabilistic and Deterministic Results ............................................. 89
11. Seismic Design Criteria .................................................................................................. 94
11.1. Response Spectra for the MCE ............................................................................. 94
11.2. Response Spectra for the OBE ............................................................................ 102
12. Selection of Time Histories ........................................................................................... 104
12.1.1. Intraslab....................................................................................... 104
12.1.2. Interface ...................................................................................... 105
12.1.3. Crustal ......................................................................................... 105
12.1.4. Selected Events ........................................................................... 105
13. Additional Studies in progress ..................................................................................... 114
13.1. PSHA Sensitivity ................................................................................................ 114
13.2. PSHA Sensitivity Calculations Conclusions ...................................................... 114
14. Other Earthquake Related Hazards ........................................................................... 118
14.1. Reservoir Triggered Seismicity .......................................................................... 118
14.1.1. Summary ..................................................................................... 121
14.2. Seismic Induced Landslide Potential .................................................................. 122
15. Conclusions and Recommendations ............................................................................ 124
15.1. Recommendations to Update Existing Reports and Advance Studies ................ 127
16. References ...................................................................................................................... 129
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LIST OF FIGURES
Figure 1-1. South-Central Alaska Regional Faults ........................................................................ 5
Figure 2-1. Regional Tectonic Terranes and Basins – Part 1 of 2 ................................................. 7
Figure 2-2. Regional Tectonic Terranes and Basins Part 2 of 2 .................................................... 8
Figure 2-3. Schematic Evolution of South-Central Alaska ......................................................... 10
Figure 2-4. Correlations of Cenozoic Tectonic, Magmatic, and Sedimentary Events in South-
Central Alaska ............................................................................................................................... 11
Figure 2-5. Geologic Map Updated With Observations from 2014 ............................................ 13
Figure 2-6. Geologic Map Updated With Observations from 2014 ............................................ 14
Figure 2-7. Tectonic Setting of South-Central Alaska During the 1964 Earthquake (modified
from (Brocher, et al., 2014) .......................................................................................................... 15
Figure 2-8. Map View of Slab Planes .......................................................................................... 17
Figure 2-9. Schematic Showing Subducting Slab Geometry....................................................... 18
Figure 2-10. Denali Fault Characterization ................................................................................. 19
Figure 2-11. Castle Mountain Fault Characterization .................................................................. 21
Figure 2-12. Northern Foothills Fold and Thrust Belt ................................................................. 22
Figure 2-13. Rupture Areas for Historical Alaskan Subduction Zone Earthquakes .................... 23
Figure 2-14. Susitna-Watana Seismic Network Location Plan and November 16, 2012 through
December 31, 2014 Seismic Events.............................................................................................. 25
Figure 2-15. NNW-SSE-Oriented Cross Section Showing Seismicity from November 16, 2012
through December 31, 2014 .......................................................................................................... 26
Figure 4-1. Glacial Ice Reconstruction Profiles ........................................................................... 33
Figure 5-1. Crustal Stress Orientations and Strain Ellipses ......................................................... 39
Figure 6-1. Map and Cross Section of Alaska Subduction Zone Earthquakes ............................ 47
Figure 6-2. Subduction Interface Model ...................................................................................... 49
Figure 6-3: Castle Mountain Fault ............................................................................................... 55
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Figure 6-4. Site Vicinity Tectonic Features ................................................................................. 58
Figure 6-5. Unfiltered Earthquake Catalog .................................................................................. 61
Figure 6-6. Declustered Earthquake Catalog ............................................................................... 61
Figure 6-7. Magnitude vs. Time Prior to 2011 ............................................................................ 62
Figure 6-8. Final Recurrence Catalog .......................................................................................... 64
Figure 6-9. Maximum Likelihood Recurrence Curves for SAB Central Areal Zone .................. 64
Figure 8-1. Crustal Fault Model................................................................................................... 71
Figure 8-2. Hazard Curves for Peak Horizontal Acceleration ..................................................... 75
Figure 8-3. Hazard Curves for 0.5 sec Spectral Acceleration ...................................................... 76
Figure 8-4. Hazard Curves for 1.0 sec Spectral Acceleration ...................................................... 77
Figure 8-5. Hazard Curves for 3.00 sec Spectral Acceleration .................................................... 78
Figure 8-6. Mean Uniform Hazard Spectra, Total Hazard .......................................................... 80
Figure 8-7. Relative Contributions, Peak Horizontal Acceleration ............................................. 81
Figure 8-8. Relative Contributions, 0.5 sec Spectral Acceleration .............................................. 81
Figure 8-9. Relative Contributions, 1.0 sec Spectral Acceleration .............................................. 82
Figure 8-10. Relative Contributions, 3.00 sec Spectral Acceleration .......................................... 82
Figure 8-11. Deaggregation for the Interface, Peak Horizontal Acceleration, 2,500-year Return
Period ............................................................................................................................................ 84
Figure 8-12. Deaggregation for the Interface,1.0 sec Spectral Acceleration, 10,000-year Return
Period ............................................................................................................................................ 84
Figure 8-13. Deaggregation for the Intraslab, 0.5 sec Spectral Acceleration, 2,500-year Return
Period ............................................................................................................................................ 85
Figure 8-14. Deaggregation for the Intraslab, 3.0 sec Spectral Acceleration, 10,000-year Return
Period ............................................................................................................................................ 85
Figure 10-1. Intraslab Deterministic Hazard Compared to the Total Hazard UHS ...................... 89
Figure 10-2. Megathrust Deterministic Hazard Compared to the Total Hazard UHS .................. 90
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Figure 10-3. Denali Fault Deterministic Hazard Compared to the Total Hazard UHS ................ 90
Figure 10-4. Castle Mountain Fault Deterministic Hazard Compared to the Total Hazard UHS 91
Figure 10-5. Fog Lake Graben Deterministic Hazard Compared to the Total Hazard UHS ........ 91
Figure 10-6. Southern Alaska Block Central Period-Dependent Deterministic Hazard Compared
to the Total Hazard UHS............................................................................................................... 92
Figure 11-1. Design Response Spectra ........................................................................................ 97
Figure 11-2. Intraslab M8.0 – 69th Percentile Design Response Spectra and Intraslab M7.5 – 84th
Percentile Design Response Spectra ........................................................................................... 100
Figure 11-3. Interface M9.3 – 88th Percentile Design Response Spectra .................................. 101
Figure 11-4. Crustal M7.0 – 84th Percentile Design Response Spectra ..................................... 102
Figure 12-1. OBE Response Spectra and Scaled Crustal Event ................................................ 113
Figure 13-1. Mmax Sensitivity for Peak Ground Acceleration ................................................. 116
Figure 13-2. Mmax Sensitivity for 1.0 s Spectral Acceleration ................................................ 116
Figure 14-1. USGS Shake Map for 2002 Denali Earthquake (USGS) ...................................... 121
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LIST OF TABLES
Table ES-1. Peak Ground Acceleration Values for the MCE, MDE and OBE ....................... ES-4
Table 4-1. Summary of Lineament Groups and Areas ................................................................ 31
Table 6-1. Fault Characterization ................................................................................................ 51
Table 6-2. Northern Foothills Fold and Thrust Belt (NFFTB) Fault Data .................................. 52
Table 7-1. Ground Motion Prediction Equations Used in PSHA ................................................ 65
Table 8-1. Site Region Faults Excluded from the PSHA Source Model ..................................... 70
Table 8-2. Geometric Fault Parameters for Susitna Source Model, as Modeled for PSHA ........ 73
Table 8-3. Fault Slip Rate and Magnitude Parameters, as Modeled for PSHA ........................... 74
Table 8-4. Uniform Hazard Spectra (g) ....................................................................................... 79
Table 9-1. Deterministic Hazard Input Parameters ...................................................................... 87
Table 9-2. Crustal Seismicity (10,000 yr) Period-Dependent Deaggregation Results Summary 87
Table 11-1. Peak Ground Acceleration Values for the MCE, MDE and OBE ............................ 94
Table 11-2. Peak Ground Acceleration and Percentile for Deterministic Response Spectra ...... 94
Table 11-3. Deterministic Seismic Input Parameters .................................................................. 96
Table 11-4. Median Vertical / Horizontal Ratios ......................................................................... 98
Table 11-5. Horizontal and Vertical Design Response Spectra for Intraslab Events .................. 99
Table 11-6. Horizontal and Vertical Design response Spectra for Interface Events ................. 100
Table 11-7. Horizontal and Vertical Design Response Spectra for Crustal Events ................... 101
Table 11-8. PGAs for Selected Return Periods ......................................................................... 103
Table 11-9. OBE Horizontal Response Spectrum ...................................................................... 103
Table 12-1. Record Parameters for Selected Slab Time Histories – M8.0 -69th Percentile
(PGA=0.81) ................................................................................................................................. 106
Table 12-2. Record Parameters for Selected Slab Time Histories – M7.5 -84th Percentile
(PGA=0.69) ................................................................................................................................. 107
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Table 12-3. Record Parameters for Selected Interface Time Histories – M9.2 -88th Percentile
(PGA=0.58) ................................................................................................................................. 108
Table 12-4. Record Parameters for Selected Crustal Time Histories – M7.0 -84th Percentile
(PGA=0.49) ................................................................................................................................. 109
Table 12-5. Estimate of Significant Duration using the Brookhaven Model ............................ 111
Table 12-6. Selected Time Histories for Feasibility Analysis– Intraslab and Crustal ............... 113
Table 12-7. Selected Time Histories for Feasibility Analysis – Interface ................................. 113
Table 14-1. Deterministic Input Parameters .............................................................................. 120
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LIST OF ACRONYMS, ABBREVIATIONS, AND DEFINITIONS
Abbreviation Definition
AEA Alaska Energy Authority
AEC Alaska Earthquake Center
AFE Annual Frequency of Exceedance
ASZ Alaskan Subduction Zone
BC British Columbia
BOC Board of Consultants
DEM Digital Earth Model
DGGS Alaska Department of Geological & Geophysical Surveys
DSHA Deterministic Seismic Hazard Assessment
FERC Federal Energy Regulatory Commission
ft Foot
GF Geologic Feature
GMPE Ground Motion Prediction Equations
in Inch
INSAR Interferometric Synthetic Aperture Radar
ILP Integrated Licensing Process
IMASW Interferometric Multichannel Analysis of Surface Waves
ISR Initial Study Report
Ka Kiloannum
km Kilometer
LiDAR Light Detection and Ranging
m Meter
M Magnitude
mm Millimeter
Mmax Maximum Magnitude
Mw Moment Magnitude
MCE Maximum Credible Earthquake
MDE Maximum Design Earthquake
mi Mile
NFFTB Northern Frontal Fold and Thrust Belt
NGA Next Generation Attenuation
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Abbreviation Definition
OBE Operating Basis Earthquake
OSL Optically-Stimulated Luminescence
PHA Peak Horizontal Acceleration
PRM Project River Mile
PSHA Probabilistic Seismic Hazard Assessment
RCC Roller Compacted Concrete
RSP Revised Study Plan
RTS Reservoir Triggered Seismicity
SA Spectral acceleration
SAB Southern Alaska Block
sec Second
SPD Study Plan Determination
UHS Uniform Hazard Spectra
USACE United States Army Corps of Engineers
USGS United States Geological Survey
Vs Shear Wave Velocity
Vs30 Average Shear Wave Velocity in the Top 100 Feet (30 Meters)
WCC Woodward Clyde Consultants (AECOM)
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EXECUTIVE SUMMARY
The proposed Susitna-Watana Dam is a hydroelectric power development project planned for the
upper Susitna River. As proposed, the project would involve the construction of a dam, reservoir
and power facilities on the Susitna River at project river mile (PRM) 187, approximately 34
miles upstream of Devils Canyon. The current proposed project is a curved gravity dam,
approximately 705 ft. high, constructed using the roller compacted concrete (RCC) methodology.
The proposed Watana Dam is regulated by the Federal Energy Regulatory Commission (FERC).
This study was performed at the request of AEA in accordance with the FERC Engineering
Guidelines for the Evaluation of Hydropower Projects (FERC, 2014) and a Draft version of
Chapter 13, Evaluation of Earthquake Ground Motions (Idriss & Archuleta, 2007) guidelines,
and with direction from the FERC.
The purpose of this report is to provide FERC with the finalized Section 16.6 of the RSP for the
current licensing initiative which was filed on December 14, 2012. Section 16.6 is described as
the Site-Specific Seismic Hazard Study and focuses on conducting deterministic and
probabilistic seismic hazard evaluations to estimate earthquake ground motion parameters at the
Project site, assessing the risk at the site and the loads that the Project facilities would be subject
to during and following seismic events, and proposing design criteria for Project facilities and
structures considering the risk level This report is a compilation of technical memorandums,
reports and the Engineering Feasibility Report:
• Fugro Consultants, Inc., (FCL), (2012), Seismic Hazard Characterization and Ground
Motion Analyses for the Susitna-Watana Dam Site Area, prepared for Alaska Energy
Authority, Technical Memorandum No. 4, Dated February 24, 2012, 146 pages and 4
Appendices.
• Fugro Consultants, Inc., (FCL), (2013), Lineament Mapping and Analysis for the Susitna-
Watana Dam Site, prepared for Alaska Energy Authority, Technical Memorandum No. 8,
Dated March 27, 2013, 61 pages plus figures, plates, and 1 appendix.
• MWH, (2013a), Preliminary Reservoir Triggered Seismicity, prepared for Alaska Energy
Authority, Technical Memorandum No. 10 v3.0, Dated March 29, 2013, 95 pages.
• MWH, (2013b), Preliminary Reservoir Slope Stability Assessment, prepared for Alaska
Energy Authority, Technical Memorandum No. 12, Dated September 18, 2013, 43 pages
and 3 Attachments.
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• MWH, (2014a), Briefing Document – Discussion of MCE and OBE, prepared for Alaska
Energy Authority, Technical Memorandum No. 14-13-BD, Dated April 30, 2014, 6 pages.
• MWH, (2014c), Engineering Feasibility Report for the Susitna-Watana Dam, prepared for
Alaska Energy Authority, December 2014.
• Fugro Consultants, Inc. (FCL) (2014a). Watana Seismic Network Station Vs30
Measurements for the Susitna-Watana Dam Site, prepared for Alaska Energy Authority,
Technical Memorandum No. 14-12-TM, Dated March 20, 2014, 51 pages and 1
Appendix.
• Fugro Consultants, Inc., (FCL), (2014b), Revised Intraslab Model and PSHA Sensitivity
Results for the Susitna-Watana Dam Site Area, prepared for Alaska Energy Authority,
Technical Memorandum No. 14-11 -TM, Dated April 25, 2014, 31 pages.
• Fugro Consultants, Inc., (FCL), (2014c), Seismic Network 2013 Annual Seismicity
Report for the Susitna-Watana Dam Site Area, prepared for Alaska Energy Authority,
Technical Memorandum No. 14-06-REP, Dated March 14, 2015, 40 pages.
• Fugro Consultants, Inc., (FCL), (2015a), Crustal Seismic Source Evaluation for the
Susitna-Watana Dam Site, prepared for Alaska Energy Authority, Report No. 14-33-REP,
Dated May 2015, 141 pages and 3 Appendices.
• Fugro Consultants, Inc., (FCL), (2015b), Seismic Network 2014 Annual Seismicity
Report for the Susitna-Watana Dam Site Area, prepared for Alaska Energy Authority,
Technical Memorandum No. 14-32-REP, Dated March 31, 2015, 55 pages.
• Alaska Earthquake Information Center (AEC). (2015), Susitna-Watana Seismic
Monitoring Project: January –June 2015 Report, prepared for the Alaska Energy
Authority, September 2015, 43 pages.
This report summarizes our current investigations and engineering to date, however given the
iterative nature of these reports some initial studies have not incorporated results from
subsequent studies.
An initial seismic hazard characterization and ground motion analysis for the proposed Project
site, this technical memorandum includes the seismotectonic setting, historical seismicity,
seismic source characterization, applicable ground motion prediction equations, probabilistic
seismic hazard analysis (PSHA), and deterministic seismic hazard analysis (DSHA) (Fugro,
2012). Subsequent lineament mapping and analysis was based on digital imagery as well as the
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seismic sources identified in the initial seismic hazard analysis (Fugro, 2013). Focused studies
were then performed for the local crustal seismic sources / faults (Fugro, 2015a) and the
subduction intraslab (Fugro, 2014b).
A long -term earthquake monitoring system was created in August-September 2012, and
expanded in 2013. Details regarding the earthquake event data are documented in various reports
(Fugro, 2014c, 2015b; AEC, 2015). The earthquake monitoring system was installed to monitor
earthquake activity and to record strong shaking of the ground in the Project area during
moderate to strong earthquakes. Data obtained from this monitoring system has been used to
refine the intraslab model (Fugro, 2014b), check the source characterization of background
sources (Fugro, 2015a), define the focal mechanisms for the larger detected earthquakes, and
provide a background level of seismicity to monitor reservoir triggered seismicity (RTS).
Studies regarding earthquake hazards such as RTS, a dam site area fault rupture, and reservoir
rim slope stability were also performed. Current literature and empirical relationships were used
to evaluate the potential for RTS (MWH, 2013a). Site investigations were performed which
included: LiDAR (Light Detection and Ranging), INSAR (Interferometric Synthetic Aperture
Radar), geologic mapping, drilling and in situ testing, instrumentation installation and
monitoring, and laboratory testing. These site investigations were used to update the terrain units
and complete a preliminary evaluation of the reservoir rim slope stability (MWH, 2013b) and
also provide necessary data to evaluate the dam site area fault rupture evaluation, included in the
Crustal Source Evaluation (Fugro, 2015a).
For Watana Dam, maximum credible earthquake (MCE) ground motions were estimated
following FERC guidelines using deterministic seismic hazard analysis (DSHA), while the
maximum design earthquake (MDE) was defined based on the 5,000-year return period ground
motions from a probabilistic seismic hazard analysis (PSHA). The operating basis earthquake
(OBE) was selected to be the 500 year return period from the PSHA. Table ES-1 summarizes the
peak ground acceleration resulting from the MCE, MDE and OBE (see additional details
regarding the seismic design criteria in MWH, 2014a, 2014b).
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Table ES-1. Peak Ground Acceleration Values for the MCE, MDE and OBE
CASE DESIGN EVENT PGA
MCE Deterministic 0.81g
MDE 5,000-yr Return Period 0.66g
OBE 500-yr Return Period 0.27g
Finally, the report concludes with a discussion on the design response spectra and the
development of time histories. Additional details on the development of time histories can be
found in the Engineering Feasibility Report (MWH, 2014b; Appendix B6).
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1. INTRODUCTION
1.1. Background
The purpose of this report is to develop an assessment of the seismic hazard potential relating to
the Watana Dam which is part of the Susitna-Watana hydroelectric project (FERC Project No.
14241) located in Matanuska Borough, Alaska.
The Susitna- Watana hydroelectric project (“Project”) will be a major development on the
Susitna River some 120 mi (193 km) north and east of Anchorage and about 75 mi (121 km)
south of Fairbanks.
As proposed, the project would involve the construction of a dam, reservoir and power facilities
on the Susitna River at Project river mile (PRM) 187, approximately 34 miles (55 km) upstream
of Devils Canyon.
An application for a FERC license for the Susitna-Watana hydroelectric project was submitted to
FERC under FERC number P-7114 in 1983, with the application being subsequently amended
under the same number in 1985. That application was withdrawn in March 1986.
The current proposed project is significantly smaller than that previously proposed, but includes
a curved gravity dam - of a height of approximately 705 ft. (215 m) – constructed using the roller
compacted concrete (RCC) methodology. This choice of dam was made after comparison of an
Earth Core Rockfill dam, a Concrete Faced Rockfill dam, and a Roller Compacted Concrete
dam.
As part of the current licensing initiative, on December 14, 2012, Alaska Energy Authority
(AEA) filed with the Federal Energy Regulatory Commission (FERC or Commission) its
Revised Study Plan (RSP), which included 58 individual study plans. Section 16.6 of the RSP
described the Site-Specific Seismic Hazard Study. This Study Plan focuses on conducting
deterministic and probabilistic seismic hazard evaluations to estimate earthquake ground motion
parameters at the Project site, assessing the risk at the site and the loads that the Project facilities
would be subject to during and following seismic events, and proposing design criteria for
Project facilities and structures considering the risk level. RSP 16.6 provided goals, objectives,
and proposed methods for data collection regarding seismic hazards.
On February 1, 2013, FERC staff issued its study plan determination (February 1 SPD) for 44 of
the 58 studies, approving 31 studies as filed and 13 with modifications. RSP Section 16.6 was
one of the 31 study plans approved with no modifications.
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Following the first study season, FERC’s regulations for the Integrated Licensing Process (ILP)
required AEA to “prepare and file with the Commission an initial study report describing its
overall progress in implementing the study plan, schedule and the data collected, including an
explanation of any variance from the study plan and schedule.” (18 CFR 5.15(c)(1)) An Initial
Study Report (ISR) on the Site Specific Seismic Hazard Study was prepared in accordance with
FERC’s ILP regulations and detailed AEA’s status in implementing the study, as set forth in the
FERC-approved RSP.
1.2. Regulators
The proposed Watana Dam is regulated by the Federal Energy Regulatory Commission (FERC).
The Watana Dam has been evaluated using FERC Engineering Guidelines for the Evaluation of
Hydropower Projects (FERC, 2011). Guidance for the evaluation of seismic hazards is provided
under a Draft version of Chapter 13, Evaluation of Earthquake Ground Motions (Idriss &
Archuleta, 2007). These guidelines were released for review and further development by the
FERC, but have been used for several years in draft form. The regulatory process for seismic
hazard evaluation defined by the FERC specifies that both probabilistic and deterministic
evaluations be conducted. This study was performed at the request of AEA in accordance with
these guidelines, and with direction from the FERC.
The “Maximum Design Earthquake” (MDE) may be selected between the 2,500 year event and
the maximum credible earthquake (MCE). The dam and appurtenances must also be designed to
resist the Operating Basis earthquake (OBE) representing the ground motions or fault
movements from an earthquake considered to have a reasonable probability of occurring during
the functional life-time of the project. All critical elements of the project (such as dam,
This document records the completed initial Site Specific Seismic Hazard Study for feasibility and
licensing. For detailed design, certain studies would be undertaken to incorporate additional data to
be collected and update the seismic hazard analysis. Tasks may include:
• Update the crustal seismic source characterization based on new data obtained for design of
the project (rock structure and shear wave measurements at the dam site)
• Update the ASZ and worldwide subduction zone data will be needed to develop appropriate
weighting of uncertain parameters such as Mmax
• Subduction fault model and source characterization
• Revise areal source zones with recorded seismicity
• Update probabilistic seismic hazard analysis (PSHA)
• Update ground motion prediction equations (GMPE)
NGA West 2
Subduction
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appurtenant structures, reservoir rim, and equipment) should be designed to remain functional
during the OBE, and any resulting damage should be easily repairable in a limited time. For a
Class I dam, t he OBE can be defined based on probabilistic evaluations, with the level of risk
(probability that the magnitude of ground motion will be exceeded during a particular length of
time) as a probabilistic event with a return period between 150 and 250 years or greater return
frequency.
For Watana Dam, MCE ground motions were estimated following FERC guidelines using
deterministic seismic hazard analysis (DSHA), while the MDE was defined based on the 5,000-
year return period ground motions from a probabilistic seismic hazard analysis (PSHA). The
OBE was selected to be the 500 year return period from the PSHA.
1.3. Objective
The goals of this study were to conduct deterministic and probabilistic seismic hazard
evaluations to estimate earthquake ground motion parameters at the Project site, assess the risk at
the site and the loads that the Project facilities would be subject to during and following seismic
events, and propose design criteria for Project facilities and structures considering the risk level.
The intent of the study is to fulfill specific objectives including, but not limited to the following:
• Identify the seismic sources along which future earthquakes are likely to occur, including
the potential for reservoir-triggered seismicity;
• Characterization of the degree of activity, style of faulting, maximum magnitudes, and
recurrence information of each seismic source;
• Develop maps and tables depicting the spatial and geometric relations of the faults and
seismic source zones together with specific distance parameters to evaluate ground
motion parameters from each source;
• Assemble available historical and instrumental seismicity data for the region, including
maximum and minimum depth of events;
• Determine the distance and orientation of each fault with respect to the site;
• Estimate the earthquake ground motions at the proposed dam site, updating previous
studies to include changes in practice and methodology since the 1980s;
• Propose the seismic design criteria for the site; and,
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• Use the appointed Board of Consultants (BOC) for independent technical review and
guidance during development of site-specific studies.
The Supporting Design Report prepared for the FERC license application will include the
seismic criteria determined herein, and the results of dam stability analysis under seismic
loading.
1.4. Study Area
The study area for the seismic hazard evaluation was necessarily large in order to include
potentially significant seismic sources throughout the region. The study area encompassed
subduction-related sources (i.e. plate interface between the North American and Pacific Plates,
which was the source of the 1964 earthquake, the epicenter of which is a significant distance
south of the Project, and intraslab sources within the down-going Pacific Plate) and all applicable
Quaternary crustal seismic sources within about 62 miles (100 kilometers) of the site (Figure
1-1). Crustal seismic sources beyond these distances are not expected to provide significant
ground motion contributions at the dam site relative to nearby sources. A more focused study
area included the dam site and reservoir areas. The study area thus included much of the
Talkeetna block and surrounding fault zones such as the Denali; Castle Mountain; Northern
Foothills fold and thrust fault zone; inferred Talkeetna fault; and Broad Pass Fault.
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Figure 1-1. South-Central Alaska Regional Faults
1.5. Limitations
This report is presented to provide an evaluation of the site specific seismic hazards and their
potential effect on the design and economic construction of the proposed project. The analyses,
conclusions, and recommendations contained in this report are based on 1) the project site
conditions as they existed at the time of this evaluation, 2) review of readily available existing
data and information obtained from public and private sources, 3) the degree to which
unconsolidated sediments mantle bedrock, 4) the duration of seismic event monitoring, and 5)
the project layout described herein. In the event that there are any changes in the nature, design
or location of the project, if additional subsurface data and seismic event data are obtained or any
future additions are planned, the conclusions and recommendations contained in this report will
need to be reevaluated by MWH in light of the proposed changes or additional information
obtained.
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2. GEOLOGIC SETTING, SEISMOTECTONIC SETTING AND
SEISMICITY
2.1. Regional Geologic Setting
The tectonic evolution of south-central Alaska is defined by long-term plate convergence, with
Mesozoic (i.e., Jurassic-Cretaceous) collisions of the Wrangellia composite terrane to North
America. The Wrangellia composite terrane itself is an accretion of the Peninsular terrane to the
Wrangellia terrane. The Wrangellia terrane generally consists of late Paleozoic flood basalts and
meta volcanic rocks; the Peninsular terrane consists of Mesozoic (Jurassic) arc volcanics,
metasediments, and plutons. The two terranes originated well south (~30° latitude) of their
current position and were sutured together in the Late Jurassic (Csejtey, et al., 1978). The
Wrangellia composite terrane, in turn, was accreted onto North America in the mid- to late-
Cretaceous when the southern plate margin of North America was roughly along the position of
the Denali fault. Between the converging Wrangellia composite terrane and North America was a
marine basin (Kahiltna Basin) that accumulated syn -collisional Jurassic-Cretaceous
sedimentation shed from the southeast direction (Kalbas, Ridgway, & and Gehrels, 2007). The
northeast-striking Talkeetna fault, located approximately 3.5 miles (5.6 km southeast of the
Watana dam site, is the eastern boundary of the Wrangellia composite terrane, with the Jurassic-
Cretaceous sedimentary rocks (i.e., Kahiltna Basin deposits) on the northwest of the fault and the
Wrangellia composite terrane rocks to the southeast of the fault (Figure 2-1 and Figure 2-2).
Thus, in terms of terrane accretion, the region of crust south of the Denali fault and northeast of
the Talkeetna fault is a large suture zone that narrows to the east, reflecting oblique plate
convergence and the long-term closing of the Kahiltna Basin. The rocks that formed in the
Kahiltna Basin have been uplifted through the Cenozoic, making up much of the Alaska Range
and northwestern Talkeetna Mountains and forming a structural inversion. Essentially, formerly
low-lying areas (i.e., basins) have now become high topography (i.e. mountains) as a result of
plate convergence and mountain-building uplift along generally northeast trending folds and
thrust faults.
Jurassic plutonism from melting of the oceanic subducting slab formed the batholitic complex of
the southeastern Talkeetna Mountains (Nelson 2009) by intruding into the Peninsular terrane
(Figure 2-2, map unit TKg). Subsequent uplift initiated northeast-directed sedimentation within
proto-Kahiltna Basin in what is now the northeastern Talkeetna Mountains (Kalbas, Ridgway, &
and Gehrels, 2007). Kahiltna Basin sediments continued to accumulate during the Cretaceous as
westward sediment transport on fluvial, shallow marine and submarine fan depositional
environments. The Kahiltna assemblage is about 1.9 to 3.1 miles thick (3 to 5 km), and consists
of turbidite sequences, chert, mudstone, sandstone, and greywackes that comprise eight distinct
lithofacies (Kalbas, Ridgway, & and Gehrels, 2007). Progression in the understanding of the
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relationships between the terrane units and tectonics has allowed a deeper understanding about
the Kahiltna Basin rocks and their significance as a recorder of long-term tectonic deformation,
in contrast to previous interpretations that generalized the complex stratigraphic unit as
“argillite” or “flysch”.
Figure 2-1. Regional Tectonic Terranes and Basins – Part 1 of 2
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Figure 2-2. Regional Tectonic Terranes and Basins Part 2 of 2
Oblique subduction of an oceanic spreading center during Paleocene to early Eocene initiated
magmatism and formation of short-lived northwest trending extensional (normal) faults shown in
Figure 2-3 (Ridgway K. , Trop, Glen, and O’Neill, & eds., 2007). Included in these volcanics are
the Cantwell and Jack River volcanic fields dated at 55 to 60 Ma, and 50 to 56 Ma, respectively
shown in Figure 2-4 (Cole, Layer, Hooks, & and Turner, 2007). To the southeast, volcanic flows
that overlie and cap the Talkeetna fault are dated at 50 Ma (Csejtey, et al., 1978). Thus, Tertiary
magmatic intrusions punctuate the Kahiltna Basin assemblage and, the Wrangellia composite
terrane, and the Talkeetna fault (Figure 2-2).
Regional crustal rotation of southern Alaska took place sometime in the early to mid-Tertiary,
with rotation of 30 to 50 degrees in the counterclockwise direction accommodated by the dextral
Denali and Castle Mountain faults to the north and south, respectively. Consequently, regional
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transpressive deformation occurred during middle Eocene to Oligocene time, generating narrow
fault-bounded basins along major strike slip faults as well as northeast trending folds (Trop &
Ridgway, 2007). The Watana Creek basin probably was formed during this time as the Talkeetna
fault re-activated as a strike slip structure from the changing crustal stress orientations (Figure
2-3 and Figure 2-4).
Post-Eocene tectonic growth of southern Alaska is controlled by the oblique collision of the
Yakutat terrane, probably 15 to 10 Ma, with construction of continental magmatic arcs (i.e., the
Wrangell volcanic field) from subduction of the Yakutat microplate, and development of large
coastal mountain ranges (e.g., St. Elias Mountains). The collision of the Yakutat microplate is
considered to have substantial influence on the deformation and counterclockwise rotation in the
interior of south-central Alaska (Haeussler P. , 2008). Subduction of the Pacific plate continued
beneath North America from Eocene onwards, with growth of the Aleutian Islands from three
main pulses of arc-wide magmatism occurring at 38 to 29 Ma, 16 to 11 Ma, and 6 to 0 Ma (Jicha,
Scholl, Singer, Yogodzinski, & Kay, 2006).
Since the latest Cenozoic through today, south-central Alaska has experienced rapid rates of
tectonic deformation driven by the obliquely convergent northwestward motion of the Pacific
Plate relative to the North American Plate. In this region, the Pacific Plate is converging with
North American Plate at a rate of 2.1 inches per year (in/yr)(54 millimeters per year (mm/yr)) at
a slightly oblique angle (Demets & Dixon, 1999) (Carver & Plafker, 2008). Consequently, rates
and magnitudes of seismicity are also accordingly high. In southern and southeastern Alaska, the
oblique convergent plate motion is accommodated by subduction of the Pacific Plate along the
Alaska-Aleutian megathrust trench, and dextral (right-lateral) transform faulting along the Queen
Charlotte and Fairweather fault zones. Transpressional deformation primarily is accommodated
by dextral slip along the Denali and Castle Mountain faults, as well as by horizontal crustal
shortening to the north of the Denali fault.
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Figure 2-3. Schematic Evolution of South-Central Alaska
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From Cole et al., 2007
Age-event diagram showing radiometric ages of volcanic rocks in the Caribou Creek volcanic field.
Figure 2-4. Correlations of Cenozoic Tectonic, Magmatic, and Sedimentary Events in South-Central Alaska
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The regional magmatism described above directly forms the rocks that make up the dam site
though plutonic intrusions and volcanism. Multiple ages of early Cenozoic (i.e., Tertiary)
volcanics intruded the Kahiltna formation, as well as the Wrangellia Terrain rocks and the
Talkeetna suture zone (e.g., (Wilson, Schmoll, Haeussler, Schmidt, Yehle, & Labay, 2009)). The
rocks present at the dam site range in mineralogical composition and texture, including diorite
intrusions, andesite, and felsic dikes and, to a lesser extent, mafic volcanic extrusive rocks
(Acres, 1982b). Previous geologic mapping reveals that the volcanic rocks have a complex field
relationship at the dam site with intrusive and extrusive rocks often occurring proximal to each
other with gradational contacts. A range of mineralogical variability within intrusional bodies is
relatively common (U.S. Army Corps of Engineers (USACE), 1979). Review of rock core
drilled for the project (Golder Associates, Inc., 2013), (MWH, 2014) as well as inspection of
field outcrops confirms the complexity of the igneous history. Both andesite and diorite rocks
include a wide range of textures and compositions. In some instances, diorite bodies locally are
cut by felsic dikes. In both outcrop and core samples, inclusions of diorite have been observed
within the andesite. No dikes were found cutting the andesite, suggesting it is the youngest
volcanic unit at the site based on these cross cutting relationships ( (Acres, 1982a); p. 6-7). The
intrusions likely occurred sometime between 50 to 60 Ma, the field observations and
relationships confirm multiple ages (or, episodes) of volcanism, intrusion, or flows of which the
specific chronology has yet to be defined. Mapping by Csejtey et al. (1978) suggests that the
dam site rocks could be of the order of 58 Ma; however, these dates were not collected on rock at
the dam site. Rock samples were collected during 2014 field investigations that may be suitable
for absolute dating purposes to establish site geochronology.
2.2. Regional Structure
The geologic mapping transect along the Susitna River, extending through the Watana dam site
area, suggests that the site area lies within a relatively coherent crustal block of Kahiltna
assemblage sedimentary rocks which are overall gently tilted to the northwest, moderately
folded, and intruded by multiple early to mid-Tertiary plutonic and volcanic rocks (Figure 2-5
and Figure 2-6). The Watana dam site area lies within an area of Tertiary intrusive rocks.
Kahiltna assemblage rocks and additional intrusive rocks downstream of the Watana dam site
near the confluence of the Tsusena Creek and Susitna River appear structurally congruent, with
an apparent absence of major cross-cutting structure or extensive penetrative deformation. There
are likewise no significant expressions of vertical uplift or tectonics along the Susitna River
transect, downstream of the Talkeetna fault and Watana Creek basin.
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Figure 2-5. Geologic Map Updated With Observations from 2014
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Figure 2-6. Geologic Map Updated With Observations from 2014
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2.3. Seismotectonics
South-central Alaska experiences significant tectonic deformation and seismicity driven by the
oblique convergent northwest motion of the Pacific Plate relative to the North American Plate.
The Talkeetna Mountains formed as a direct result of the convergence of these plates as the
Pacific Plate was subducted below the North American Plate as shown in Figure 2-7.
Figure 2-7. Tectonic Setting of South-Central Alaska During the 1964 Earthquake (modified from (Brocher, et al., 2014)
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The Alaska-Aleutian subduction zone is one of the longest and most tectonically active plate
boundaries in the world. It extends for nearly 2,500 miles (4,000 km) from south-central Alaska
to the Kamchatka peninsula, and has produced some of the world’s strongest earthquakes – such
as the 1964 magnitude (M) 9.2 Great Alaskan (or Good Friday) earthquake. The subduction
zone has three tectonic regimes: continental subduction in the east, an island arc along the central
Aleutian volcanic chain, and oblique subduction and transform tectonics in the west (Nishenko &
Jacob, 1990). The eastern continental subduction zone, in the vicinity of Prince William Sound,
is significant in the evaluation of the seismic hazards at the Watana Dam site. In this region, the
Pacific Plate is converging with the North American Plate at a rate of 2.1 in/yr (54 mm/yr) at a
slightly oblique angle ( (Demets & Dixon, 1999); (Carver & Plafker, 2008)).
It has been recognized that the Alaska-Aleutian subduction zone is segmented in central Alaska,
and may be broken into independent fragments (Ratchkovski & Hansen, 2002). In addition, it
has been recognized that the Alaskan-Aleutian subduction zone’s eastern termination lies within
100 km northeast of the Susitna-Watana site (Fuis, et al., 2008). The precise location and
geometric character of the slab edge are not well determined. Ruppert and Hansen (2010),
define three major sections of the slab, which they termed the McKinley, Kenai, and Kodiak
Blocks. The dam site is located within the McKinley Block as shown in Figure 2-8, this figure
was developed based on the work performed and will be discussed later in this report. A
schematic of the subducting slab, which has a shallow dip (Carver & Plafker, 2008) and a typical
forearc basin is shown on Figure 2-7. Small-magnitude seismicity observed from Susitna-
Watana seismograph network data occurs within a 7.6 mi (12.3 km) thickness at the top of the
slab as shown in Figure 2-9. In central Alaska, the slab is approximately 30.5 mi (50 km) thick
(Zhao, Christensen, & Pulpan, 1995). Larger (M 7+) slab earthquakes are likely to rupture deeper
into the slab than the zone defined by the recorded microseismic event in Figure 2-9.
Further south, transform motion along the eastern edge of the subducting slab is accommodated
by the Fairweather and Queen Charlotte (not shown) fault zones.
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Figure 2-8. Map View of Slab Planes
PROJECT SITE
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Figure 2-9. Schematic Showing Subducting Slab Geometry
The dam site is located within a distinct geologic domain referred to as the Talkeetna block. The
Talkeetna block is bounded by the Denali fault system to the north, the Castle Mountain fault to
the south, the Wrangell Mountains to the east and the northern Aleutians and Tordrillo Mountains
volcanic ranges to the west (Figure 2-1). Major stress is released along the Denali and Castle
Mountains bounding faults during earthquakes resulting in movement (i.e., strain). However, it
is less clear how stress and strain are accommodated to the east and west. There is a relative
absence of large historical earthquakes within the Talkeetna block as well as a lack of mapped
faults with documented Quaternary displacement ( (Koehler, 2013); (Koehler R. , Farrell, Burns,
& and Combellick, 2012) ). The absence of earthquakes and mapped Quaternary faults within
the block implies that the block is behaving rigidly with little to no internal deformation.
The Talkeetna suture zone refers to the broad area of crust northwest of the Talkeetna thrust fault
and south of the Denali fault (labeled in Figure 2-1.) Glen et al. (2007) describe the Talkeetna
suture zone as a deep crustal structure bounding the northwestern edge of the Wrangellia
Terrane; thus the Talkeetna fault is the southeastern boundary of the suture zone.
The Denali fault predominantly shows right-lateral, strike-slip fault motion; and in plan the fault
has an arcuate shape and defines the northern margin of the Talkeetna block as shown in Figure
2-1.
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The Denali fault has been a major structural component of Alaska since it formed during the Late
Jurassic to early Cretaceous Period (Ridgway K. D., Trop, Nokleberg, Davidson, & Eastham,
2002). Offsets of 56 Ma metamorphic and intrusive rocks suggests at least 249 mi (400 km) of
total right lateral displacement (Nokleberg, Jones, & Silberling, 1985). Offset is also constrained
in the Denali region where the 38 million year old Mt. Foraker pluton is displaced 24 mi (38 km)
from the McGonagal Pluton (Reed & Lanphere, 1974).
In 2002, movement on the Denali fault produced an M 7.9 earthquake, the largest strike-slip
earthquake to occur in North America in almost 150 years (Eberhart-Phillips, et al., 2003).
Detailed studies of offset glacial features along the fault following the earthquake have
demonstrated a westward decrease in the Quaternary slip rate along the fault ( (Matmon, et al.,
2006); (Mériaux, et al., 2009)), as shown in Figure 2-10.
Figure 2-10. Denali Fault Characterization
Along the north and south sides of the Denali fault lie two zones of deformation. To the north is
the Northern Foothills Fold and Thrust Belt (NFFTB), a zone of variably dipping, but generally
Quaternary thrust faults and folds that accommodates transpressional deformation along the
north side of the Alaska Range (Figure 2-10). The westward reduction in Denali fault slip rate is
considered to be predominantly the result of strain partitioning onto the NFFTB ( (Haeussler P. ,
An overview of the neotectonics of interior Alaska—Far-field deformation from the Yakutat
Microplate collision, 2008); (Mériaux, et al., 2009) ).
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The other zone of deformation adjacent to the Denali fault lies south of the fault where several
thrust faults splay from the Denali fault’s central section as shown on Figure 2-10. Most of these
faults are recognized as Tertiary terrane-bounding features in which Mesozoic or Paleozoic rocks
are thrust over Tertiary sediments and volcanics (Haeussler P. , 2008). Rupture along the
previously unmapped Susitna Glacier thrust fault during the 2002 Denali fault earthquake
highlighted the potential for seismogenic activity in this area, in contrast to the relatively sparse
mapping of Quaternary faults south of the Denali fault. This concept is well expressed in the
Neotectonic Map of Alaska fault explanatory note (Plafker, Gilpin, & Lahr, Neotectonic map of
Alaska, 1994).
The Castle Mountain fault defines the southern margin of the Talkeetna block. This fault is
described by some as a dextral oblique strike-slip fault whose western segment is defined by a 39
mi (62 km) long Holocene fault scarp. Recent field and LiDAR-based geomorphic observations
by Koehler et al. (2014) support the inference that the Castle Mountain fault is a high angle
oblique reverse fault. The eastern section is primarily evident in bedrock, and there is no
indication of Holocene surface rupture as shown in Figure 2-11 . Paleoseismic studies, by
Haeussler et al. (2002), on the western section demonstrate four earthquakes on the fault in the
past 2,800 years, with a recurrence interval of approximately 700 years. More recent work by
Koehler et al. (2014), suggest only two earthquakes in the Holocene indicating that the
recurrence interval could be longer than previously thought. Despite the apparent lack of
Holocene surface rupture on the eastern section, this section of the fault is spatially associated
with historic seismicity as high as M 5.7 (Lahr, Page, Stephens, & Fogleman, 1986).
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Figure 2-11. Castle Mountain Fault Characterization
2.4. Historical Seismicity
The region within the 124 mi (200 km) radius of the Watana dam site is seismically active as
indicated by the occurrence of earthquakes with a magnitude greater than or equal to M 5 (AEC
seismicity catalog). The greatest number of these are deep (> 25 mi (40 km) depth) events with
magnitudes up to M 7.1, that likely are associated with the subducting Pacific Plate, and a
smaller number of events to the southeast that likely are associated with tectonic under-plating of
the Yakutat block. The remaining events are crustal earthquakes occurring at depths of about 19
mi (30 km) or less. The largest of those crustal earthquakes is the 2002 M 7.9 Denali fault
earthquake (initiated on the Susitna Glacier fault), with an epicenter approximately 59 mi (95
km) from the dam site. Several of the M ≥ 5 events are associated with the Denali fault
including: the M 6.7 foreshock of the 2002 earthquake (Nenana Mountain earthquake), several
2002-2003 aftershocks up to M 5.8, and six additional events up to M 6.4.
Events up to M 7.2 are located in the Northern foothills fold and thrust belt and the Minto Flats
seismic zone. The Northern foothills fold and thrust belt includes the Kantishna seismic cluster,
the Northern foothills thrust, and the Molybdenum Ridge fault (Figure 2-12). An M 5.7 event in
1984 is associated with the Castle Mountain fault (Lahr, Page, Stephens, & Fogleman, 1986).
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Many events cannot be spatially correlated with a documented Quaternary fault, including an M
7.2 earthquake in 1912.
Seven historical earthquakes are documented within 31 mi (50 km) of the site (AEC catalog).
Four of these earthquakes occurred at depths between 30 to 60 mi (49 to 97 km), which places
them within the subducting slab. The largest slab event within 31 mi (50 km) of the site has a
magnitude of M 5.4. Three earthquakes are located at upper crustal depths (13-22 mi [21-36
km]), the largest of which has a magnitude of M 6.2. These three earthquakes occurred between
1929 and 1933, and spatially are not associated with any known Quaternary fault, though they
may be inaccurately located, or have poor depth control, due to the lack of regional seismograph
stations at that time.
Figure 2-12. Northern Foothills Fold and Thrust Belt
2.4.1. 2002 Denali Fault Earthquake
The M 7.9 2002 Denali fault earthquake is the largest onshore strike-slip earthquake in North
America in the past 150 years (Eberhart-Phillips, et al., 2003). The earthquake initiated on the
previously unmapped Susitna Glacier thrust fault (Figure 2-10) with a 30 mi (48 km) surface
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rupture and up to 36 ft. (11 m) of displacement (Crone, Personius, Craw, Haeussler, & Staft,
2004). The earthquake then propagated eastward rupturing 140 mi (226 km) of the central Denali
fault and 41 mi (66 km) of the Totschunda fault. Average slip along the Denali fault was
approximately 16 ft. (5 m), with a maximum slip of 29 ft. (8.8 m) west of the junction with the
Totschunda fault (Haeussler, et al., 2004). The earthquake caused no fatalities and minimal
damage to infrastructure, likely due to the sparse population density near the fault. The estimated
intensity of the earthquake at the Watana Dam site was Modified Mercalli scale VI (USGS,
2003).
2.4.2. 1964 Great Alaskan Earthquake
The M 9.2, March 28, 1964 Great Alaskan earthquake had an epicenter directly south of the 124
mile (200 km) radius site region; however, the subsurface rupture area extends nearly beneath the
site region (Figure 2-13). The isoseismal map of the event shows the Watana Dam site
experienced ground shaking with Modified Mercalli scale VII intensity (Stover & Coffman,
1993). The earthquake is the second largest recorded in the world since instrumental recordings
began in the late 1800s (the one larger event was the 1960 M9.5 Chilean earthquake).
Figure 2-13. Rupture Areas for Historical Alaskan Subduction Zone Earthquakes
The 1964 earthquake ruptured approximately 500 mi (800 km) of the Aleutian megathrust with
left-lateral reverse-slip motion, and produced approximately 66 ft. (20 m) of maximum
displacement (Christensen & Beck, 1994). The earthquake was felt over 700,000 square miles in
Alaska and Canada (Hake & Cloud, 1966) with an intensity of MM VII estimated at the Watana
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Dam site (Stover & Coffman, 1993). Coseismic vertical displacements affected an area of about
200,000 square miles. Prince William Sound experienced up to 38 ft. (11.5 meters) of uplift, and
7.5 ft. (2.3 meters) of inland subsidence (relative to sea level) occurred (Plafker, 1969). Fifteen
fatalities were attributed to the earthquake and 113 fatalities from the ensuing tsunami. In
Anchorage, the earthquake destroyed structures up to 6-stories high and triggered numerous
destructive landslides.
2.4.3. 1912 Delta River Earthquake
A widely felt 1912 earthquake, commonly referred to as the Delta River earthquake, was
relocated by Doser (2004) to a location within 6 mi (10 km) of the Denali fault, though with 95%
error bounds of about 62 mi (100 km) in the east-west direction and 44 mi (70 km) north to
south. Carver et al. (2004) interpreted healed tree damage as having resulted from surface
deformation during the 1912 event. However, paleoseismic studies at several sites along the
Denali fault do not show any evidence for a surface rupturing 1912 event ( (Schwartz &
(DFEWG), 2003); (Plafker, Carver, Cluff, & and Metz, 2006); (Koehler, Personius, Schwarz,
Haeussler, & Seitz, 2011b) ). Therefore, the event is considered as being unassociated with a
particular known crustal fault.
2.5. Susitna-Watana Seismic Network
An earthquake monitoring network was created in August-September 2012, and expanded in
2013. The system was installed to monitor earthquake activity and to record strong shaking of
the ground in the Project area during moderate to strong earthquakes. The entire network system
at build-out consisted of four 6-component strong motion and broadband stations, three 3-
component broadband stations, and a GPS station, co-located with a 6-component seismograph
station at the dam site.
The monitoring network remained in operation through mid-June 2015. The system coverage is
within about 31 mi (50 km) of the proposed dam site – all as shown on Figure 2-14. Data for the
analysis of seismic events in the Project area were obtained in real-time for processing, and
archived by IRIS (Incorporated Research Institutions for Seismology). The seismograph stations
were operated as part of the Alaska Seismographic Network by the Alaska Earthquake
Information Center (AEC). These seismograph stations have increased the station density in the
region, leading to greater magnitude and detection capabilities, a decrease in magnitude of
completeness, and greater location accuracy. The increase in recorded events has led to a better
picture of shallow crustal seismicity and intraslab seismicity associated with the subducting
Pacific Plate below the proposed dam site. Due to funding constraints, stations WAT2, WAT3,
WAT4, and WAT 5 were removed in mid-June, 2015.
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Figure 2-14. Susitna-Watana Seismic Network Location Plan and November 16, 2012 through
December 31, 2014 Seismic Events
From November 16, 2012 to the end of 2014 the earthquake monitoring system has recorded a
total of 2,523 earthquakes which were located within a region roughly 50 mi (80 km) east-west
and 30 mi (50 km) north-south of the site.
The earthquakes in the Project area form two distinct groups, crustal events between 0 and 25 km
depth and intermediate depth events below 18.6 mi (30 km) in the subducting Pacific plate. This
can be seen clearly in the cross-section (Figure 2-15).
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Figure 2-15. NNW-SSE-Oriented Cross Section Showing Seismicity from November 16, 2012 through December 31, 2014
Since network installation, the largest earthquake within the subducting plate, a magnitude 4.6
earthquake, occurred on November 29, 2014 at a depth of 37.9 mi (62 km) and located 39.5 km
(24.5 miles) ESE of the dam site. The largest earthquake magnitude within the crust, magnitude
3.8, occurred on July 24, 2013 at a depth of 6.9 mi (11.1 km) and was located 8.8 mi (14.2 km)
NW of the dam site. About 10 small aftershocks (M=0.9-2.3) were located within a few days of
the 3.8 event.
Details of seismicity patterns, rates, and focal mechanisms refer to reports by Fugro (2015a) and
AEC (2015).
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3. PRELIMINARY SEISMIC HAZARD ANALYSIS APPROACH
At the beginning of the studies recorded in this report, a preliminary deterministic and
probabilistic seismic hazard evaluation was undertaken to update the seismic hazard studies from
the 1980s by Woodward Clyde Consultants ( (Woodward-Clyde Consultants (WCC), 1980);
(Woodward Clyde Consultants (WCC), 1982)) and R&M (2009) including an update of the site-
specific seismic source model (Fugro 2012). Initial ground motion parameters were developed
based on a desk-top study. The methods follow general guidance defined according to Chapter
13 of the Federal Energy Regulatory Commission’s Engineering Guidelines.
The task included research, compilation, and review of relevant scientific literature, studies, and
maps necessary to update the geologic and seismologic understanding of the region. This
included review of the existing seismic studies and published literature and fault rupture maps
following the 2002 Denali earthquake. This assignment also included contacting technical
experts in the Alaskan seismotectonics to elucidate the current understanding of shallow crustal
seismic sources and Quaternary faulting in the Talkeetna Terrain. Based on this literature review,
a geological and seismotectonics database and library were created.
When the initial task had been completed, a comprehensive study plan approach was
implemented to allow for the final deterministic and probabilistic seismic hazard assessment
(DSHA, PSHA). Subtasks included the following:
• Updating evaluations of geologic, seismologic, and seismotectonic literature for the
Project study area to identify data gaps and uncertainties that may require further
evaluations.
• Updating seismicity catalogue for evaluation of seismicity rates, depths, magnitudes, and
focal mechanisms. This includes evaluation of recent and ongoing data collected by the
Alaska Seismographic Network and augmented by the additional seismic stations
installed in the Project area as part of the long term earthquake monitoring program.
• Developing a seismotectonic model that identifies and characterizes seismic sources of
engineering significance to the Project.
• Conducting geologic studies using newly acquired Light Detection and Ranging (LiDAR)
and Interferometric Synthetic Aperture Radar (INSAR) datasets to aid in the
identification and evaluation of potential seismic sources and geohazards.
• Collecting field geologic data for characterization of potential seismic sources and
surface displacement hazards.
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• Assessing surface fault displacement hazard to evaluate the significance (likelihood and
amount) of potential ground surface displacement from faulting in the area of the Project,
including beneath the dam, if such a feature is present.
• Performing sensitivity studies on selected surface tectonic features, faults and lineaments,
identified and being considered as potential seismic sources of engineering significance
on the design of the Project.
• Monitoring and detection of local earthquakes to understand the seismic hazards in the
Project area.
The initial site specific seismic hazard for the Watana Dam site was completed on February 24,
2012 (Fugro 2012). This initial seismic hazard assessment was performed prior to acquisition of
the LiDAR data, lineament mapping and analysis, evaluation of crustal seismic sources,
acquisition of earthquake event data in the Project, etc. For design, the site specific seismic
hazard analysis and calculations at the Watana Dam site should be revisited and further
evaluation of the ASZ and worldwide subduction zone data will be needed to assess the full
range and weights for larger Mmax estimates and to develop a basis for estimation of an
appropriate characterization of Mmax for the intraslab source for use in deterministic
evaluations.
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4. CRUSTAL SEISMIC SOURCE EVALUATION SUMMARY
4.1. General
As described in Section 3, a preliminary seismic hazard source model and probabilistic ground
motion assessment was prepared in 2012 based on desktop review of prior studies and recent
literature (Fugro, 2012). Subsequent to the preliminary seismic hazard ground motions
assessment, this study completed lineament mapping based on interpretation of recently
acquired, detailed, topographic data (i.e., INSAR- and LiDAR-derived DEM data). The mapped
lineaments were assembled into lineament groups, and evaluated in the office using semi-
qualitative criteria to reject or select lineament groups for further investigation during the
summer field season of 2013 (Fugro, 2013). In total, 22 lineament groups and three broader
lineament areas were advanced to the field investigation phase that took place over parts of
summer of 2013 and 2014 (Fugro, 2015a).
The purpose of the lineament mapping and crustal evaluation was two-fold:
1. to identify potential crustal seismic sources that could appreciably contribute to the
seismic hazard at the proposed hydroelectric project and affect dam design; and
2. to identify faults and assess their potential for surface fault rupture at the proposed dam
site area.
A primary objective of the lineament field investigation was to document and interpret available
field evidence for the presence or absence of potential crustal seismogenic sources (faults) along
features identified through previous lineament mapping, and evaluate the features’ significance
with respect to Quaternary faulting and their potential as seismic sources of significance to the
Susitna-Watana Project seismic hazard evaluations. The crustal seismic source evaluation was
vetted through peer-review of technical memoranda and selected field sites by the Alaska
Department of Geological & Geophysical Surveys (DGGS), as well as meetings with the Board
of Consultants.
4.2. Methods
The desktop lineament mapping and analysis report (Fugro, 2013) describes the approach and
results for mapping of individual lineaments across the Project area, that is, within a 62 mi (100
km) radius from the dam site. For that effort, criteria were established to provide a basis for
delineating lineament groups (that is, aggregates of individual lineaments) that appear to have
sufficiently extensive lateral continuity and geomorphic expression consistent with an origin by
tectonic processes. Additional criteria were developed to exclude lineament groups that were
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created by erosional or depositional processes (i.e. non-tectonic lineaments), lineament groups
that are chiefly related to lithologic controls (i.e., differential erosion), lineament groups that did
not meet length and distance criteria, and lineaments that did not show consistent senses of
displacement along strike. In total, 22 lineament groups and three broader lineament areas were
advanced to further field investigation and evaluation
The selected lineament groups were assessed in the field based on geomorphological
characteristics observed in the field as well as geologic relationships around the lineaments
(Fugro, 2015a). As guidelines for the field teams conducting the evaluation of individual
lineament groups, a series of questions were developed as an aid to focus observations made
during the field investigation. To evaluate the field data, a set of questions and criteria similar to
those used for evaluation of the desktop findings were developed. The principal objective of
these criteria is to guide judgments regarding the lineaments’ origins in order to evaluate their
potential association with Quaternary faulting and potential crustal seismogenic sources.
The lineament groups were visually inspected in the field to identify positive evidence for (or
against) tectonic deformation of the Quaternary deposits (as present in the field) that may
overlie, or project toward, the lineaments. The ground-based geologic data collection included
walking of parts of mapped lineaments, photo documentation, exposure and logging of shallow
soil pits, local mapping, collection of relevant structural measurements (strike, dip), and
comparison of existing geologic mapping to field exposures and findings.
Geologic observations made during this recent study included examination of prominent
geologic outcrops that seem to have been un-recognized in previous mapping. This effort is
intended to indicate confirmation or disagreement with existing mapping, and to provide a level
of transparency as to where outcrops are present or absent, and from which locations outcrop-
based interpretations are possible. Field geologic transects were completed to document styles,
distributions, and extent of structural deformation sub-regionally and near the dam site. Sub-
regional field transects were completed where outcrops were accessible in terms of helicopter
landing sites, river water level conditions, and availability of outcrop exposures. The transects
were completed chiefly along the Susitna River, Watana Creek, and to a much lesser extent,
Tsusena Creek. The transect data were synthesized with regional mapping to characterize the
significance of structural features such as terrane bounding faults and deformation of
sedimentary strata. These observations, in turn, allow development of a conceptual tectonic
model that provides a consistent framework that helps explain the presence or absence, as well as
significance, of the structural geologic features at the dam site (Section 5).
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4.3. Results
All lineament groups targeted for field work received a low-altitude aerial observation and
ground inspection was completed at selected locations where features of interest were identified.
Based on these investigations, the overall evaluation and grouping of the lineament groups and
features are summarized in Table 4-1 below.
Table 4-1. Summary of Lineament Groups and Areas
Category Category Description Lineament Groups
I
Lineament groups that were not advanced for field investigation in
2013 based on Fugro (2013) desktop evaluations. Most were not
rigorously inspected during field activities.
4, 10, 11, 13, 14, 15, 16, 18, 24, 25,
North-South Features near
Talkeetna River-Susitna River
Confluence
IIa
Lineament groups evaluated during 2013-2104 field studies, and
judged to be non-tectonic (dominantly erosional, depositional, or
jointing/bedding in origin). No further field investigation is
recommended for evaluation as potential crustal seismic sources.
1, 2, 3a, 3b, 5, 12a, 17a, 21a, 21b,
22, 23, 26, select Reger et al.
(1990) features, Susitna feature,
Watana lineament
IIb
Lineament groups evaluated during 2013-2014 field studies, and also
judged to be of non-tectonic origin, but which appear to be spatially
associated with previously mapped bedrock faults. No evidence of
Quaternary faulting was observed, and no field investigation work is
recommended for evaluation as potential crustal seismic sources
6, 7, 8, 9, 12b, 17b, 17c, 19, 20,
Broad Pass area, Clearwater
Mountains area, select Reger et al.
(1990) features, Talkeetna fault at
T-1 and T-2
III
Lineament groups that have defensible justification based on current
field investigations for consideration or inclusion as crustal seismic
sources in an updated seismic source model.
27 (Sonona Creek fault), Castle
Mountain extension
Category I includes several lineament groups not advanced for further field study based
primarily on distance from the site considerations derived from the evaluations (Fugro, 2013).
Many of the lineament groups investigated are judged to be dominantly erosional in origin, or to
a lesser extent, related to rock bedding or jointing, are not associated with tectonic faults, and are
thus assigned to Category IIa. A second set of lineament groups do appear to be coincident with
previously mapped pre-Quaternary (i.e., bedrock) faults, but are also interpreted as erosional in
origin as no evidence was found for offset or deformation of Quaternary deposits or surfaces.
These are assigned to Category IIb.
Category III lineaments have defensible justification for consideration or inclusion as crustal
seismic sources in an updated seismic source model, and consist of lineament group 27 (Sonona
Creek fault) and lineaments of the Castle Mountain extension. The results of our field
investigations did not identify any specific features with evidence of late Quaternary faulting
within at least 25 mi (40 km) of the Watana dam site. For most of this area, the time and
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detection limits of the imagery and field investigations imply post-glacial time limits of about
12,000 to 15,000 years, and detection of surface offsets of more than about 1 m extending over
several kilometers. For the area near Watana dam site where detailed LiDAR data was the basis
for this evaluation, potential detection limits of surface fault displacements are much lower
(about 0.5 m over several hundred meters).
For crustal seismic evaluations, the ages from regional glacial chronologic correlations imply
that the vast majority of the landscape within about 62 mi (100 km) of Watana dam site was
covered beneath glacial ice or glacial lakes as late as about 17 ka, with a slow reduction in ice
and lake extent through 12 to 11 ka (Fugro, 2015a). The final stages of deglaciation in the
middle Susitna Basin near the Watana dam site appear to be recorded by the large areas of
stagnant ice deposits extending north from the Susitna River between Tsusena and Deadman
Creeks (Acres, 1982b) to Tsusena Butte (Figure 4-1). Samples for optically-stimulated
luminescence (OSL) dating of this last stage of deglaciation were collected in 2014 from an
exposure along Deadman Creek about 2.5 mi (4 km) northeast of the Watana Dam site. The OSL
ages suggest that deglaciation and stagnation of the larger Deadman Creek ice lobe in the middle
Susitna River valley, and thus, geomorphic surfaces on which a record of potential surface
faulting might be preserved, must be older than about 14 to 15 ka (Fugro 2015a).
Despite the apparent absence of geologic evidence for late Quaternary faulting in the broader
region, updates to the seismic source model (Fugro, 2012) may consider inclusion of portions of
some new sources. Such an updated source model would consider the findings and limitations
from this evaluation, seismicity recorded since 2012, and other data, although some seismic
sources may be constrained to very low slip rates as defined by the crustal seismic study (Fugro,
2015a).
Synthesis of regional geology and seismology, sub regional mapping transects, and site data all
indicate that major faults, typical of active crustal seismic sources capable of primary surface
rupture associated with major earthquakes in the contemporary tectonic environment, are absent
from the Watana dam site area. The evaluation of potential crustal seismic sources has not
identified any specific features with evidence of late Quaternary faulting within at least 25 miles
(40 km) of the Watana dam site. This is consistent with the observations that the reservoir area is
structurally coherent with lack of pervasive penetrative deformation (see also Section14.1). This
is also consistent with previous fault studies completed in the dam site area.
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Figure 4-1. Glacial Ice Reconstruction Profiles
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5. DAM SITE AREA FAULT RUPTURE EVALUATION
5.1.1. General
Permanent ground deformation from surface fault rupture can occur as primary, secondary, or
sympathetic (triggered) rupture. Primary rupture is ground displacement associated with the
main trace of a seismogenic fault. Secondary rupture is ground displacement from a fault that is
structurally connected to the seismogenic fault, but is not the main seismogenic source.
Sympathetic rupture is ground displacement from neither the main seismogenic source nor a
secondary fault, but occurs principally from the effects of co-seismic strong ground shaking.
Potential sources of surface fault rupture hazard that were considered and characterized to the
extent possible at the proposed Watana dam site consist of: 1) crustal seismic source faults with
surface expression which transect the dam foundation directly or extend nearby, 2) buried or
“blind” crustal seismic source faults with no direct surface expression, or 3) features, proximal to
the dam site, not active in the contemporary stress regime that could be potentially reactivated
through mechanisms of reservoir triggered seismicity. Each of these potential sources of surface
fault rupture hazard was evaluated based on differing aspects and combinations of the existing
geological, geophysical, and seismological data. Evaluation of crustal scale seismic source
faults, either those with surface expression or “blind” structures, which are the source of primary
or secondary fault rupture hazards underscores the importance of regional data because the
source dimensions of these structures requires features with scales on the order of tens of
kilometers. Evaluation of potential fault reactivation emphasizes knowledge of the existence,
extent, and orientation of potential faults in the immediate site vicinity because of the potential
significance to the dam. One common element for evaluation of each source of potential fault
rupture hazard is the existence and characteristics of faults within the dam foundation. In an
absence of known seismogenic faults at the dam site, the evaluation of fault rupture hazard
focuses on the possibility of displacement along existing planes of weakness in the bedrock.
5.1.2. Methodology
The approach for evaluating surface rupture hazard at the dam site relies on four principal lines
of independent data and analyses:
1. Assessment of the contemporary tectonic framework (stress field) of the site region as an
indication of the potential for reactivation of site geologic features.
2. Geomorphic evaluation of Quaternary and post-glacial faulting (i.e., lineament mapping
and analyses) to assess whether potential seismogenic faults are present near the site
vicinity,
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3. Field geologic transects to assess styles and patterns of structural deformation near the
site, and
4. Assessment of results of site-specific investigations of geologic structure in the dam
foundation.
Collectively, these four lines of independent and relatively indirect evidence are integrated to
develop the evaluation of (or supporting argument for no) fault rupture hazard at the dam site.
This approach is in accordance with accepted methods and practices currently used for similar
evaluations on projects involving major dam projects or critical facilities that pose potential
hazard to the public and environment.
The evaluation collectively considers regional tectonic history, sub-regional deformation patterns
observed in Mesozoic and Cenozoic rocks around the site, emplacement of intrusions and
volcanics at the dam site, crustal stress orientations from earthquake focal mechanisms, known
active faulting, plate motions, and GPS data, geomorphic landform evaluations, and current
understanding of geologic features at the dam site. The surface fault rupture evaluation assesses
the weight of evidence in relation to three topical areas:
• The regional and subregional evidence of Quaternary faulting,
• The presence or absence of faults and large-scale shear features at the dam site proper,
• The qualitative potential for reactivation of geologic structures at the dam site within the
current tectonic framework.
Regional and sub-regional evidence of Quaternary faulting through geomorphic evaluation of
post-glacial faulting is the strongest argument to address late Quaternary faulting at the dam site.
The evaluation of post-glacial faulting consisted of carefully inspecting and analyzing the
detailed LiDAR elevation data in the dam site area (and vicinity) to identify evidence of tectonic
geomorphology suggestive of faulting. In addition, field investigations were conducted to verify
the results of desktop based LiDAR lineament mapping (refer to Fugro, 2015a).
Data on the potential existence and characteristics of faults and shear features in the dam
foundation are discussed in the report on lineament mapping and analysis by Fugro (2013), and
are further evaluated in the framework of the regional seismic source evaluations and sub-
regional mapping near the dam site (Fugro, 2015a).
To evaluate the contemporary tectonic framework of the dam site, the updated information from
the Susitna-Watana Seismic Network and the AEC regional network, as well as published
literature, have been reviewed (AEC, 2014). This includes data on crustal stress orientations
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from earthquake focal mechanisms, known active faulting, plate motions and GPS data,
geomorphic landform evaluations, and current understanding of geologic features at the dam site.
5.1.3. Regional Evidence
The evaluation of potential crustal seismic sources has not identified any specific features with
evidence of late Quaternary faulting within at least 25 mi (40 km) of the Watana dam site.
Within this region, faults depicted on existing geologic maps were evaluated through field and
imagery analyses for evidence of late Quaternary faulting and multiple types of imagery were
reviewed to define lineaments, which were then evaluated through field investigation for
evidence of potential Quaternary faulting (Fugro, 2013, 2015a). The area along the Susitna
River, and extending at least 3 mi (5 km) north and south in proximity to the dam site and deeper
portions of the proposed reservoir, was also imaged with high-resolution LiDAR and aerial
photography. This data improved resolution and potential detection capability to reveal the
geomorphic expression and thus, the existence of potential late Quaternary faults. These efforts
indicate that at least over the past 12,000 to 15,000 years – the time since deglaciation of much
of the area – there is no evidence for major surface-rupturing earthquakes from crustal scale
seismic sources within the dam site region (25 mi (40 km) radius).
Over longer periods, the crustal seismic source evaluation also indicates an absence of significant
zones of uplift or vertical deformation localized along specific surface or blind fault structures.
Recurrent large earthquakes on blind faults, e.g. M ~6.5 or larger, with repeated dip slip motion
over many events, eventually result in recognizable geomorphic features and topographic uplift
which persists in the landscape proximal to these features. Thus, even for features with uplift
rates as low as 0.004 in/yr (0.1 mm/yr), a fault slip rate associated with large earthquake
recurrence approaching 10,000 years, would result in relative uplift of about 3,300 ft. (1000 m)
over a period of 10 million years. For comparison, the topographic relief along the northwestern
side of Mount Watana to the Fog Lakes area, taken as a proxy for maximum uplift in that area, is
about 1,650 ft. (~500 m). Maximum topographic relief along even short, relatively linear
sections of hills surrounding the Fog Lakes basin near the Watana dam site is primarily less than
about 1,000 ft. (~300 m). For example, the Susitna Glacier fault, which was a “blind” initiating
fault plane of the 2002 Denali M7.9 earthquake, ruptured the ground surface near the base of
south-facing mountains that have about 1,500 ft. (~460 m) of relief. This illustrated the premise
that blind or previously undetected Quaternary faults produce noticeable long-term topographic
uplift near the “buried” fault tip even if the ground expression of surface rupture is not
recognized. No such high-relief topography is present either at the dam site or in the site vicinity
that would be a basis on which to postulate the presence of a nearby blind fault that might
transect the site footprint.
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The contemporary stress regime, as defined by current plate tectonic models, GPS observations,
earthquake focal mechanisms, and Quaternary faulting indicates that the Watana dam site area is
subject to northwest-southeast oriented sub-horizontal compressive stress associated with the
long-term ongoing subduction of the Pacific Plate in south central Alaska. Crustal deformation
associated with the plate interactions has been accommodated primarily along the Denali fault, as
right-lateral motion, at a relatively constant rate over the past 10 million years (Freymuller, et al.,
2008). Between the Denali fault and the Castle Mountain fault, geologic evidence suggests that
the intervening Talkeetna Block – a region including the Watana dam site between the Copper
River Basin to the east and the Susitna Basin to the west – has been relatively stable. This is
consistent with the sub-regional mapping described above that indicates only gentle structural
deformation (folding) and a relative paucity of penetrative faulting. Paleomagnetic data from
volcanic rocks with ages of 30 to 50 million years indicates an absence of significant internal
rotation or deformation within the Talkeetna Block (Figure 2-1). Likewise, the extent and
distribution of these Tertiary volcanic rocks across the landscape of the Talkeetna Block argues
against the existence of large-scale vertical or lateral fault displacements within the area.
Within the contemporary stress regime of the Talkeetna Block, the primary modes of tectonic
deformation appear to involve right-lateral strike slip structures with east-northeast strikes (sub-
parallel to the closest portion of the Denali and Castle Mountain faults), and with dip slip or
compressional shortening along structures with northeast strikes or elongations (roughly
perpendicular to the regional direction of crustal shortening), Figure 5-1. Structures with these
orientations would be oriented roughly parallel to the overall structural grain of the pre-existing
tectonic terrains and rock units within the Talkeetna Block. Secondary modes of tectonic
deformation might involve left-lateral strike-slip motions along north to north-northwest striking
faults, or potentially lessor amounts of extensional deformation along structures with northwest
strikes. Because evidence suggests the dam site region is dominated by compression (Figure
5-1), extensional features are expected to be relatively less common and would primarily be
expected as second or third order local structures, found locally in association with structural
complexities of the primary east-northeast or northeast striking structures, instead of northwest-
southeast trending structures that dominate the dam site (Fugro, 2015a).
5.1.4. Sub-Regional Geologic Transects
For evaluation of primary bedrock crustal structure, two sub-regional transects, one oriented
roughly east-west along the Susitna River, and a second oriented roughly northeast-southwest
along Watana Creek, provide the most complete bedrock exposures near the Watana dam site.
These transects demonstrate that the Watana dam site lies within a relatively coherent structural
block of folded Kahiltna Basin rocks which have been extensively intruded by mid to early
Cenozoic igneous units. Data from these transects, and evaluation of existing geologic mapping,
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does not define any apparent crustal scale faults within at least 3 mi (5 km) of the Watana dam
site.
The most significant crustal fault structure in the area is the northeast-striking fault-bounded
basin along Watana Creek that accommodated Tertiary sedimentation. Structural and
stratigraphic data suggests that this basin most likely formed tectonically as an extensional
graben in a right step-over between two strands of the Talkeetna fault, which was active at the
time as a right lateral strike slip fault (essentially, a syntectonic depocenter) (Panel G on Figure
2-3). The dips, apparent section thickness, and extent of the Watana Creek basin sediments
suggest vertical displacements of at least a few hundred meters, which would imply possible
lateral offsets of at least a few kilometers. The Watana Creek basin contains non-marine
sediments and undated volcanic flows that are tentatively correlated by Csejtey et al. (1978) to
the Paleocene Chickaloon Formation of the Matanuska Valley. There appears to be a lack of
sedimentary detritus from the surrounding more than 50 million year old dioritic and granitic
sediments exposed in the surrounding the area, which in aggregate suggest a relatively older age
for this period of strike slip faulting associated with the Talkeetna fault. The mid to early
Cenozoic age of faulting implied by this data are consistent with existing mapping, which shows
that the Talkeetna fault does not appear to offset or significantly displace plutonic rocks distally
to the southwest of the Watana dam site (e.g., (Woodward Clyde Consultants (WCC), 1982);
(Wilson, Schmoll, Haeussler, Schmidt, Yehle, & Labay, 2009)). It is also consistent with new
mapping in the Talkeetna Mountains Quadrangle that shows an absence of continuity for the
Talkeetna fault south of the Susitna River (Twelker, et al., 2014).
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Figure 5-1. Crustal Stress Orientations and Strain Ellipses
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Published regional mapping does not depict any other faults that would intersect the sub-regional
geologic transects or within at least 3 mi (5 km) of the Watana dam site ( (Csejtey, et al., 1978);
(Wilson, Schmoll, Haeussler, Schmidt, Yehle, & Labay, 2009)). Some earlier studies suggested the
possibility of structural control of the east – west trending sections of the Susitna River near the
Watana dam site (e.g. (Gedney & Shapiro, 1975); and Watana lineament of (Woodward Clyde
Consultants (WCC), 1982)) based on regional-scale lineament evaluations. However, the recent
field mapping evaluations have not revealed evidence for such structures and dam site drilling
investigations in 2014, which included two inclined boreholes drilled from opposite banks of the
river (DH14-9b and DH14-10) beneath the river channel, through bedrock, encountered no large-
scale shears or fault beneath the river nor east-west oriented features.
Previous mapping conducted for Watana dam site has depicted or inferred several nearby potential
faults of crustal scale ( (Woodward Clyde Consultants (WCC), 1982); (Acres, 1982b)) as shown on
Figure 2-5 and Figure 2-6. These faults are depicted with maximum map lengths of about 0.5 to 3
mi (0.8 to 5 km) and are primarily inferred extensions of shear features found in river valley wall
exposures upstream and downstream of the Watana dam site, and extended kilometers northwest to
apparently similar features in the nearest bedrock exposures along Susitna River tributaries and
Tsusena Creek. Bedrock exposures in the intervening areas are covered by Quaternary deposits, and
geomorphic evaluations based on the detailed LiDAR data and ground reconnaissance do not
disclose evidence of the fault continuity or offset of the Quaternary units. This fault is
approximately 0.5 mi (0.8 km) upstream of the Watana dam site and correlates to Geologic Feature 1
(GF1) of Acres (1982b).
An additional north-northeast trending fault is shown by Acres (1982b) upstream of the dam site
near the mouth of Deadman Creek (Figure 2-5 and Figure 2-6); however, no detailed description of
the fault was provided. No exposure of this “fault”, or of structures with similar orientations in the
Kahiltna Basin rocks near the dam site were observed during mapping for the sub regional transects
along the Susitna River in 2014. Moreover, there is no expression in the LiDAR data set of this fault
along possible extensions to the northeast, and no indications of this structure in the Susitna River
canyon exposures to the southwest or at the dam site. As depicted by Acres (1982b), much of the
trace of this fault lies beneath the Susitna River channel or beneath Quaternary glacial deposits.
Near the confluence of Deadman Creek and the Susitna River, the mapped location of this “fault”
was inferred from widely spaced outcrops at river level observed during 2014 mapping. However,
additional outcrops of Kahiltna Basin rocks observed from aerial traverses and evident in the LiDAR
data set to the north and east of the confluence suggest the “fault” is more likely the intrusive contact
zone between the Tertiary intrusive rocks and the Cretaceous Kahiltna Basin rocks, with an irregular,
not planar geometry.
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Mapping in 2014 identified two additional minor faults in bank exposures of the Cretaceous rocks
along the north bank of the Susitna River at approximately 3.5 mi (~5.5 km) upstream of Deadman
Creek. Neither fault can be traced beyond the bank exposures, and there is no indication of these
faults is evident along strike in the detailed LiDAR data set. The two faults are located about 165 ft.
(50 m) apart from each other and have strike and dip of 303°, 42° S and 300°, 32° S; thus, the faults
trend northwest-southeast similar to the structural fabric observed (e.g., geologic features) at the dam
site. Bed separation measured on the shallow dipping fault plane was 4 inches (10 cm) on both
faults. Net slip estimated based on fault plane slickensides and a dipping bed offset by the fault
indicates less than 3 ft. (1 m) of net slip; hence these are considered minor faults. Based on the sub-
regional transects, these faults appear to represent a distinctly different style and orientation of
faulting compared to that expressed by the geologic features observed at the Watana dam site.
Overall, the Cretaceous rocks appear to be a structurally coherent block, not disrupted by major
faults and there is no expression of these faults in the overlying Quaternary deposits.
5.1.5. Dam Foundation Geologic Features
Based on recent site mapping and re-interpretation of previous mapping (i.e., (Acres, 1982b)), the
principal geologic features (GF) that underlie the dam footprint are:
• Geologic Feature GF4
• Geologic Feature GF5
• Other similar but unnamed geologic features
o An unnamed structure delineated as underlying part of the dam foot print on the north
bank of the Susitna River, approximately 220 ft. (67 m) downstream from GF5
o Another unnamed feature mapped 580 ft. (177 m) downstream of GF5 on the north bank
of the Susitna River.
Each of these geologic features was evaluated for their significance as potential fault rupture hazards
(Fugro, 2015a).
5.1.6. Summary of Dam Foundation Fault Rupture Evaluation
In the evaluation of fault rupture hazards in the dam foundation, the approach used involved separate
lines of enquiry that took into consideration various independent types of evidence. The evaluation
assessed the weight of evidence in relation to: a) the regional and subregional evidence of
Quaternary faulting, b) the presence of significant faulting or shear zones at the dam site, and c) the
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qualitative potential for reactivation of geologic structures at the site within the current tectonic
framework.
The evaluation found that one of the more compelling findings is the absence of crustal scale surface
faults or apparent “blind” structures within several miles (kilometers) of the dam site. From detailed
evaluations of new imagery data, evaluations of local and regional scale mapping, and field
investigations, no evidence has yet been revealed of potential Quaternary faulting within at least 15
mi (~25 km) of the Watana dam site. Thus, this information strongly suggests that potential sources
of primary or secondary, surface fault rupture at the dam site are absent. Further, geomorphic
evaluations based on the detailed LiDAR data within about 3 mi (5 km) of the site has not identified
any expression or continuity of potential faults or specific geologic features extending from the site
area that would be indicative of deformation of Quaternary deposits. Given the absence of
potentially active crustal scale seismic sources in the immediate site vicinity, the potential existence
of small and minor structural features in the dam foundation bedrock does not indicate an elevated
potential for a fault rupture hazard.
From sub-regional transects and evaluation of the existing mapping within about 3 mi (5 km) of the
dam site suggest that the Watana dam site lies within a relatively coherent block of relatively gently
folded Kahiltna Basin rocks that have been cross cut and locally disrupted by early Tertiary igneous
and volcanic rocks. The intrusive process likely resulted in numerous alteration zones, fractures, and
shears, but does not appear to be associated with nearby fault structures of significant crustal extent.
The few short faults near the dam site depicted by Acres (1982b) are mostly likely similar features,
and not post-intrusive, crustal scale faults. The closest major Tertiary structure appears to be the
fault-bounded depositional basin along Watana Creek, approximately 8.5 mi (13.5 km) upstream of
the Watana dam site.
The orientation of discontinuities and narrow shear features mapped at the site chiefly have
northwest strikes and steep, vertical to near-vertical dips ( (U.S. Army Corps of Engineers (USACE),
1979); (Acres, 1982); (Harza-Ebasco, 1984); Fugro, 2015a). Based on review of the 2012 and 2014
drill hole logs, the bedrock encountered is pervasively fractured, with jointing prevalent in each and
every boring. The joints are high-angle, and are reported as 70° dip or greater. Based on geologic
mapping and oriented discontinuities in rock core in recent drill holes, it appears that thin shear
zones present are high-angle features of about 80° dip. This is relatively consistent with the near
vertical shears exposed in outcrop.
Regarding specific features that may lie within the dam footprint, existing data show a dominant
structural fabric of northwest strikes and high-angle dips. Site mapping and overlapping drill holes
beneath the Susitna River appear to exclude structures with orientations parallel to the river channel
at the site. Those joints and shear zones that do cross the dam footprint appear to be relatively
discontinuous along strike and are challenging to map and correlate from outcrop to outcrop.
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Although not directly observed at the surface, geologic mapping within gullies, particularly those on
the right abutment (GF4B and GF5), indicate that gullies likely formed initially by the preferential
erosion of weak and relatively narrow fracture zones that have been widened and enhanced to their
present dimensions by erosion due to stress relief and freeze thaw processes and/or block movement.
Thus, the subset of geologic features that are depicted to transect the dam footprint appear to be
relatively minor structures, with potentially limited bedrock continuity or persistence, and appear to
have dominant orientations that are least favorable to reactivation in the contemporary stress regime.
The following is a summary of the principal findings and lines of evidence in relation to potential
surface fault rupture:
1. The contemporary stress regime, as defined by current plate tectonic models, GPS
observations, earthquake focal mechanisms and Quaternary faulting, indicates that the
Watana dam site area is subject to northwest-southeast oriented sub-horizontal compressive
stress associated with the long-term ongoing subduction of the Pacific Plate in south central
Alaska. Crustal deformation associated with the plate interactions has been accommodated
primarily along the Denali fault, as right-lateral motion, at a relatively constant rate over
the past 10 million years. Between the Denali fault and the Castle Mountain fault, geologic
evidence suggests that the intervening Talkeetna Block, a region including the Watana dam
site, has been relatively stable.
2. Paleomagnetic data from volcanic rocks with ages of 30 to 50 million years indicates an
absence of significant internal rotation or deformation within the Talkeetna Block.
Similarly, the extent and distribution of Tertiary volcanic rocks across the Talkeetna Block
argues against the existence of large-scale vertical or lateral fault displacements within the
area.
3. Within the current stress regime of the Talkeetna Block, the primary modes of tectonic
deformation appear to involve right-lateral strike slip structures with east-northeast strikes,
and with dip slip or compressional shortening along structures with northeast strikes or
elongations (roughly perpendicular to the regional direction of crustal shortening).
Structures with these orientations would be oriented roughly parallel to the overall
structural grain of the pre-existing tectonic terrains and rock units within the Talkeetna
Block. Secondary modes of tectonic deformation might involve left-lateral strike-slip
motions along north to north-northwest striking faults, or potentially smaller amounts of
extensional deformation along structures with northwest strikes. Because regional
evidence suggests the dam site region is dominated by compression, extensional features
are expected to be relatively less common and would primarily be expected as second or
third order structures found locally in association with structural complexities of the
primary east-northeast or northeast striking structures.
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4. Detailed evaluations of new imagery data, evaluations of local and regional scale mapping,
and field investigations have not identified any evidence of potential Quaternary faulting
within at least 15 mi (25 km) of the Watana dam site. These data strongly suggest that
potential sources of primary or secondary surface fault rupture at the dam site are absent.
5. Evaluation of existing mapping within the dam site area, and data from sub-regional
transects along the Susitna River do not support the existence of major crustal faults near
the dam site. Mapped shear zones within this area appear to be primarily associated with
the mid-early Tertiary intrusive rocks, similar to those at the site.
6. Geomorphic evaluations based on the detailed LiDAR data within the dam site area have
not identified any expression or continuity of potential faults or specific geologic features
extending from the site area that would be indicative of deformation of Quaternary
deposits. This indicates that although shear features may be present in the foundation, there
is evidence to support lack of surface displacement along these features in the last 12,000
to 15,000 years (Fugro, 2015a).
7. Recurrent large earthquakes on blind faults, e.g. M ~6.5 or larger, with repeated dip-slip
motion over many events, produce and eventually result in recognizable geomorphic
features and topographic uplift which persists in the landscape. No such high-relief
topography is present at the dam site, which would be a basis on which to postulate the
presence of a nearby blind fault or seismic source in the site vicinity.
8. Bedrock beneath the proposed dam, powerhouse, spillway and appurtenance structures
consists of fresh to slightly weathered, blocky, strong to very strong diorite that is locally
altered and fractured and includes minor shears and shear zones. Fracture zones, shear
zones, and alteration zones tend to trend in a northwest-southeast direction, parallel to the
predominant joint set (JS1) and to a lesser extent to a more north–south direction associated
with a secondary set (JS3). On the south abutment just upstream of the proposed dam,
several narrow northwest trending shear zones are cross-cut by a felsic dike and at least one
healed fracture cuts across the shear zones. Together with the observations of healed shear
and alteration zone, these observations suggest that many of the fracture and shear zones
are likely associated with mid-early Tertiary intrusive processes and are not related to the
contemporary seismotectonic regime.
9. Inclined drilling beneath the Susitna River, encountered generally fresh to slightly
weathered, strong diorite. Although some widely spaced, narrow fracture zones and minor
shear zones were intersected in the drill holes, no significant geologic structure was
revealed beneath the river. This supports the interpretation that the river at the dam site is
not controlled by a major through-going fault or shear zone.
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10. Investigations were made of previously identified “geologic features”, shear and/or fracture
zones greater than 5 ft. (1.5 m) in width, several of which may cross beneath the dam site.
It is now considered that the prominence of these features, particularly those that would be
encountered in the dam and spillway foundations, has been over-represented in geologic
characterization conducted in previous studies. Further, the subset of geologic features that
are depicted to transect the dam footprint appear to be relatively minor structures, with
potentially limited bedrock continuity or persistence, and appear to have dominant
orientations that are least favorable to reactivation in the contemporary stress regime.
In conclusion, therefore, it is considered that the potential for any reactivation of the geologic
features that might transect the dam footprint must be considered extremely low given the following:
• The apparent lack of continuity and small scale of structural geologic features at the site
(shear zones) upon which surface fault rupture could conceivably take place;
• The dominant northwest-southeast trend is unfavorably oriented with respect to the
contemporary tectonic stress regime, as the primary mode of tectonic deformation appear to
involve right-lateral strike slip structures with east-northeast strikes, ;
• The absence of nearby crustal scale fault structures and neotectonic or paleoseismic evidence
of Quaternary faulting; and
• The absence of Quaternary faults mapped with about 15 mi (25 km) of the dam site.
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6. SEISMIC SOURCE CHARACTERIZATION
The seismic source characterization was developed as an update to WCC (1980) model. The seismic
source model summarized herein addresses deep (subducting plate) and shallow (crustal) seismic
sources, as well as background (aerial) seismic sources.
6.1. Subduction Zone
The Alaska subduction zone (ASZ) is one of the world’s most seismically active subduction
zones. Relative plate motion between the Pacific and North American Plates increases from
about 2 in/yr (5.4 cm/yr) at its eastern end near the Talkeetna Mountains to about 3 in/yr (7.8
cm/yr) at the west end of the Aleutian arc (Carver & Plafker, 2008) (Figure 2-13). The ASZ is
also termed the Alaska-Aleutian Arc. East of longitude 170 degrees West, the Pacific Plate is
subducting beneath continental crust, while to the west it subducts beneath oceanic crust that was
trapped after initiation of the arc in the middle Eocene. This results in a more shallow-dipping
plate interface to the east than to the west.
Earthquakes associated with the ASZ are of two main types: large “megathrust” events due to
accumulated frictional strain along the interface of the two plates (most notable being the 1964
M 9.2 earthquake, described in Section 2.4.2), and those occurring within the down going Pacific
Plate as it descends into the mantle. These “intraslab” earthquakes, considered capable of reaching
magnitudes of M 7.5 and higher, are due to factors such as spreading ridge push, gravitational
pull of the plate due to density contrasts between it and the mantle, and metamorphic reactions due
to increasing temperature and pressure within the down going plate.
The dam site area lies at the eastern end of the ASZ. At this location the plate interface has an
extremely low dip, almost flat (Figure 6-1). The northern boundary of the interface is at a
depth of about 22 mi (35 km) and lies about 50 mi (80 km) southeast of the site. To the northwest
of this line intraslab earthquakes are produced as the plate dips more steeply as it descends into the
mantle. Beneath the site the top of the plate is at a depth of about 31 mi (50 km) (Figure 6-1).
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Figure 6-1. Map and Cross Section of Alaska Subduction Zone Earthquakes
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6.1.1. Interface
The interface between the North American and Pacific Plates is the source of the largest magnitude
earthquakes in the source model. Due to studies of the 1964 M 9.2 earthquake, seismic
refraction/reflection surveys (e.g. (Brocher, et al., 1994)), and research results from a regional
seismograph network operated by the Alaska Earthquake Information Center (AEC) (Ratchkovski &
Hansen, 2002), the geometry of the down going plate within a few hundred km of the site is fairly
well known. In this region, Wesson et al. (2007) following cross sections of relocated seismicity
shown in Ratchkovski and Hansen (2002), modeled the interface as shown in Figure 6-2. In this
figure the up-dip 12 mi (20 km) contour is seen to the southeast. The contour representing the down-
dip boundary of the plate interface is seen to the northwest. To the southwest its depth is 25 mi (40
km), reflecting the steeper interface dip to the west, and at longitude 151 degrees West, it begins to
shoal from 25 mi (40 km) to 20 mi (33 km) to the end of the interface at the northeast end. This
reflects the slightly tilted interface seen in Figure 6-1.
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Figure 6-2. Subduction Interface Model
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6.1.2. Intraslab
Intraslab earthquakes occur within the down going Pacific plate, after it breaks contact with the
North American Plate in the megathrust zone, and assumes a steeper dip as it descends into the upper
mantle. Notable earthquakes of this type include the M 6.5 1965 and M 6.8 2001 Nisqually,
Washington earthquakes associated with the Cascadia subduction zone. As seen in Figure 6-1, in the
dam site region this megathrust zone consists of two parts: an intermediate zone dipping about 25
degrees between depths of 31 and 50 mi (50 and 80 km), and a deeper zone from 50 to 93 mi (80 to
150 km) that dips more steeply. The physical sources of these earthquakes include ridge push from
oceanic spreading ridges, gravitational pull of the slab due to density contrasts between it and the
surrounding mantle, and chemical reactions due to increasing pressure and temperature.
6.2. Crustal Faults
The following section discusses all faults within 125 mi (200 km) of the dam site with evidence of
historical or Quaternary activity, as well as suspicious faults that may or may not be active structures
(Figure 1-1, Table 6-1 and Table 6-2). The primary compilation of faults in Alaska, and the initial
basis for the seismic source model included is the “Neotectonic Map of Alaska” by Plafker et al.
(1994). Quaternary faults identified after the Plafker et al. (1994) map and presented in published
literature have also been included in the model. A Quaternary fault and fold database of Alaska has
been compiled by the Alaska Division of Geological & Geophysical Surveys (Koehler R. , Farrell,
Burns, Combellick, & Weakland, 2011a), however, final publication of the database was subsequent
to the source model development (Fugro, 2012). Table 8-1 and Table 8-2, numerates the closest
distance from the site area to the faults listed in Table 6-1 and Table 6-2.
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Table 6-1. Fault Characterization
Fault Name Section Age from Plafker et al. (1994) Age from Other Sources Sense of Slip Dip Seismogenic Depth (km) Slip Rate (mm/yr) Recurrence (years)
Castle Mountain fault
Eastern Castle
Mountain -
Caribou fault
Eastern Castle
Mountain = Historic;
Caribou =
Pleistocene
Historic based on 1984 seismicity
(Lahr, Page, Stephens, & Fogleman, 1986) RL - Reverse
76° N (Lahr, Page,
Stephens, & Fogleman,
1986) ;
80°-90° (Fuchs, 1980)
? lateral: 0.5 - 0.6 (Fuchs, 1980)
thrust:? N/A
Western Castle
Mountain Holocene
Holocene ( (Haeussler, Best, & Waythomas, 2002); (Willis, Haeussler,
Bruhn, & and Willis, 2007)
RL - Reverse
70°-90° N
(Haeussler, Bruhn, &
Pratt, 2000)
20 [based on
1984 EQ
Lahr et al., (1986) ]
lateral: 2.8-3.6, [preferred rate of 3.0 -
3.2] (Willis et al., (2007)); 2.9 mm/yr
(Wesson et al. (2007)); 0.45-0.63
(Koehler and Reger, (2011)) thrust: 0.07-0.14
700 (Haeussler, Best, &
Waythomas, 2002)
Denali fault
Eastern Holocene Holocene (Matmon, et al., 2006) RL ? ? 8.4 +- 2.2 mm/yr (Matmon, et al., 2006)
380 (mean ages from (Plafker,
Carver, Cluff, & and Metz,
2006)and DFWG summarized in
Koehler et al., (2011))
Central Holocene/
Suspicious Historic - 2002 M7.9 (Eberhart-Phillips, et al., 2003). RL 75°-90°
(Haeussler, et al., 2004).
12 [from 2002
aftershocks (Ratchkovski,
Wiemer, & Hansen,
Seismotectonics of the Central
Denali Fault, Alaska, and the
2002 Denali Fault Earthquake
Sequence, 2004)]
14.4 +- 2.5 mm/yr (Matmon, et al.,
2006), 85 km W = 13.0 +- 2.9 mm/yr
255-283 km W = 9.4 +- 1.6 mm/yr
323 km W = 6.7 +- 1.2 mm/yr (Mériaux,
et al., 2009) (distance relative to
Totschunda junction)
400 (mean age from DFWG
summarized in Koehler et al.,
(2011))
Western Holocene/
suspicious N/A RL ? ? ? ?
Pass Creek - Dutch
Creek fault N/A Late Pleistocene Holocene (Haeussler P. , An overview of the neotectonics of interior Alaska—
Far-field deformation from the Yakutat Microplate collision, 2008) Normal ? ? 1.72 mm/yr min slip rate based on scarp
height of Willis and Bruhn (2006) 1340 max (Willis & Bruhn, 2006)
Sonona Creek fault N/A N/A Holocene (Williams & Galloway, 1986) ? ? ? ? ?
East Boulder Creek
fault N/A Late Pleistocene Holocene (1994) ? ? ? ? ?
Matanuska Glacier
fault N/A N/A Holocene (Haeussler & Anderson, 1995) Right normal ? ? ? ?
Susitna Glacier fault N/A N/A Historic 2002 M7.9 (Eberhart-Phillips, et al., 2003) Reverse
19-48
(Crone et al., (2004)and
Ratchovski et al.,)
? ?
~4000
(Crone, Personius, Craw,
Haeussler, & Staft, 2004)
Broxson Gulch fault N/A Neogene Cenozoic (Ridgeway et al., (2002)) Reverse 5-40
(Stout & Chase, 1980) ? ? ?
McCallum-Slate Creek
fault N/A Late Pleistocene Early Pliocene (Weber & Turner, 1977) Reverse ? ? ? ? ?
Bull River fault N/A suspicious N/A Reverse ? ? ? ?
Foraker fault N/A ? N/A Reverse ? ? ? ?
Broad Pass fault N/A ? N/A Reverse ? ? ? ?
Notes: See Table 6-2 for Northern foothills fold and thrust belt faults
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Table 6-2. Northern Foothills Fold and Thrust Belt (NFFTB) Fault Data
Fault Name Age from Plafker
et al. (1994)
Age from Bemis et
al. (2015) Sense of Slip Dip (direction)
Billy Creek fault suspicious Holocene LL - Reverse
(NW up) >60 (?)
Canteen fault Late Pleistocene Holocene LL - Reverse
(NW up)
>60
(direction?)
Cathedral Rapids fault N/A Holocene Reverse (S up) 15-60 (S)
Ditch Creek fault N/A Quaternary Reverse (SW up) >60? (SW?)
Donnely Dome fault Late Pleistocene Holocene Reverse (S up) 45-90 (S)
Dot "T" Johnson fault N/A Holocene Reverse (S up) 15-45 (S)
East Fork fault Holocene Holocene Reverse (N up) >60 (S?)
Eva Creek fault N/A Quaternary Reverse (N up) >60
(direction?)
Glacier Creek fault N/A Quaternary Reverse (S up) 30-60 (S)
Gold King fault - Section A N/A Late Pleistocene Reverse (S up) 15-30 (S)
Gold King fault - Section B N/A Quaternary Reverse (S up) 10-30 (N)
Granite Mountain fault A Late Pleistocene Holocene LL - Reverse (NE up) >60
(direction?)
Granite Mountain fault B Late Pleistocene Quaternary Reverse (S up) 30-60 (SW)
Healy Creek fault Late Pleistocene Late Pleistocene Reverse (N up) 60-90 (N)
Kansas Creek fault N/A Quaternary RL - Reverse (S up) >30 (S)
Macomb Plateau fault N/A Quaternary Reverse (S up) 15-60 (S)
McGinnis Glacier fault Holocene Reverse (SW up) >45 (SW?)
Molybdenum Ridge Fault N/A Holocene Reverse (S up) 15-45 (S)
Mystic Mountain fault Neogene Late Pleistocene RL - Reverse ( S up) >30 (S)
Nern Foothills thrust N/A Late Pleistocene Reverse (S up) 15-45 (S)
Panoramic fault N/A Late Pleistocene Reverse (NE up) >60 (?)
Park Road fault N/A Late Pleistocene Reverse (N up) 30-90 (N)
Peters Dome Fault N/A Quaternary Reverse (S up) 15-45 (S)
Potts fault N/A Quaternary Reverse (NE up) >60? (?)
Red Mountain fault N/A Late Pleistocene Reverse (S up) 30-60 (?)
Rex fault N/A Late Pleistocene Reverse (S up) >30
Stampede fault N/A Late Pleistocene Reverse (N up) 15-30 (N)
Trident fault N/A Late Pleistocene Reverse (SE up) >30 (SE)
Trident Glacier fault N/A Quaternary Reverse (S up) 30-60? (S)
Notes:
(1) Fault Data from the NFFTB summarized from Bemis et al. (2015)
(2) LL = left lateral, RL = right lateral.
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6.2.1. Denali Fault
The Denali fault is a right-lateral fault with an arcuate shape striking to the northwest in the
east, and an increasingly westerly and southwesterly strike to the west (Figure 2-10). A typical
geometry evoked for the fault includes an eastern section located east of the junction with
the Totschunda fault, a central section between the Totschunda junction and an asperity in
the fault strike near Denali, and a western section west of Denali. The western termination
of active faulting is considered in the source model to be at latitude 154.7°W based on
Wesson et al. (2007), who propose that slip tapers to 0 mm/yr at this location. Western
continuation of the fault beyond this point would not be expected to have significant impact on
the site ground motions due to the large distance, ~202 mi (~324 km), to the western end of the
Denali fault. The fault sections outlined above serve solely for geographic reference, as there
is no evidence that the section boundaries would inhibit seismic rupture.
The largest historical earthquake on the fault is the 2002 M7.9 Denali fault earthquake, which
had 211 mi (340 km) of total surface rupture. The earthquake initiating in the west and
ruptured a 30 mi (48 km) long section of the previously unrecognized Susitna Glacier thrust
fault. Slip propagated primarily eastward rupturing 140 mi (226 km) of the Central Denali fault.
At the eastward limit of slip on the Central Denali fault the rupture stepped southeastward
rupturing 66 km of the Totschunda fault (Haeussler, et al., 2004).
Subsequent studies of Quaternary slip rates along the fault using cosmogenic exposure dating
of offset moraines and other glacial features show a westward reduction in slip rate on the
Denali fault (Figure 2-10). Matmon et al. (2006) calculate an 8.4 ± 2.2 mm/yr slip rate for the
eastern Denali fault, and a 6.0 ± 1.2 mm/yr on the Totschunda fault. The slip rates of the
Totschunda and Eastern Denali faults sum to 14.4 ± 2.5 mm/yr at the eastern part of the
Central Denali fault section. The preferred slip rates on the central Denali fault west of the
Totschunda fault junction are: 53 mi (85 km) west of the site is 13.0 ± 2.9 mm/yr (Mériaux, et
al., 2009), 255-283 km west is 9.4 ± 1.6 mm/yr (Matmon, et al., 2006), 323 km west is 6.7 ±
1.2 mm/yr (Mériaux, et al., 2009), and 390 mi (626 km) west is 0 mm/yr (Wesson, Boyd,
Mueller, Bufe, Frankel, & Petersen, 2007). The westward reduction in slip rate is widely
considered to be the result of the partitioning of slip onto the Northern foothills fold and
thrust belt (Figure 2-12) ( (Bemis, Wldon, & Carver, 2015); (Haeussler P. , 2008); (Matmon, et
al., 2006); (Mériaux, et al., 2009) ). These slip rates are in line with measurements of strain
accumulation via geodetics, 6.5 to 9 mm/yr (Fletcher, 2002), and INSAR, 10 mm/yr (Biggs,
Wright, Lu, & Parsons, 2007). The westward reduction in slip rate is also consistent with the
westward decrease in displacement during the 2002 earthquake (Haeussler, et al., 2004).
Paleoseismic studies performed by the Denali Fault Working Group after the 2002
earthquake found that the penultimate slip events were of similar magnitude to the 2002 event
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(Schwartz & (DFEWG), 2003). Carver et al. (2004) used tree ring counts from damaged trees
near the Delta River to propose that the penultimate event was a M7.2 earthquake on July 6,
1912. Results from test pits adjacent to the Delta River by Plafker et al. (2006) suggest two
paleo-events at 310 to 460 years before present, and 650 to 780 years before present.
Trenching by the Denali Fault Working Group produced the following Denali fault
earthquake chronology:
• The 2002 rupture trace had earthquakes between 350 and 600 years before present, and
between 715 and 1,080 years before present;
• West of the 2002 earthquake the fault ruptured between 110 and 380 years before
present, and between 560 and 670 years before present;
• East of the Totschunda – Denali fault junction the fault experienced three events
between 110 and 356 years before present; ≥560 and 690 years before present; and
≤1,020 and 1,230 years before present (summarized in (Koehler, Personius,
Schwarz, Haeussler, & Seitz, 2011b)).
Koehler et al. (2011b) trenched a site along the 2002 rupture trace and found that the
penultimate event at this location was after 560 to 670 years before present. None of the
paleoseismic trenching studies found evidence for the 1912 Delta River earthquake discussed
in Carver et al. (2004)and Doser (2004).
6.2.2. Castle Mountain Fault
The Castle Mountain fault is an active, oblique strike-slip fault with a western and eastern
section (Figure 6-3). The eastern section is combined with the Caribou fault of Plafker et al.
(1994) due to their parallel geometry and the designation by Plafker et al. (1994) that both
sections have evidence for Quaternary displacement. The eastern section-Caribou fault is
primarily recognized in bedrock, has no evidence for Holocene surface rupture, and has historic
seismicity to Mb 5.7 (1984 EQ documented in (Lahr, Page, Stephens, & Fogleman, 1986) ). The
western section is defined by a 39 mi (62 km) Holocene fault scarp (north side up); and has no
known historic seismicity greater than M5 (Flores & Doser, 2005). The fault trace was mapped
in detail by Detterman et al. ( (1974) and (1976) ), and also by Haeussler (1998). Detterman et al.
(1974) document a near surface fault dip of 75 degrees northward and seismic reflection data
shows the fault to be steeply dipping (70 to 90 degrees) at depth (Haeussler, Bruhn, & Pratt,
2000).
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Figure 6-3: Castle Mountain Fault
Paleoseismic investigations of the Castle Mountain fault have yielded varying Quaternary
slip rates and interpretations of deformational style. Detterman et al. (1974) proposed a
maximum age for the most recent event of 1860 ± 250 years based on a radiocarbon ages of
a displaced soil horizon exposed in a trench across a 6.9 ft. (2.1 m) high scarp. Detterman et al.
(1974) also document 23 ft. (7 m) of horizontal displacement of a linear sand ridge across
the fault. Trenching by Haeussler et al. (2002) on the western section identified 4 earthquakes
on the fault (including one event on an adjacent fault strand) in the past 2800 years with a
recurrence interval of approximately 700 years. The most recent rupture occurred 730-610
years before present. Haeussler et al. (2002) determined a shortening rate of 0.07 to 0.14
mm/yr but no lateral offset was observed in the trenches. Wi llis et al. (2007) use an offset post-
glacial outwash channel on the western section to constrain a lateral slip rate of 2.8 mm/yr to 3.6
mm/yr), with a preferred rate of 3.0 mm/yr to 3.2 mm/yr Koehler and Reger (2011) propose that
a lateral slip rate of 0.45 to 0.63 mm/yr may be more appropriate for the western section.
Fuchs (1980) proposed a post-Eocene slip rate of 0.5 to 0.6 mm/yr for the eastern section.
6.2.3. Pass Creek – Dutch Creek Fault
The Pass Creek – Dutch Creek fault is a northeast-striking, south side down, normal fault
bounding the northern edge of the Peters Hills Basin (Haeussler P. , 2008) (Figure 1-1).
The Peters Hills basin is a small Neogene basin that may be a piggyback basin in the hanging
wall of a “Broad Pass fault” (see discussion on the Southern Denali fault zone below)
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(Haeussler P. , 2008). The Pass Creek – Dutch Creek fault forms a 21 ft. (6.5 m) tall scarp that
displaces Holocene sediments, and creates a vegetation lineament on the north side of the
Skwentna River. The last significant rupture on the fault had > 6.5 ft. (>2 m) of uplift and cut
a moraine with a radiocarbon age of 1340 ± 60 years before present (Willis & Bruhn, 2006).
6.2.4. Sonona Creek Fault
The Sonona Creek fault is located in the western Copper River basin. The structure is mapped
by Williams and Galloway (1986) as a 4 mi (7 km) long, northeast-striking fault with north side
down sense of displacement, offsetting Late Pleistocene glaciolacustrine sediments. No
information is provided by Williams and Galloway (1986) to indicate further the age, sense or
amount of displacement on this fault. Although resolution is low, topographic height of the scarp
appears limited, suggesting that at least the vertical slip rate is relatively low. As a singular
surface rupture along a potentially active fault, the length of the scarp is relatively short. The
presently available aerial imagery (Google Earth) of the fault is permissive of extensions and
certainly do not rule out extensions of this fault in either direction.
6.3. Zones of Distributed Deformation
Zones of distributed deformation are regions with poorly characterized or suspected active
faults; where the Quaternary geologic and fault mapping may be incomplete, and/or the
slip-rate and recurrence of individual faults is poorly understood. The site region includes
two areas classified as zones of distributed deformation: the Northern foothills fold and thrust
belt, and the Southern Denali faults.
6.3.1. Northern Foothills Fold and Thrust Belt Zone
The Northern foothills fold and thrust belt (NFFTB) is a zone of Quaternary faults and folds
along the north side of the Alaska Range ( (Bemis & Wallace, 2007); (Bemis, Wldon, &
Carver, 2015)) (Figure 2-12, Table 6-2). The zone is primarily comprised of variably
dipping thrust faults with dominantly north vergent deformation. Bemis and Wallace
(2007), propose that much of the NFFTB is underlain by a gently south-dipping basal
detachment that may daylight at the Northern foothills thrust along the northern margin of the
NFFTB in the vicinity of the Nenana River. The surface trace of the basal detachment is
not identified in the western and eastern margins of the NFFTB. The western margin of
the NFFTB is marked by the termination of uplifted topography northwest of the Peters
Dome fault and the Kantishna Hills anticline. The eastern margin is constrained by the
paleoseismic investigation by Carver et al. (2010)which found no evidence for Quaternary
deformation east of the Cathedral Rapids fault in the vicinity of Tok, Alaska. Quaternary
deformation is presumed across the uplifted region within the NFFTB; however, rates of
Quaternary deformation along individual faults are still poorly constrained (Bemis, Wldon, &
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Carver, 2015). Mapping of deformation in Pleistocene gravels by Bemis (2010) suggests
that the region west of the Nenana River has a maximum shortening rate of 3 mm/yr. Bemis
et al. (2015) used offset Nenana gravel along the Granite Mountain fault to suggest horizontal
shortening 1-4 mm/yr of in the eastern NFFTB. Meriaux et al. (2009) proposes that the
partitioning of slip from the Denali fault onto the NFFTB could produce convergence rates up
to about 4 mm/yr in the eastern NFFTB end, and about 12 mm/yr to the west. Due to the
apparent variability in slip rates longitudinally across the NFFTB the zone is divided into a
western and eastern zone.
6.4. Talkeetna Block Structures
The region is characterized by bedrock faults and distributed deformation associated with
Cretaceous accretion of the Wrangellia Terrane ( (Csejtey, et al., 1978), (1992); (Ridgway K.
D., Trop, Nokleberg, Davidson, & Eastham, 2002) ) and post-accretionary right-lateral bulk
shear in the Tertiary, (O’Neill, Schmidt, & Cole, 2005); (Glen, Schmidt, & and Morin, 2007b)).
To date, no direct geologic evidence to conclusively evaluate the late Quaternary fault activity
in the Talkeetna Block exists, although new mapping is underway by the DGGS (Twelker, et al.,
2014). The proximity of the Talkeetna block structures to the Watana dam site area requires a
thorough discussion of the previously mapped faults with respect to the seismic hazard
characterization (Fugro, 2013; 2015a.
6.4.1. Talkeetna Thrust Fault / Talkeetna Suture
A previously mapped through-going structure within the Talkeetna block is the Talkeetna thrust
fault along the eastern margin of the Talkeetna suture zone (e.g. Figure 2-1 and Figure 2-2). The
Talkeetna thrust is mapped as a northeast-striking, southeast-dipping fault by Csejtey et al.
(1978), WCC (1982), and Wilson et al. (1998) (Figure 6-4). The fault juxtaposes Triassic and
Permian metavolcanic and metasedimentary Wrangellia terrane rocks on the south against late
Jurassic through Cretaceous sedimentary rocks of the Kahiltna Assemblage on the north. The
mapped trace of the fault, depicted as concealed and inferred along much of its extent, projects in
the northeast toward the younger Broxson Gulch thrust and is mapped as being obscured in the
southwest by Tertiary igneous rocks. The approximate fault trace follows a broad topographic
lineament striking northeast across the Talkeetna Mountains (Figure 6-4); however, the precise
location of the fault (expressed as a lineament) is obscured along much of its length by Tertiary
igneous rocks and Quaternary sediments. The investigation by WCC (1982) found indeterminate
geologic evidence for conclusively evaluating Quaternary activity along the fault.
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Figure 6-4. Site Vicinity Tectonic Features
The Talkeetna fault was recognized as a major tectonic feature near the Watana dam site by
Kachadoorian and Moore (1979) and WCC (1982) although no evidence of Quaternary faulting
was located during their either investigation. This study reached similar conclusions, based on
the initial literature review for seismic source characterization and subsequently based on field
investigation, mapping, and lineament mapping using LiDAR-derived DEM’s (Fugro, 2015a).
The WCC (1982) investigations included paleoseismic trenching at two locations along the
suspected map trace of the Talkeetna fault: trench T-1 and trench T-2. Trench T-1 is located
directly southwest of the Fog Lakes, and lies about 9 mi (15 km) southwest from the proposed
dam site (number 2 on Figure 6-6), although the trench location was not directly atop the map
trace. Trench T-2 is located much farther to the southwest, about 40 mi (65 km) from the
proposed dam site, and is slightly west of the confluence of the Talkeetna River and Iron Creek
(number 1 on Figure 6-4).
Recent geologic mapping by Twelker et al. (2014) as part of the Talkeetna Mountains C-4
quadrangle map updates the T-1 trench site and broader area geology at 1:50,000-scale based on
field mapping, analysis of gravity and electromagnetic data, and mineralogic analysis. Key
conclusions from their mapping effort are that the Talkeetna fault is not expressed in bedrock
geology as a single, continuous fault. Rather, it is now characterized from geophysical-based
bedrock interpretation and mapping as a series of complex, high angle, northeast-trending fault
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strands, and strands of the Talkeetna fault themselves appear to be cross-cut and truncated by
north-northwest trending bedrock faults providing evidence suggesting that the Talkeetna fault is
not active in the contemporary stress regime.
6.4.2. Susitna Lineament
The Susitna lineament is a pronounced northeast-southwest trending lineament located near the
dam site area (Figure 6-6). Gedney and Shapiro (1975) described the feature as a fault based on
differential K-Ar cooling data in the Talkeetna Mountains and seismicity. However, subsequent
mapping by Csejtey et al. (1978) found no evidence for a major fault in the location of the
Susitna lineament. This study concluded similarly (Fugro, 2015a). The lineament is mapped
near Butte Lake by Smith et al. (1988) and through the central Talkeetna Mountains near the dam
site by Clautice (1990). Fault and lineament mapping by Wilson et al. (2009) shown no northeast
trending faults in the vicinity of the Susitna lineament but do show several short lineament
segments (5-7.5 mi [8-12 km]) that are adjacent to, and parallel with, the previously mapped
lineament trace (Figure 6-4). WCC (1982) interpreted the Susitna lineament to be a bedrock
feature not related to faulting, except for possible erosion along a minor shear zone parallel to the
fault. This conclusion was based on bedrock and surficial mapping, a magnetometer survey, and
paleoseismic trenching along the trace of the lineament.
Glen et al. (2007b) describes the Susitna lineament as a series of 6- to 12-mi (10 to 20 km) long
en echelon segments stepping eastward along strike to the north. They report east side down
motion on the lineament which exposes Eocene volcanic rocks and Miocene and Oligocene
sedimentary rocks in the Fog Lakes and Watana basins. O’Neill et al. ( (2003b) and (2005) )
suggest that the en echelon pattern of the lineament may be the result dextral motion during post-
accretion right-lateral bulk shear. Reger et al. (1990) do not depict the Susitna feature as a photo-
geologic lineament and the map shows no direct evidence to support faulting or offset of late
Pleistocene glacial till deposits or kame-esker deposits (Reger map units Qd3 and Qk,
respectively).
6.4.3. Shorter Structures Proximal to the Dam Site
In addition to the Talkeetna fault and the Susitna lineament, there are numerous northeast-
and northwest-striking bedrock faults and lineaments in the Talkeetna block. Several of these
structures, proximal to the Watana d am site area (i.e. the northwest-striking shear zones), were
studied in detail by WCC (1982) and this study. The results and are applied in Section 5 of this
report, and fully discussed in Fugro (2015a).
The northeast-southwest structural fabric likely originated during Cretaceous accretionary
deformation (Csejtey et al., (1978) and (1982); Ridgeway et al., (2002) ). Post-accretionary
deformation driven by Tertiary right lateral bulk shear in the Talkeetna block has been
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proposed by several studies (O’Neil et al. (2005), Glen et al., (2007a) and (2007b) ). These
studies suggest that Tertiary trans-tensional deformation reactivated northeast-southwest
oriented structures and produced several grabens and half grabens including the Watana
Creek lowland, and Fog Lakes lowland. Based on regional geophysical data, Glen et al.
(2007a) propose that the Fog Lakes lowland is structurally bounded on the west by the
Susitna lineament, and to the east by a series of range front normal faults (e.g. Talkeetna fault)
defining a Fog Lakes graben (Figure 6-4). Whether or not this is true, the seismic source model
includes the hypothesized Fog Lakes graben as a model element because of its geographic
position with respect to the dam site.
6.5. Crustal Seismicity
6.5.1. Earthquake Catalog
6.5.1.1. Earthquake Data Source and Magnitudes
A catalog of earthquakes for the study area was compiled starting with the Alaska
Earthquake Center (AEC) earthquake catalog. The catalog contains earthquakes of M 3 and
above, down to a depth of 62 miles (100 km), and from 1899 through December 31, 2010.
The base AEC catalog was supplemented with the undeclustered (includes aftershock
earthquakes) USGS catalog from the 2007 Alaska hazard maps (Wesson, Boyd, Mueller,
Bufe, Frankel, & Petersen, 2007). In addition, the earthquake locations, depths, and/or
magnitudes were updated using the results of relocation studies by Doser (2004), Doser et
al. (2002), Doser et al. (1999), and Ratchkovski et al. (2003). The AEC catalog mb, ML, and
MS magnitudes were converted to moment magnitude (Mw) following the relations of
Ruppert and Hansen (2010), which apply to earthquakes from 1971 to the present.
Earthquakes prior to 1971 were assigned Mw magnitudes according to: (1) the relocation
studies noted above, (2) the 2007 USGS Alaska catalog, or (3) following the relation which
agrees with the magnitudes used by the USGS. The updated catalog (Figure 6-5) was
declustered (remove aftershocks) using the Gardner-Knopoff algorithm (Gardner & Knopoff,
1974). An aftershock exclusion zone was used to identify likely aftershocks of the 2002 Denali
earthquake (Figure 6-6). Earthquakes within the exclusion zone, post- dating the 2002 event
were removed from the catalog. The 2002 Denali earthquake itself is also removed from the
catalog as it is directly associated with the Denali fault, and, therefore, is inappropriate to
remain in the catalog database used to derive aerial source zone earthquake recurrence. This
event is accounted for in hazard calculations through the characterization of the Denali fault
discussed in Section 6.2.1.
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Figure 6-5. Unfiltered Earthquake Catalog
Figure 6-6. Declustered Earthquake Catalog
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6.5.1.2. Earthquake Magnitude Completeness
In order to analyze catalog completeness as a function of magnitude, a “Stepp” plot was
constructed which shows the event rate per year as a function of time since the present. This is
shown in Figure 6-7 using 5-year bins, and indicates completeness for M 3 since 1970, M 4
and 5 since 1965, and M 6 since 1930. M 7 events are few, thus, completeness for these should
rely on population density and reporting. Wesson et al. (2007) estimated completeness for
M 4.5 since 1964, M 6 since 1932, and M 6.9 since 1898 for their Alaska catalog. Because the
results shown in Figure 6-7 are consistent with Wesson et al. (2007) completeness estimates,
the Wesson et al. (2007) completeness estimates are adopted for this study. The M 3
completeness since 1970 is consistent with seismic network information in Ruppert and Hansen
(2010). The large bump in rates around 1977 is due to the ML to M conversion formula in
Ruppert and Hanson (2010). This formula added 1.13 units to ML values between 1977 and 1989.
While this conversion appears to be poorly constrained, it doesn’t affect the selection of
completeness periods.
The magnitude completeness was for the preliminary PSHA completed in 2012. The
completeness analyses have been updated based on the project seismicity network data (Fugro,
2015b).
Figure 6-7. Magnitude vs. Time Prior to 2011
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6.5.2. Crustal Source Zones
Crustal thickness in the southern Alaska block (SAB) source zones is estimated to be 16 mi
(25 km) in the West and Central zones, and 31 mi (50 km) in the East zone (see Section 3.5
and 3.6; Fugro, 2012). However, the West and Central zones experience earthquakes
associated with the subducting slab below 14 mi (23 km) depth (see Fugro, 2012, Figure
27). The maximum depth for earthquakes in the recurrence catalog in the West and Central
zones is reduced to 14 mi (23 km) to exclude these apparent slab events, but the source model
for these zones allows earthquakes down to 16 mi (25 km), reflecting the uncertainty in
crustal thickness. In comparison, WCC (1982) apparently used a seismogenic crustal
thickness in their Talkeetna terrane of 12 mi (20 km). The Eastern zone is located off the
northwest edge of the subducting slab so events as deep as 31 mi (50 km) are included in
the recurrence catalog, and the source model allows for earthquakes down to 31 mi (50 km) as
well.
The majority of seismicity in NFFTB zones is located above 12 mi (20 km) depth, and most
events below 12 mi (20 km) have high vertical location errors (> 3 mi [5 km]) (see Fugro, 2012,
Figure 28), thus seismicity in the recurrence calculations is constrained to a maximum depth of
12 mi (20 km).
6.6. Earthquake Recurrence from Seismicity
Earthquake catalogs used for SAB and NFFTB areal source zone recurrence calculations are
shown in Figure 6-8. Fewer earthquakes are shown in Figure 6-8 than in Figure 6-6 because of
removal of some events after filtering for the completeness periods discussed in Section 6.5.1.
The location of the 1912 Denali earthquake, as relocated by Doser (2004), is directly north of the
Denali fault. Considering the large location error, the 1912 event conservatively is included in
the SAB Central zone recurrence catalog instead of the NFFTB West zone. The truncated
exponential recurrence model was used for these areal zone sources, with recurrence parameters
computed by the maximum likelihood method. Figure 6-9 presents the maximum likelihood
recurrence curves for the SAB Central aerial source zone (background source for dam site).
Magnitudes for the PSHA calculations range from 5.0 to 7.0.
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Figure 6-8. Final Recurrence Catalog
Figure 6-9. Maximum Likelihood Recurrence Curves for SAB Central Areal Zone
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7. GROUND MOTION PREDICTION EQUATIONS
Ground motion prediction equations (GMPEs) transform magnitude, distance, and other
ground motion-related parameters into ground motion amplitude distributions for a wide range
of vibrational frequencies. Such equations are continually being developed and refined as
more strong motion accelerograms become available. For this project, three types of GMPEs
were drawn upon: those for crustal sources, those for plate interface sources, and those for
intraslab sources. The GMPEs and their weights for the three source categories were selected
and are shown in Table 7-1. It is recognized that the GMPEs used to calculate the hazard in
Fugro (2012) have since been updated and the revised seismic hazard analysis will use the
current GMPEs along with an updated source models.
Table 7-1. Ground Motion Prediction Equations Used in PSHA
Sources GMPE Abbreviation Weight
Megathrust
BC Hydro, 20111 BCH11 0.50
(Zhao, et al., 2006) ZH06 0.25
(Atkinson & Macias, 2009) AM09 0.25
Intraslab
BC Hydro, 20111 BCH11 0.50
(Zhao, et al., 2006) ZH06 0.25
(Atkinson & Boore, 2003) AB03 0.25
Crustal
(Abrahamson & Silva, 2008) AS08 0.25
(Chiou & Youngs, 2008) CY08 0.25
(Campbell & Bozorgnia, 2008) CB08 0.25
(Boore & Atkinson, 2008) BA08 0.25
Note: (1) as provided by N. Abrahamson, August 2011.
7.1. Subduction Zone
For the megathrust and intraslab GMPEs BCH11 is preferred model because it is based on a
much larger data set that includes all of the data used by Zhao et al. (2006), and uses the
Atkinson and Macias (2009) simulation result to constrain the break in the magnitude
scaling at high magnitudes. The Atkinson and Boore (2003) relation uses the “global” version,
as opposed to the Cascadia version. The crustal source GMPEs consist of four NGA GMPEs,
each weighted equally.
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7.2. Crustal
The crustal source GMPEs consist of four NGA West1 GMPEs, each weighted equally. The
Idriss 2008 NGA West1 relationship was not used in this assessment because the distance range
was outside the applicable range for this equation. Any updates to the PSHA should use NGA
West2 GMPEs.
7.2.1. Shear Wave Velocity
All of the GMPEs in Table 7-1 employ VS30 (average shear-wave velocity in the top 100 ft. (30
m) as a site condition parameter for linear and non-linear site response, either explicitly or as a
site category indicator. Based on the initial data review, the hazard was computed using a
reference VS30 of 2,625 ft/s (800 m/s), as this is the range that was constrained by the empirical
data acquired prior to the seismic hazard computations. Interferometric Multichannel Analysis of
Surface Waves (IMASW) was performed at a later date to estimate shear wave velocities (Vs) at
seven existing and proposed seismic recording stations operated for the Susitna-Watana
Hydroelectric Project (Fugro, 2014a). Existing seismicity station, WAT-1, is located at the dam
site. This station had a VS30 value on rock which was calculated to be 3,556 ft/s (1,084 m/s). The
adopted value of 2,625 ft/s (800 m/s) which was used in the hazard calculations is considered to
be a conservative velocity value that should be updated when the hazard study is revised.
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8. PROBABILISTIC SEISMIC HAZARD ANALYSIS
8.1. Methodology
The basic PSHA methodology employed here follows the precepts of Cornell (1968). The
programs used were Fugro Consultants, Inc. codes faultsource_31 version 3.1.228 for fault
sources, mrs5.2 version 5.2.228 for areal sources, and agrid1.1 version 1.1.228 for gridded
seismicity sources. Earlier versions of these codes were vetted under the PEER PSHA Code
Verification Workshop (Wong, Thomas, & Abrahamson, 2004).
8.2. Inputs
8.2.1. Subduction Zone
8.2.1.1. Intraslab Model
For the intraslab source the Wesson et al. (2007) model was used, which consists of gridded
seismicity for two depth levels, 31 to 50 mi (50-80 km), and 50 to 75 mi (80- 120 km), and a
magnitude range of M 5 to M 7.5. Following Wesson et al. (2007), the depth for the 31 to 50 mi
(50-80 km) sources was set to 37 mi (60 km), and 56 mi (90 km) for the 50 to 75 mi (80-120 km)
points. A correlation distance of 31 mi (50 km) was used by Wesson et al. (2007) to smooth the
seismicity. Correlation distance is defined as the standard deviation of the Gaussian smoothing
function (Frankel A. D., 1995). The subduction intraslab, as modeled by Wesson et al. (2007),
used a truncated exponential recurrence model.
A sensitivity study was performed for the intraslab seismic source (Fugro, 2014b) by comparing
different Mmax selections, VS30 values, and slab geometries, however the PSHA with the
recommended changes to that source (Fugro 2012) has not been revised. Discussion on the
refined intraslab model is included in Fugro (2014b).
8.2.1.2. Interface Model
For the purposes of this study, the megathrust, or plate interface, geometry was modeled as a
single plane (seen as the rectangle in Figure 6-2) dipping 2.6 degrees to the northwest with upper
(southeast) and lower (northwest) depth bounds of 12 and 22 mi (20 and 35 km). This geometry
also roughly corresponds to the estimated rupture extent of the 1964 event (Figure 2-13). The
geometric parameters of this plane, including distances to the site, are listed in Table 8-2.
Following Wesson et al. (2007), the largest megathrust event is modeled as a repeat of the M 9.2
1964 event. A time-independent (Poissonian) annual rate of 1/560 is assigned, based on
paleoseismic investigations (Carver & Plafker, 2008). The Poissonian rate of this magnitude
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event has been ultimately decreased due to inclusion of time- dependent models (Fugro, 2012,
Appendix A).
Also following Wesson et al. (2007), the M 7-8 interface earthquakes are modeled as being
exponentially distributed according to rates calculated from the Wesson et al. (2007) earthquake
catalog. This catalog resulted from a hierarchical compilation of several catalogs, resolution of
magnitudes to the Mw scale, and declustering to remove dependent events. The a and b
Gutenberg-Richter recurrence values for this source were taken from Wesson et al. (2007).
Earthquakes in this magnitude range were modeled as occurring on the fault plane shown in
Figure 6-2. The interface earthquakes in the M 5-7 range are modeled as “gridded, smoothed
seismicity.” As described in Wesson et al. (2007), this model is created by sorting this seismicity
into 0.1 degree bins, and performing Gaussian smoothing with a correlation distance (Frankel, et
al., 1996) of 46 mi (75 km) (Fugro, 2012, Figures 16 and 17). The grid sources were placed at a
depth of 3 mi (5 km), as in Wesson et al. (2007). Although this depth is not realistic, given that
the megathrust lies 12 to 19 miles (20 to 30 km) beneath the site, this depth was retained to
maintain consistency with the Wesson et al. (2007) model. In addition to the truncated
exponential model, the interface magnitude probability density function included the maximum
moment magnitude with a Mmax 9.2 and no uncertainty. It is recommended that the interface be
updated to a more realistic model in subsequent studies.
8.2.2. Crustal Sources
A number of crustal faults were not included the PSHA source model (Table 8-1). Five crustal
faults are included in the PSHA with the source characterization parameters contained in Table
8-2 and Table 8-3. Two of these faults, the Denali and Castle Mountain, have been included
in the previous USGS source model (Wesson, Boyd, Mueller, Bufe, Frankel, & Petersen,
2007). The source characterizations for these faults have been updated, and include time-
dependent alternatives for the Denali fault, as well. The Pass Creek-Dutch Creek fault was not
included in the USGS source model, but previously was identified as a Quaternary fault in
Plafker et al. (1994). A conservative slip rate distribution is included for this fault. The
Sonona Creek and Fog Lakes graben are potential sources within the Southern Alaska block
newly considered in this evaluation because of their potential proximity to the Watana Dam
site. Evidence to support full seismic source characterization for both sources was incomplete
at the time of the initial PSHA (Fugro, 2012). However, a Crustal Source Characterization has
since been completed (Fugro, 2015a). Although, updates included in this summary report are not
included in the seismic hazard calculation, there are no published estimates for Quaternary
displacement on either of these faults. T he structures were included in this preliminary
evaluation with a full probability of activity, to test the sensitivity of their inclusion to the
hazard estimates for the site. Thus, slip rate distributions for these sources span about two
orders of magnitude, and range from 0.004 to 0.01 in/yr (0.01 to 0.3 mm/yr) to test a range of
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values that would reflect relative inactivity to an activity rate which is similar to the lower
range of slip rates on the Castle Mountain fault.
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Table 8-1. Site Region Faults Excluded from the PSHA Source Model
Fault Name Distance from Site (km) Distance from Site (miles)
Faults in the Southern Alaska Block – South of Denali fault
Broad Pass fault 63.8 39.6
Broxson Gulch fault 62.6 38.9
Bull River fault 78.2 48.6
Cathedral Rapids fault 244.1 151.7
East Boulder Creek fault 101.6 63.1
Foraker fault 135.5 84.2
Matanuska Glacier fault 136.4 84.8
McCallum-Slate Creek fault 153.9 95.6
McGinnis Glacier fault 147.0 91.3
Susitna Glacier fault 77.8 44.1
Faults in the Northern fold and thrust belt – North of Denali fault
Billy Creek fault > 70 > 45
Canteen fault > 70 > 45
Ditch Creek fault > 70 > 45
Donnely Dome fault > 70 > 45
Dot "T" Johnson fault > 70 > 45
East Fork fault > 70 > 45
Eva Creek fault > 70 > 45
Glacier Creek fault > 70 > 45
Gold King fault - Section A > 70 > 45
Gold King fault - Section B > 70 > 45
Granite Mountain fault A > 70 > 45
Granite Mountain fault B > 70 > 45
Healy Creek fault > 70 > 45
Fault Name Distance from Site (km) Distance from Site (miles)
Kansas Creek fault > 70 > 45
Macomb Plateau fault > 70 > 45
Molybdenum Ridge fault > 70 > 45
Mystic Mountain fault > 70 > 45
Northern Foothills thrust > 70 > 45
Panoramic fault > 70 > 45
Park Road fault > 70 > 45
Peters Dome fault > 70 > 45
Potts fault > 70 > 45
Red Mountain fault > 70 > 45
Rex fault > 70 > 45
Stampede fault > 70 > 45
Trident fault > 70 > 45
Trident Glacier fault > 70 > 45
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8.2.2.1. Denali Fault
The occurrence of an M 7.9 earthquake on the Denali fault in 2002 (Section 6.2.1) led to a
number of scientific investigations that greatly improved the characterization of this fault for
seismic hazard studies.
As shown in Figure 8-1, two geometric models are considered for this study: one in which
a repeat of the 2002 rupture occurs and the segment to the west ruptures independently;
and another model in which the entire fault length ruptures. Each of these scenarios has a
maximum magnitude of 7.9, and a maximum moment model is used for each (Table 8-3). The
two scenarios are weighted equally.
Figure 8-1. Crustal Fault Model
Following Wesson et al. (2007), the modeled slip on the Denali fault decreases monotonically
from 14.4 mm/yr at the east end to zero at the west end. The estimated slip rate as a function
of distance along the fault is shown in Figure 18 (Fugro, 2012). This was accounted for in
the PSHA as follows: In modeling ruptures on a fault, a rupture that has less area than the fault
itself is assumed to occur at any location with equal probability. Such ruptures are modeled by
placing them sequentially along strike and up and down dip with some spacing interval (0.6
mile [1 km] in this study). A rupture will consequently correspond to a portion of the fault
along the x-axis in Figure 18 (Fugro, 2012). The slip rate assigned to that rupture will,
therefore, be the average slip rate along that portion of the fault. Alternative models of the
Denali fault in which the fault extends farther west are not considered, because these models
would primarily extend the fault beyond 200 mi (320 km) from the site and the initial
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sensitivity evaluations (Fugro, 2012), indicate no significant change in the ground motion
results from this type of change to the source model.
Time-dependent occurrence rate models also were employed for the Denali fault. The
rationale is that since the 2002 rupture occurred so recently, another such event should be less
likely than the average rate in the near future. By similar reasoning, an earthquake on the
segment west of the 2002 rupture, which has not occurred for about 600 years, is more
likely to occur in the near future than the average. Details and results of the time-dependent
analysis are contained in Appendix A (Fugro, 2012).
8.2.2.2. Castle Mountain Fault
The Castle Mountain fault, described in Section 6.2.2, is modeled as two scenarios: a
segmented model where the east and west segments rupture independently, and an
unsegmented model where the entire fault length ruptures in one earthquake. The fault
geometry and location are shown in Figure 8-1. These scenarios are weighted equally.
To account for the uncertainty of the western segment slip rate (i.e. Haeussler et al., (2002);
Koehler and Reger, (2011); Willis et al., (2007)) two slip-rate scenarios are used (Table
8-3 also equally weighted. The higher slip-rate scenario reflects the rates used in prior USGS
hazard models (Wesson, Boyd, Mueller, Bufe, Frankel, & Petersen, 2007), while the lower
slip-rate scenario reflects more recent investigations (e.g., Koehler and Reger, (2011)).
8.2.2.3. Pass Creek-Dutch Creek Fault, Sonona Fault, and Fog Lake Graben F aults
The Pass Creek-Dutch Creek, Sonona and Fog Lake graben faults are described in Sections
6.2.3, and 6.2.4, respectively, and shown in map view in Figure 8-1. Their geometric
properties are listed in Table 8-2, and maximum magnitudes and slip-rate distributions in Table
8-3. No alternative models were employed for these sources.
8.2.2.4. Crustal Areal Zones
Five areal source zones were included in the model. Details of the geometric properties and
recurrence calculations for them are presented in Section 6.5.2 and Fugro (2012).
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Table 8-2. Geometric Fault Parameters for Susitna Source Model, as Modeled for PSHA
Fault Length (km)
Area1
(km2)
Depth Range (km) Dip
Rupture
Distance2
(km)
JB Distance3
(km)
Farthest
Distance (km)
ASZ - megathrust
interface model 319.9 102,500 20.0 to 35.0 2.6 78.4 70.2 529.4
Denali - 2002 rupture 307.5 4612 0.0 to 15.0 90.0 86.0 86.0 312.3
Denali - West segment 386.4 5795 0.0 to 15.0 90.0 71.2 71.2 324.0
Denali - entire fault 726.0 10,889 0.0 to 15.0 90.0 71.2 71.2 356.5
Castle Mtn fault 189.6 3856 0.0 to 20.0 80.0 99.8 97.8 186.1
Castle Mtn West fault
high 61.4 1253 0.0 to 20.0 80.0 136.9 135.4 186.1
Castle Mtn West fault
low 61.4 1253 0.0 to 20.0 80.0 136.9 135.4 186.1
Castle Mtn East fault 128.2 2602 0.0 to 20.0 80.0 99.8 97.8 138.0
Pass Creek-Dutch
Creek fault 65.6 1552 0.0 to 20.0 60.0 106.8 104.9 170.4
Sonona fault 36.9 749 0.0 to 20.0 80.0 71.5 69.2 91.6
Fog Lake graben north 60.9 1230 0.0 to 20.0 80.0 6.9 3.5 49.4
Fog Lake graben south 47.7 969 0.0 to 20.0 80.0 9.5 6.1 34.3
Notes:
(1) Magnitude-area formula for strike-slip faults from UCERF2 (Field et al., 2009), all others from Wells & Coppersmith
(1994
(2) Rupture distance is the closest distance to the fault plane. (3) JB (Joyner-Boore) distance is the closest horizontal
distance to surface projection of the fault plane.
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Table 8-3. Fault Slip Rate and Magnitude Parameters, as Modeled for PSHA
Fault Slip Rate
(mm/yr)
Mean
(mm/yr)
Slip Rate Distribution Type
Maximum
Magnitude
Magnitude
Models
Denali System
Unsegmented 0.0 – 14.4 N/A tapered 7.9
West of 2002
9.8 – 0.0 N/A tapered 7.9 Maximum Moment
(Normal Distribution,
sigma=0.2) 2002 rupture
segment1 14.4 – 9.8 N/A tapered 7.9
Eastern segment Not modeled separately due to distance from site
Southern Alaska Crustal faults
Sonona Creek 0.1 – 0.5 0.3 triangle,sym 7.0
Maximum Moment
(Normal Distribution,
sigma=0.2)
Pass Creek –
Dutch Creek 0.5 – 1.5 1.0 triangle,sym 7.0
Fog Lake graben
north 0.01 – 0.3 (0.1)2 0.14 triangle,asym2 7.0
Fog Lake graben
south 0.01 – 0.3 (0.1)2 0.14 triangle,asym2 7.0
Castle Mountain fault scenarios
Segmented Model (weight 0.5)
Castle Mtn east 0.5 0.5 none 7.2
Maximum Moment
(Normal Distribution,
sigma=0.2)
Castle Mtn west
(weight 0.5) 0.4 – 0.6 0.5 uniform 7.2
Castle Mtn west
(weight 0.5) 2.1 – 3.6 2.9 uniform 7.2
Unsegmented Model (weight 0.5)
Castle Mtn
combined 0.4 – 0.6 0.5 triangle,sym 7.6
Maximum Moment
(Normal Distribution,
sigma=0.2)
Notes:
(1) 2002 rupture segment includes the 72 km of the Totschunda fault ruptured in 2002
(2) Apex value for asymmetric triangular distribution in parentheses.
8.3. Results
8.3.1. Hazard Curves
A hazard curve consists of a ground motion level (in g) on the x-axis, and the mean annual
frequency of exceeding that ground motion on the y-axis. Mean hazard curves were developed
for four spectral response periods, peak horizontal acceleration (PHA), and 0.5, 1.0, and 3.0
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seconds acceleration response at 5% damping. The PHA plot is shown in Figure 8-2, Figure 8-3,
Figure 8-4, and Figure 8-5. The major sources have been grouped together for purposes of
presentation.
Figure 8-2. Hazard Curves for Peak Horizontal Acceleration
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Figure 8-3. Hazard Curves for 0.5 sec Spectral Acceleration
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Figure 8-4. Hazard Curves for 1.0 sec Spectral Acceleration
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Figure 8-5. Hazard Curves for 3.00 sec Spectral Acceleration
8.3.2. UHS
A uniform hazard spectrum (UHS) is developed from the suite of total hazard curves, each
of which is calculated for a specific spectral period (or its inverse, spectral frequency) at the
specified damping level. The spectrum is keyed to a return period, which is the inverse of
annual frequency of exceedance. For example, to construct a 10,000-year uniform hazard
spectrum, the ground motion level for PHA (0.00 or 0.01 spectral period) at the 0.0001
(1/10,000) y-axis level of that hazard curve is tabulated. The same is done for the hazard for
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the other spectral periods. The spectral period is then plotted on the x-axis, and the tabulated
ground motion level on the y-axis. These spectra, therefore, indicate the ground motion
amplitudes across the entire range of periods for a common hazard level.
Mean uniform hazard spectra for the total hazard were developed for return periods of 100,
250, 1000, 2,500, 5,000, and 10,000 years. These results are shown in Figure 8-6, values are
provided in Table 8-4.
Table 8-4. Uniform Hazard Spectra (g)
Period (sec) 100 yrs 250 yrs 1,000 yrs 2,500 yrs 5,000 yrs 10,000 yrs
0.01 0.1270 0.1991 0.3671 0.5222 0.6641 0.8271
0.02 0.1397 0.2184 0.3997 0.5662 0.7179 0.8918
0.03 0.1578 0.2467 0.4506 0.6370 0.8064 1.0004
0.05 0.1855 0.2898 0.5275 0.7437 0.9394 1.1631
0.075 0.2417 0.3784 0.6914 0.9807 1.2461 1.5527
0.10 0.2895 0.4545 0.8344 1.1897 1.5184 1.9008
0.15 0.3007 0.4732 0.8782 1.2609 1.6152 2.0264
0.20 0.2780 0.4383 0.8181 1.1764 1.5067 1.8874
0.25 0.2430 0.3837 0.7175 1.0325 1.3231 1.6586
0.30 0.2140 0.3391 0.6373 0.9200 1.1816 1.4844
0.40 0.1717 0.2746 0.5204 0.7540 0.9703 1.2201
0.50 0.1387 0.2233 0.4255 0.6179 0.7969 1.0038
0.75 0.0938 0.1532 0.2963 0.4351 0.5661 0.7209
1.0 0.0713 0.1179 0.2304 0.3421 0.4496 0.5791
1.5 0.0466 0.0774 0.1529 0.2305 0.3085 0.4054
2.0 0.0345 0.0569 0.1125 0.1709 0.2313 0.308
3.0 0.0221 0.0364 0.0713 0.1093 0.1490 0.1995
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Figure 8-6. Mean Uniform Hazard Spectra, Total Hazard
8.3.3. Deaggregations
A deaggregation of the ground motion hazard was performed, based on the principles
outlined in McGuire (1995) and Bazzurro and Cornell (1999). Bazzurro and Cornell
(1999) provide a comparative review of different techniques and their implications.
They point out that there is a tradeoff between matching the target spectrum precisely, and
identifying the most likely event to produce the target motions. McGuire’s (1995) method
of collecting contributions that equal the target motion for each GMPE was applied here,
and the deaggregation therefore is focused on matching the target spectrum.
Source deaggregation plots are shown in Figure 8-7, Figure 8-8, Figure 8-9, and Figure 8-10,
one for each of the four spectral response periods (PHA, 0.5 sec, 1.0 sec, 3.0 sec). Only
sources contributing 5% or more at any ground motion level are plotted, so the minor sources
are not shown. In Figure 8-7, Figure 8-8, Figure 8-9, and Figure 8-10, the 100- and 10,000-
year ground motion levels are shown, and in some cases, intermediate return periods. These
values can be found in the appropriate cell in Table 8-4.
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Figure 8-7. Relative Contributions, Peak Horizontal Acceleration
Figure 8-8. Relative Contributions, 0.5 sec Spectral Acceleration
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Figure 8-9. Relative Contributions, 1.0 sec Spectral Acceleration
Figure 8-10. Relative Contributions, 3.00 sec Spectral Acceleration
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The PHA hazard is dominated by the Alaska subduction zone intraslab at all return
periods (Figure 8-7). This reflects the high rate of M 5 – 7.5 events produced by this source.
The results are mixed for the 0.5 spectral acceleration (SA), with intraslab seismicity
dominating at return periods less than 2,500 years, and megathrust seismicity dominating at
longer periods (Figure 8-8). A similar result occurs for the 1.0 second SA, but with megathrust
activity dominating for the 1,000-year return period and longer (Figure 8-9). For 3.0-
second response SA, the Alaska subduction zone sources, Denali fault, and areal sources
contribute equally for the 100 year return period, but the Alaska subduction zone megathrust
dominates at all longer return periods (Figure 8-10).
Plots showing magnitude, distance, and epsilon contributions to the total hazard were
produced for the two major sources (the Alaska subduction zone interface and intraslab), for
three return periods (2,500, 5,000, and 10,000 years). Epsilon is the number of standard
deviations above or below the median, from which a ground motion amplitude (for a given
magnitude and distance) is contributing.
Figure 8-11 shows megathrust results for PHA, 2,500-year return period, and the BC
Hydro 2011 GMPE. The plot shows the dominant contribution from the M 9.2 earthquake,
with minor contributions from M 7 events. Figure 8-12 shows the same results, but for
1.0-second response and a return period of 10,000 years. For these results, the distance to the
megathrust is evident, 48 mi (78 km) or greater. Figure 8-13 shows contributions to the total
hazard from the intraslab source, for 0.5-second response, 2,500-year return period, and the
Zhao et al. (2006) GMPE. Since the dam site lies above this source, an exponential-appearing
decrease with distance is evident. Figure 8-14 shows that the intraslab contributions are
coming only from higher magnitude events near the site, at the extreme tails of the ground
motion distributions.
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Figure 8-11. Deaggregation for the Interface, Peak Horizontal Acceleration, 2,500-year Return Period
Figure 8-12. Deaggregation for the Interface,1.0 sec Spectral Acceleration, 10,000-year Return Period
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Figure 8-13. Deaggregation for the Intraslab, 0.5 sec Spectral Acceleration, 2,500-year Return Period
Figure 8-14. Deaggregation for the Intraslab, 3.0 sec Spectral Acceleration, 10,000-year Return Period
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9. DETERMINISTIC SEISMIC HAZARD ANALYSIS
9.1. Methodology
The regulatory process for seismic hazard evaluation defined by FERC (Section 1.2)
specifies that both probabilistic and deterministic evaluations be conducted. Draft guidance for
the deterministic evaluation outlined in Section 5.1 of Idriss and Archuleta (2007) provides the
general framework followed here.
The seismic source characterization (Section 4) and the PSHA results (Section 8.3) provide
a basis for selecting critical seismic sources for the deterministic evaluation. These critical
sources are selected primarily based on consideration of magnitude, distance, and their
relative contributions of each source in the PSHA analyses. Other seismic sources in the
region may have smaller magnitudes at similar or comparable distances to this group of
sources, and are therefore not included in the deterministic evaluation.
9.2. Inputs
Four critical fault sources are identified: (1) subduction interface, (2) intraslab, (3) Denali fault,
and, (4) Fog Lake graben (Table 9-1). For these fault sources, the same maximum magnitudes
used in the both the probabilistic and deterministic evaluations. Distances are measured
from the site to the closest approach of the fault source as modeled in the PSHA model,
except for the intraslab source. The intraslab source distance is estimated from cross sections
which show seismicity associated with the down-going slab beneath southern Alaska and the
site (Figure 6-1). Recurrence estimates associated with the largest events on the fault sources
likely vary by more than an order of magnitude. Recurrence for the deterministic magnitudes
on the Denali fault, subduction interface and intraslab sources are most likely less than 1,000
years. Slip rates on these sources are high (greater than 0.004 in/yr (0.1 mm/yr)).
Recurrence for an M 7 on the Fog Lake graben source is unknown, but potentially greater
than 10,000 years. Slip rates on this source are also unknown, but are included in the seismic
source model as a range from 0.01 to 0.5 mm/yr. As a conservative approach and
sensitivity test in the preliminary PSHA, a probability of activity of 1.0 is used for this
source, but lack of data and evaluation of the Fog Lake graben source indicate that the
present probability of activity should be considerably less than 1.0.
Diffuse seismicity occurring throughout the region is not associated with specific faults, but
modeled in the PSHA as background source zones with a maximum magnitude of M7.3. The site
area location is near the center of the Southern Alaskan block (SAB) central zone and a
deterministic evaluation for this seismic source zone is derived from the 10,000-yr return
period deaggregation results from the PSHA (Table 9-2). These results indicate that different
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magnitude and distance pairs result for each spectral period. Thus, a separate deterministic
magnitude – distance pair could be selected for each of the four spectral periods of interest.
Table 9-1. Deterministic Hazard Input Parameters
Source Magnitude
(Mw)
Rupture Distance
(km)
JB Distance (km)
Epsilon Depth
(km)1
Ground Motion Prediction Equations [weight]
Megathrust 9.2 78 n/a n/a 35
ZH06 [0.25]
AM09 [0.25]
BCH11 [0.50]
Denali fault 7.9 71 71 n/a 0 – 15
BA08 [0.25]
CY08 [0.25]
CB08 [0.25]
AS08 [0.25]
Intraslab 7.5 50 n/a n/a 45
ZH06 [0.25]
AB03 [0.25]
BCH11 [0.50]
Fog Lake
graben 7.0 7.0 3.5 n/a 0 – 20
BA08 [0.25] CY08 [0.25]
CB08 [0.25]
AS08 [0.25]
Castle Mtn.
fault 7.6 100 98 n/a 0 – 20
BA08 [0.25]
CY08 [0.25]
CB08 [0.25]
AS08 [0.25]
Notes: Depth range indicates top and bottom of faults, individual depths indicate the rupture depth
Table 9-2. Crustal Seismicity (10,000 yr) Period-Dependent Deaggregation Results Summary
Period (sec) Return Period Mbar Dbar Epsbar Mod_MD Mod_MDE
0.0 10k 6.14 17.00 1.20 5.38 - 10.0 5.63 - 12.5 - 1.20
0.5 10k 6.44 21.70 1.23 6.25 - 11.3 6.50 - 18.8 - 0.90
1.0 10k 6.61 23.98 1.29 6.48 - 11.3 6.65 - 17.5 - 0.90
3.0 10k 6.84 24.10 1.48 6.95 - 11.3 7.13 - 18.8 - 0.90
Note: These inputs are the average of Next Generation ground motion prediction equations Abrahamson and Silva (2008),
Boore and Atkinson (2008), Campbell and Borzorgnia (2008), and Chiou and Youngs (2008).
9.3. Results
Deterministic ground motion estimates were developed for five critical seismic sources based
on maximum magnitude estimates, site to source distances, and the weighted GMPE’s used
for each source in the initial PSHA analyses. The deterministic sources are the subduction
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interface, subduction intraslab, Denali fault, Castle Mountain fault, Fog Lake graben faults,
and a 10,000-year return period earthquake for the background source derived from
deaggregation of the PSHA results.
The deterministic ground motion evaluation uses multiple GMPEs appropriate for each type of
seismic source with weights shown in Table 9-1. The same weighting of GMPE’s is used in the
deterministic evaluation as was used in the PSHA.
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10. COMPARISON OF PROBABILISTIC AND DETERMINISTIC
RESULTS
The FERC guidelines ( (FERC, 2011); (Idriss & Archuleta, 2007) ) recommend comparison
of the deterministic results to the total UHS from the probabilistic evaluation. The weighted
deterministic results, both median and 84th percentile, are shown as individually for each
critical source in comparison to the total UHS from the probabilistic evaluation (Figure 10-1
to Figure 10-6). The guidelines recommend use of 84th percentile values for the highly
active sources, but use of median values for sources with low average slip rates (Section 5.1
in Idriss and Archuleta (2007))..
Figure 10-1. Intraslab Deterministic Hazard Compared to the Total Hazard UHS
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Figure 10-2. Megathrust Deterministic Hazard Compared to the Total Hazard UHS
Figure 10-3. Denali Fault Deterministic Hazard Compared to the Total Hazard UHS
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Figure 10-4. Castle Mountain Fault Deterministic Hazard Compared to the Total Hazard UHS
Figure 10-5. Fog Lake Graben Deterministic Hazard Compared to the Total Hazard UHS
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Figure 10-6. Southern Alaska Block Central Period-Dependent Deterministic Hazard Compared
to the Total Hazard UHS
The deterministic evaluation indicates that the largest values of ground motions at the site are
associated with the subduction interface and intraslab sources due to their large magnitude,
relatively short distance, and GMPE’s used for these sources. For the intraslab source, the
deterministic results are generally similar to the 10,000-yr UHS, except at periods greater
than 0.5 sec (Figure 10-1). At periods of about 3 sec, the intraslab source contribution
corresponds to the 2,500-yr UHS. In contrast, for the subduction interface source, the
deterministic results are near the 2,500-yr UHS for periods less than 0.2 sec, but are near the
10,000-yr UHS for periods greater than 2 sec (Figure 10-2).
The median and 84th percentile results for the crustal sources indicate that these sources are
relatively less significant compared to the subduction zone sources. The Denali fault source
84th percentile results correspond to the 100-yr UHS for periods up to about 0.2 sec and are
below 1,000-yr UHS at periods up to 3 sec (Figure 10-3). The Castle Mountain fault 84th
percentile results are lower than the Denali fault, and are below the 100-yr UHS for periods
up to about 1 sec and below 250-yr UHS for periods up to 3 seconds (Figure 10-4).
For most periods, the Fog Lake graben source 84th percentile falls at the 2,500-yr UHS
(Figure 10-5). Deterministic contributions from the seismicity in the SAB central source zone,
Red symbols are Southern Alaska Block Central areal source deterministic
hazard (see Table 9-2 for magnitudes, distances and epsilon values). Black
lines are total hazard UHS.
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including 84th percentile estimates, also plot between the 250-yr and 1,000-yr UHS hazard
(Figure 10-6).
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11. SEISMIC DESIGN CRITERIA
For Watana Dam, maximum credible earthquake (MCE) ground motions were estimated
following FERC guidelines using deterministic seismic hazard analysis (DSHA), while the
maximum design earthquake (MDE) was defined based on the 5,000-year return frequency
ground motions from a probabilistic seismic hazard analysis (PSHA). The operating basis
earthquake (OBE) was selected to be the 500 year return period from the PSHA. Table 11 -1
summaries the peak ground acceleration (PGA) resulting from the MCE, MDE and OBE. It
should be noted that the MCE is represented by four different response spectra from three
different sources: the subduction zone events – interface and intraslab and crustal events. The
intraslab is represented by a M7.5 and M8.0. Additional details regarding the PGA and
deterministic percentile selected for these events is presented in Table 11 -2 and summarized in
the remainder of this section.
Table 11-1. Peak Ground Acceleration Values for the MCE, MDE and OBE
CASE DESIGN EVENT PGA
MCE Deterministic 0.81g
MDE 5,000-yr Return Period 0.66g
OBE 500-yr Return Period 0.27g
Table 11-2. Peak Ground Acceleration and Percentile for Deterministic Response Spectra
DESIGN EVENT CASE Crustal Interface Intraslab
Fog Lake Alaskan Subduction Zone
MCE - DSHA Magnitude 7.0 9.2 7.5 8.0
PGA(g) [percentile] 0.49 [84th] 0.58 [88th] 0.69 [84th] 0.81 [69th]
11.1. Response Spectra for the MCE
Based on the Seismic Hazard Characterization and Ground Motion Analyses for the Watana Dam
site area, (Fugro, 2012), the seismic hazard at the dam site encompasses contributions from three
different sources: the subduction zone events – interface and intraslab (also referred to as the
slab), and crustal events. Response Spectra and time histories were developed for each type of
event to evaluate the difference in frequency content.
Prior to the completion of the crustal seismic source evaluation, the crustal event was selected to
be a M 7.0 event on the Fog Lake graben located at a distance of 4.4 mi (7 km). In the Crustal
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Seismic Source Evaluation (Fugro, 2015a) the evaluation of potential crustal seismic sources has
not identified any specific features with evidence of late Quaternary faulting within at least 25 mi
(40 km) of the Watana dam site; however, this event is a conservative representation of the
background crustal event from the PSHA (see Figure 10-5 and Figure 10-6).
Guidance furnished by FERC, Evaluation of Earthquake Ground Motions, was followed and a
deterministic spectrum was used (Idriss & Archuleta, 2007). Table 11 -3, contains the
deterministic parameters for each of the selected events, and Figure 11 -1, illustrates the response
spectrum. The 2,500, 5,000, and 10,000 year return period uniform hazard spectra are also
included on Figure 11 -1; this data is from the seismic hazard analysis Report (Fugro, 2012). It
should be noted that the VS30 used in the probabilistic seismic hazard assessment is 2,625 ft.
(800 m/s).
The 84th percentile or above was used for all of the events, except the M 8.0 event for the slab,
where the 69th percentile is used. The interface event was scaled up at the fundamental period
of the dam (0.55 seconds) to match the 5,000 year return period, resulting in the 88th percentile,
see Figure 11 -1.
As an update to the Vs30 value in the Seismic Hazard Characterization and Ground Motion
Analyses for the Watana dam site area, current field data indicate a VS30 of 3,556 ft. (1,100 m/s)
which was used, in Revised Intraslab Model and PSHA Sensitivity Results (Fugro, 2014b) and in
the deterministic calculations, from which the time histories were based on. The probabilistic
analysis was run for a lower Vs30 value (800m/s) prior to the field data measurements. When the
probabilistic analysis is recomputed the new Vs30 value of 3,556 ft. (1,080 m/s) should be used.
This change in Vs30 is expected to decrease the uniform hazard spectra (UHS) derived from the
PSHA results, as an increase in Vs30 decreases the ground motion.
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Table 11-3. Deterministic Seismic Input Parameters
CASE Crustal Interface Intraslab
Fog Lake Alaskan Subduction Zone
Magnitude 7.0 9.2 7.5 8.0
Hypocentral distance (km) - - 50
RRUP (km) 7.0 (RJB=3.5) 78 -
VS30 (m/s) 1,080 m/s
Type of faulting Normal Reverse Normal
Dip (degrees) 80 - -
Seismogenic Depth (km) 20 - -
Width (km) 20.3 - -
Z1.0 (km)
Z2.5 (km)
Z TOR (km) 0.5
Hanging Wall YES - YES YES
PGA(g) [percentile] 0.49 [84th] 0.58 [88th] 0.69 [84th] 0.81 [69th]
Ground Motion Prediction
Equation [weight]
BA08 [0.25]
CY08 [0.25]
CB08 [0.25]
AS08 [0.25]
BCH11 [0.5]
ZH06 [0.25]
AM09 [0.25]
BCH11 [0.5]
ZH06 [0.25]
AB03 [0.25]
Note:
km – kilometer(s)
Source: Deterministic Seismic Hazard Analysis; (Fugro, 2012)
Acronyms: BA08= Boore and Atkinson 2008; CY08=Chiou and Youngs 2008; CB08=Campbell and Borzorgnia 2008;
AS08=Abrahamson and Silva 2008; BCH11=BC Hydro 2012; ZH06= Zhao 2006; AM09=Atkinson and Macias 2009,
AB03=Atkinson and Boore 2003
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Figure 11-1. Design Response Spectra
A vertical response spectrum was developed by computing a vertical to horizontal ratio
following the guidance of Gülerce and Abrahamson (2011). The applicability to subduction zone
events was based on the work performed by Gregor et al. (2012). The magnitude and distance
pair used for the deterministic analysis was used as the input parameters to develop the vertical
to horizontal ratios. The ratios correspond to median values and are presented in Table 11 -4.
The vertical and horizontal response spectra are shown in Table 11 -5 through Table 11 -7 for each
event type together with plots shown in Figure 11 -2 through Figure 11 -4.
0
0.5
1
1.5
2
2.5
3
0.01 0.1 1Spectral Acceleration (g) Period (s)
10,000 Year Return Period -Vs30=800m/s
5,000 Year Return Period -Vs30=800m/s
2,500 Year Return Period - Vs30=800m/s
Intraslab M8, D50km Vs30=1100m/s - 69th
Intraslab M7.5, D50km Vs30=1100m/s - 84th
Interface M9.2, D78km Vs30=1100m/s - 88th
Crustal M7.0, D7km Vs30=1100m/s - 84th
Fundamental Period
Range 0.55-0.62 s
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Table 11-4. Median Vertical / Horizontal Ratios
Period (s) Slab M8.0 Slab M7.5 Interface Crustal
0.010 0.658 0.652 0.579 0.728
0.020 0.659 0.652 0.579 0.728
0.030 0.715 0.708 0.600 0.791
0.050 0.719 0.710 0.582 0.902
0.075 0.695 0.684 0.642 0.921
0.100 0.670 0.657 0.638 0.798
0.150 0.652 0.640 0.634 0.660
0.200 0.656 0.647 0.648 0.597
0.250 0.668 0.661 0.656 0.586
0.300 0.683 0.678 0.669 0.590
0.400 0.713 0.713 0.696 0.597
0.500 0.729 0.732 0.709 0.596
0.750 0.838 0.842 0.802 0.655
1.000 0.825 0.829 0.776 0.634
1.500 0.820 0.824 0.794 0.648
2.000 0.791 0.795 0.782 0.639
3.000 0.773 0.777 0.787 0.643
4.000 0.805 0.808 0.836 0.683
5.000 0.816 0.820 0.848 0.693
7.500 0.816 0.820 0.848 0.693
10.00 0.816 0.820 0.848 0.693
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Table 11-5. Horizontal and Vertical Design Response Spectra for Intraslab Events
Period M8.0 69th Percentile M7.5 84th Percentile
T (s) Horizontal
Acceleration (g)
Vertical
Acceleration (g)
Horizontal
Acceleration (g)
Vertical
Acceleration (g)
0.01 0.8075 0.531 0.6870 0.4479
0.02 0.8553 0.564 0.7337 0.4784
0.03 1.0121 0.724 0.8675 0.6142
0.05 1.2464 0.896 1.0668 0.7574
0.075 1.7055 1.185 1.4522 0.9933
0.1 2.0342 1.363 1.7286 1.1357
0.15 2.1449 1.398 1.8046 1.1549
0.2 1.9965 1.310 1.6686 1.0796
0.25 1.7432 1.164 1.4413 0.9527
0.3 1.5443 1.055 1.2646 0.8574
0.4 1.2174 0.868 1.0162 0.7246
0.5 0.9581 0.698 0.8078 0.5913
0.75 0.6262 0.525 0.5289 0.4453
1 0.4679 0.386 0.3960 0.3283
1.5 0.2835 0.232 0.2326 0.1917
2 0.1986 0.157 0.1603 0.1274
3 0.1226 0.095 0.0935 0.0726
Note: Deterministic Inputs shown in Table 11 -3.
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Figure 11-2. Intraslab M8.0 – 69th Percentile Design Response Spectra and Intraslab M7.5 – 84th
Percentile Design Response Spectra
Table 11-6. Horizontal and Vertical Design response Spectra for Interface Events
Period M9.2 88h Percentile
T (s) Horizontal Acceleration (g) Vertical Acceleration (g)
0.01 0.5754 0.3332
0.02 0.6011 0.3481
0.03 0.6328 0.3797
0.05 0.6697 0.3898
0.08 0.8857 0.5686
0.10 1.0832 0.6911
0.15 1.2221 0.7748
0.20 1.1724 0.7597
0.25 1.0975 0.7199
30.30 1.0472 0.7005
0.40 0.9222 0.6419
0.50 0.8005 0.5675
0.75 0.6308 0.5059
1.00 0.5298 0.4111
1.50 0.3848 0.3056
2.00 0.2964 0.2318
3.00 0.1914 0.1506
0.00
0.50
1.00
1.50
2.00
2.50
0.01 0.1 1 10Spectral Acceleration (g) Period (s)
Horizontal- Intraslab M8.0
Vertical - Intraslab M8.0
Horizontal -Intraslab M7.5
Vertical -Intraslab M7.5
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Note: Deterministic Inputs shown in Table 11 -3.
Figure 11-3. Interface M9.3 – 88th Percentile Design Response Spectra
Table 11-7. Horizontal and Vertical Design Response Spectra for Crustal Events
Period M7.0 84h Percentile
T (s) Horizontal Acceleration (g) Vertical Acceleration (g)
0.01 0.4910 0.3574
0.02 0.5022 0.3656
0.03 0.5487 0.4340
0.05 0.6859 0.6187
0.08 0.9005 0.8294
0.10 1.0523 0.8397
0.15 1.2028 0.7938
0.20 1.1882 0.7094
0.25 1.0686 0.6262
0.30 0.9567 0.5645
0.40 0.8077 0.4822
0.50 0.6615 0.3943
0.75 0.4455 0.2918
1.00 0.3476 0.2204
1.50 0.2289 0.1483
0.00
0.50
1.00
1.50
2.00
2.50
0.01 0.1 1Spectral Acceleration (g) Period (s)
Horizontal - Interface M9.2
Vertical - Interface M9.2
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Period M7.0 84h Percentile
T (s) Horizontal Acceleration (g) Vertical Acceleration (g)
2.00 0.1611 0.1029
3.00 0.0965 0.0620
Note: Deterministic Inputs shown in Table 11 -3.
Figure 11-4. Crustal M7.0 – 84th Percentile Design Response Spectra
11.2. Response Spectra for the OBE
According to the published guidelines ( (ICOLD), (FEMA), Alaska Dam Safety, and (U.S. Army
Corps of Engineers (USACE))) the normal choice of operating basis earthquake (OBE) would be
the earthquake that can reasonably be expected to occur within the service life of the project, that
is, with a 50 percent probability of exceedance during the service life. (This corresponds to a
return period of 144 years for a project with a service life of 100 years.) For the Susitna-Watana
Project, such an event would equate to a PGA of the order of 0.16g, which could be regarded as
unacceptably low by the general public who are not conversant with civil and structural design
guidelines. Table 11 -8 shows the PGAs for selected return periods.
0.00
0.50
1.00
1.50
2.00
2.50
0.01 0.1 1 10Spectral Acceleration (g) Period (s)
Horizontal- Crustal M7.0
Vertical - Crustal M7.0
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Table 11-8. PGAs for Selected Return Periods
Return Period, years PGA
100 0.13g
150 0.16g
500 0.27g
1000 0.37g
MWH recommends that the OBE be selected as the 500 year event, equating to a PGA of 0.27g.
The dam structure will be evaluated under this event and from a structural perspective all
facilities will be able to continue to operate without interruption or significant repair. The
horizontal response spectrum for the OBE is presented in Table 11 -9.
Table 11-9. OBE Horizontal Response Spectrum
Period (s) 500-yr Return Period
0.010 0.273
0.020 0.299
0.030 0.337
0.050 0.396
0.075 0.518
0.100 0.623
0.150 0.651
0.200 0.605
0.250 0.551
0.300 0.490
0.400 0.382
0.500 0.311
0.750 0.212
1.000 0.167
1.500 0.110
2.000 0.080
3.000 0.051
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12. SELECTION OF TIME HISTORIES
Pre-selection of time histories was completed by searching COSMOS, PEER, K-NET (Japanese
Earthquake Database), and a database run by the University of Chile and the Chile Ministry of
the Interior and Public Safety for ground motions that had magnitude, distance and record
properties similar to the controlling events.
Ideally, the selected time histories should have the same source, style-of-faulting, magnitude,
distance, site conditions and directivity condition as the event for which the evaluation is being
performed. However, in practice, it is not always possible to find a perfect match.
The criteria used to select the events are discussed below.
12.1.1. Intraslab
The catalog search for slab events included motions recorded during the El Salvador 2001 M7.6
event (14 recordings), Japan 2003 M7.1 event (412 of recorded motions), Chile 2005 M7.9 event
(10 recordings) and available records for the Japan 2011 M7.0 event (504 recordings).
The number of slab ground motions considered was then narrowed down to those events that had
a recording distance from 50 to 115 kilometers (km) and included all three components. The
design distance was 50 km and it was initially chosen to select those events that fell between
+/- 50 km, however this limited the database to a total of 28 events. By increasing the maximum
distance to 115 km and additional 24 events were able to be included. The closest event distance
was 72 km away, so the distance range was revised to 72-115 km. Chile M7.9 events were not
able to be used because no records fell within 115 km; the closest distance was recorded to be
135 km. This narrowed down the database to 52 events, 21 from the 2003 M7.1 Japan event, 20
from the 2011 M7.0 Japan event and 11 from the El Salvador event. Those 52 events were then
visually compared to the design response spectra and those with similar spectral shapes were
spectrally matched. By putting together a catalog of those strong motion events that occurred on
the slab, it is believed that the ground motion parameters (e.g., duration and equivalent number
of cycles) should be representative of the target slab scenario. Currently, there is a very limited
amount of earthquake events, and strong motions, available that can be comparable to the large
slab scenario (i.e. M8.0), which leads to high scaling factors. If the selection of time histories
includes the record properties of the scaled ground motions, then the time histories could be
scaled by large factors without affecting the average response (Watson -Lamprey & Abrahamson,
2005).
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12.1.2. Interface
The catalog search for interface events included motions recorded during Japan 2011 M9.0 event
(1400 of recorded motions) and available records for the Chile 2010 M8.8 event (55 recordings).
The design distance is 92 km and it was chosen to select those events that fell between +/- 50 km
(42-142 km). This narrowed down the database to 148 events; 138 from the 2011 M9.0 Japan
event and 10 from the 2010 M8.8 Chile event. Those 148 events were then visually compared to
the design response spectra and those with similar spectral shapes were spectrally matched. By
putting together a catalog of those strong motion events greater than M8.8 that occurred on the
interface, it is believed that the ground motion parameters (e.g., duration and equivalent number
of cycles) should be representative of the target slab scenario.
12.1.3. Crustal
The search for the crustal time history was performed using the PEER NGA West 1 Database
(Pacific Earthquake Engineering Research (PEER), 2007) for those events having a magnitude
ranging between 6.5 and 7.5 at distances of 0 to 15 km, contained all three components and a
VS30 greater than or equal to 400 m/s. After using the search criteria the time histories were then
narrowed down to those motions that had similar spectral shape.
12.1.4. Selected Events
Once the ground motions with similar spectral shape were compiled select recorded strong
motions were synthetically modified to match the target spectra. The record parameters for the
seed events are presented for the slab, interface and crustal events in Table 12-1 through Table
12-4.
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Table 12-1. Record Parameters for Selected Slab Time Histories – M8.0 -69th Percentile (PGA=0.81)
Event Title Station
Arias
Intensity
(m/s)
Predominan
t
Period
(sec)
Predominan
t
Freq.
(Hz)
Significant
Duration (s) No.
Cycles 5–95% 5–75%
Japan
(M7.1)
05/26/2003
Rrup=107km
MYG009EW
Seed 0.19 1.53 0.65 33.8 12.8 83
Spectrally
Matched 16.05 0.22 4.46 33.5 12.6 79
MYG009NS
Seed 0.18 0.22 4.53 39.7 13.7 91
Spectrally
Matched 16.06 0.23 4.34 36.6 14.6 87
MYG009UD
Seed 0.08 0.71 1.40 36.0 16.1 135
Spectrally
Matched 8.12 0.21 4.72 36.1 16.6 118
El Salvador
(M7.6)
01/13/2001
Rrup=112 km
MONTEW
Seed 1.12 0.57 1.77 17.3 10.4 44
Spectrally
Matched 9.10 0.57 1.77 20.6 11.2 39
MONTNS
Seed 1.14 0.25 3.95 18.2 11.2 33
Spectrally
Matched 9.41 0.25 3.95 20.0 12.9 40
MONTUD
Seed 0.77 0.25 3.95 19.6 14.1 79
Spectrally
Matched 5.67 0.25 3.95 24.6 16.0 55
El Salvador
(M7.6)
01/13/2001
Rrup=114 km
STTEC090
Seed 7.71 0.29 3.43 10.8 5.5 36
Spectrally
Matched 6.98 0.29 3.43 11.7 6.7 27
STTEC180
Seed 6.54 0.16 6.16 14.3 6.9 52
Spectrally
Matched 9.15 0.16 6.08 17.6 8.9 49
STTECUP
Seed 2.80 0.31 3.19 15.8 11.3 47
Spectrally
Matched 3.64 0.10 9.87 15.9 11.6 47
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Table 12-2. Record Parameters for Selected Slab Time Histories – M7.5 -84th Percentile (PGA=0.69)
Event Title Station
Arias
Intensity
(m/s)
Predominan
t
Period
(sec)
Predominan
t
Freq.
(Hz)
Significant
Duration (s) No.
Cycles 5–95% 5–75%
Japan
(M7.1)
05/26/2003
Rrup=107km
MYG009EW
Seed 0.19 1.53 0.65 33.8 12.8 83
Spectrally
Matched 11.50 0.37 2.71 34.1 12.8 72
MYG009NS
Seed 0.18 0.22 4.53 39.7 13.7 91
Spectrally
Matched 11.07 0.26 3.90 36.6 14.1 92
MYG009UD
Seed 0.08 0.71 1.40 36.0 16.1 135
Spectrally
Matched 5.89 0.13 7.79 36.0 16.5 120
El Salvador
(M7.6)
01/13/2001
Rrup=112 km
MONTEW
Seed 1.12 0.57 1.77 17.3 10.4 44
Spectrally
Matched 6.60 0.24 4.12 21.3 11.8 45
MONTNS
Seed 1.14 0.25 3.95 18.2 11.2 33
Spectrally
Matched 6.85 0.20 4.99 20.3 12.9 32
MONTUD
Seed 0.77 0.25 3.95 19.6 14.1 79
Spectrally
Matched 4.06 0.25 4.04 25.6 16.1 60
El Salvador
(M7.6)
01/13/2001
Rrup=114 km
STTEC090
Seed 7.71 0.29 3.43 10.8 5.5 36
Spectrally
Matched 4.76 0.20 4.94 12.6 6.9 27
STTEC180
Seed 6.54 0.16 6.16 14.3 6.9 52
Spectrally
Matched 6.79 0.15 6.79 17.8 9.4 46
STTECUP
Seed 2.80 0.31 3.19 15.8 11.3 47
Spectrally
Matched 2.54 0.10 9.87 16.8 11.8 44
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Table 12-3. Record Parameters for Selected Interface Time Histories – M9.2 -88th Percentile (PGA=0.58)
Event Title Station
Arias
Intensity
(m/s)
Predominan
t
Period
(sec)
Predominan
t
Freq.
(Hz)
Significant
Duration (s) No.
Cycles 5–95% 5–75%
Chile
(M8.8)
02/27/2010
Rrup=85km
CURIEW
Seed 10.55 1.45 0.69 49.3 37.0 102
Spectrally
Matched 13.06 0.23 4.35 51.1 37.3 76
CURINS
Seed 2.85 0.73 1.37 53.1 40.1 277
Spectrally
Matched 17.11 0.55 1.83 57.3 43.0 175
CURIUD
Seed 10.88 0.42 2.40 50.7 38.2 134
Spectrally
Matched 6.06 0.42 2.40 51.8 38.9 128
Japan
(M9.0)
03/11/2011
Rrup=105km
ATK 023EW
Seed 0.39 1.09 0.92 93.8 57.0 87
Spectrally
Matched 15.2 1.09 0.92 100.3 59.8 83
ATK 023NS
Seed 0.49 0.78 1.28 94.3 56.8 129
Spectrally
Matched 19.09 0.54 1.84 102.2 61.8 106
ATK 023UD
Seed 0.23 0.43 2.35 95.2 61.3 130
Spectrally
Matched 7.47 0.43 2.35 95.1 61.6 139
Japan (M9.0)
03/11/2011
Rrup=130km
CHB012EW
Seed 2.03 0.27 3.66 62.6 35.6 58
Spectrally
Matched 10.27 1.35 0.74 72.8 45.8 46
CHB012NS
Seed 2.63 0.34 2.94 57.7 33.0 86
Spectrally
Matched 12.05 0.30 3.38 73.1 43.5 47
CHB012UD
Seed 0.62 4.88 0.21 66.0 36.3 84
Spectrally
Matched 4.68 4.88 0.21 69.2 40.3 89
Note:
ATK 023 - Japanese Interface Earthquake Record – March 2011
CHB012 - Japanese Interface Earthquake Record – March 2011
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Table 12-4. Record Parameters for Selected Crustal Time Histories – M7.0 -84th Percentile (PGA=0.49)
Event Title Station
Arias
Intensity
(m/s)
Predominant
Period
(sec)
Predominant
Freq.
(Hz)
Significant
Duration (s) No.
Cycles 5–95% 5–75%
California
Loma Prieta
(M6.93)
10/18/89
Rrup=9.2km
GIL067
Seed 0.90 0.37 2.68 5.0 1.6 20
Spectrally
Matched 1.36 0.22 4.56 5.8 2.0 30
GIL337
Seed 0.70 0.37 2.71 4.8 1.3 18
Spectrally
Matched 1.34 0.45 2.23 5.5 1.8 23
GILUP
Seed 0.17 0.44 2.28 7.5 2.8 22
Spectrally
Matched 0.71 0.30 3.38 7.1 3.3 27
Iran
Dayhook
(M7.1)
09/16/1978
Rrup=13.9km
AUL000
Seed 0.06 0.59 1.69 19.0 12.7 36
Spectrally
Matched 3.67 0.48 2.10 19.6 13.9 35
AUL270
Seed 0.07 0.36 2.80 19.2 13.1 34
Spectrally
Matched 3.47 0.32 3.15 19.9 13.6 27
AUL-UP
Seed 0.02 0.44 2.28 19.3 13.1 30
Spectrally
Matched 1.65 0.45 2.22 19.0 13.6 30
Italy
Irpinia
(M6.9)
11/23/1980
Rrup=9.5km
DAYLN
Seed 1.42 0.39 2.56 12.3 6.7 35
Spectrally
Matched 2.56 0.95 1.05 13.6 7.5 32
DAYTR
Seed 1.36 0.43 2.31 12.4 6.9 15
Spectrally
Matched 2.05 0.77 1.30 11.9 6.4 24
DAYUP
Seed 0.65 0.18 5.54 14.8 8.3 68
Spectrally
Matched 1.93 0.18 5.55 15.2 8.9 56
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12.1.4.1. Spectral Matching Approach
Time histories were developed using spectral matching techniques. The spectral matching
approach uses a time domain approach (RSPMatch, (Abrahamson N. , 2012)) with the goal of
modifying a given time history to be spectrum compatible with a given target spectrum but
without any significant modification to the non-stationary characteristic of the original time
history.
Spectral matching adjusts the time series in the time domain by adding wavelets to the initial
time series. A formal optimization procedure for this type of time domain spectral matching was
first proposed by Kaul (1978) and was extended to simultaneously match spectra at multiple
damping values by Lilhanand and Tseng (1987); (1988). While this procedure is more
complicated than the frequency domain approach, it has good convergence properties and in
most cases preserves the non-stationary character of the reference time history.
Several passes were performed using the RSPMatch program until the fit to both the spectral
shape and displacement time history were acceptable.
12.1.4.2. Results – Selected Ground Motions
The time histories selected for the intraslab, crustal and interface events are presented in Table
12-1 through Table 12-4. The record properties for the seed and output time history are also
summarized in the tables.
The Arias intensity for the crustal events was calculated using empirical correlations developed
from the NGA West 1 dataset (N. Abrahamson, personal communication 2014). Equally
weighting the five ground motion prediction equations resulted in a median Arias intensity of
0.65 m/s and an 84th percentile Arias intensity of 1.48 m/s. The Arias Intensity for the horizontal
components of the spectrally matched crustal time histories range from approximately 1.34 to
3.67 m/s.
The Brookhaven Model (Silva, Abrahamson, Toro, & Costantino, 1996) was used to estimate the
significant duration between 5 and 75 percent for each of the four response spectra. The rupture
distance was used as input to the Brookhaven Model for the crustal and interface events, and the
hypocentral distance was used for the intraslab event (N. Gregor, personal communication,
August 29, 2014). The Brookhaven Model was originally developed for crustal events, but has
been shown to work adequately for the interface and is the best model available for the intraslab
(N. Abrahamson, personal communication 2014). The results are summarized in Table 12-5.
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Table 12-5. Estimate of Significant Duration using the Brookhaven Model
Event Horizontal Duration (5-75%), seconds Vertical Duration (5-75%), seconds
16th percentile 84th percentile 16th percentile 84th percentile
Intraslab Mw 7.5 6.7 20.9 7.8 20.4
Intraslab Mw 8.0 9.6 29.8 9.7 25.3
Crustal Mw 7.0 3.4 10.6 3.7 9.6
Interface Mw 9.2 25.6 79.3 19.7 53.3
Overall, the durations of both the seed and spectrally matched time histories generally fall within
the 16th to 84th percentile predicted by the Brookhaven Model. The exceptions are the vertical
interface motion recorded at station AKT023 and both horizontal motions for the M8 intraslab
event recorded at station STTEC. The spectrally matched vertical motion at station AKT023 had
a higher significant duration of 61.6 seconds (5-75 percent) compared to the 84th percentile from
the Brookhaven Model of 53.3 seconds. The spectrally matched horizontals from station GIL
crustal motion were 1.4 seconds which are slightly lower than the 16th percentile predicted value
of 3.4 seconds. The spectrally matched horizontals from station STTEC intraslab motion were
6.7 seconds and 8.9 seconds which are slightly lower than the 16th percentile predicted value of
9.6 seconds. Both the GIL and STTEC events will need to be revised in subsequent studies.
In Appendix B6 to the Engineering Feasibility Report (MWH, 2014) contains the earthquake
records plots. In Figure 1 through Figure 5 show plots of acceleration, velocity, normalized
displacement, response spectra, Husid plots, and Fourier amplitude spectra for each time history
component before and after spectral matching. Each motion has two horizontal components and
one vertical component.
Acceleration, velocity, and normalized displacement are plotted in Appendix B6, Figure 1 in blue
(labeled SEED) for the first horizontal component (MWH, 2014). The spectrally matched
acceleration, velocity, and normalized displacement time histories are shown in red. The plots of
acceleration, velocity, and normalized displacement are overlaid so that they can easily be
compared. The purpose of these plots is to confirm that the spectrally matched time history
remains similar to the original input motion and that extraneous wavelets are not being added to
the motion. Appendix B6, Figure 2 is a plot of the acceleration, velocity, and displacement for
the SEED motion and Appendix B6, Figure 3 is the same for the spectrally matched motion.
Appendix B6, Figure 4 illustrates the match to the horizontal design spectrum (black line labeled
TARGET), with the recorded motion shown in blue and the spectrally matched motion shown in
red. The overall goal of spectral matching is to achieve a fit as close as possible to the design
response spectrum. It is important to note that the fit to the lower periods (0.01s to ~0.02s) for
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some events has more variability about the target spectrum; this result is limited by the sampling
rate of 100-200 samples per second and has little impact on the structure, as most dams are
impacted by periods greater than 0.1 seconds.
The Arias normalized intensity (also called a Husid plot) for the initial and spectrally matched
acceleration time history is plotted at the top in Appendix B6, Figure 5; the bottom plot in
Appendix B6, Figure 5 is the Fourier amplitude spectra. Again, the blue line is the seed or initial
motion and the red line is the spectrally matched motion. These plots show that the Arias
intensity is not significantly different from the original input motion. The Fourier amplitude
spectra plot illustrates that the frequency content was not significantly modified in the frequency
range of 0.1 to 10 Hz (period range of 0.1 to 10 seconds).
The same presentation order is followed for each component and motion – acceleration, velocity,
and normalized displacement; SEED acceleration, velocity, and displacement; spectrally
Matched acceleration, velocity, and displacement; response spectra; and Husid plot (top), Fourier
amplitude spectra (bottom).
Twelve sets of three component spectrum compatible time histories were developed: three sets
were developed for the intraslab event M8.0-69th percentile, three sets were developed for the
intraslab event M7.5-84th percentile, three sets for the M9.2-Interface-84th percentile, and three
sets for the crustal M7.0-84th percentile.
12.1.4.3. Time Histories Used in the Analysis
For the feasibility analysis, one spectrally matched time history was selected from each of the
design events (two intraslab, one interface and one crustal). Two events were selected from the
intraslab to represent the different magnitude levels, 7.5 or 8.0; as this part of the seismic hazard
assessment is still in progress.
In total, four sets of time histories containing three records each have been developed for the
slab, interface and crustal events using spectral matching techniques. All of the ground motions
are based on the deterministic analyses using a VS30 of 1,100 m/s. The intraslab event utilized
two different earthquake records, one was from the El Salvador Mw7.6 and the other was from
the Japan Mw7.0.
For the MCE, the following time histories shown in Table 12-6 have been used:
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Table 12-6. Selected Time Histories for Feasibility Analysis– Intraslab and Crustal
El Salvador (M 7.6) STTEC Mw 7.5 – 84th percentile Intraslab
Japan 2011 (M 7.0) MYG 009 Mw 8.0 – 69th percentile Intraslab
Loma Prieta, California (M 6.93) GIL Mw 7.0 – 84th percentile Crustal
Based on comments received from the Board of Consultants to increase the design response
spectra, the following response spectra for the interface event in Table 12-7 was also used for the
analysis:
Table 12-7. Selected Time Histories for Feasibility Analysis – Interface
Chile 2010 CURI Mw 9.2 – 88th percentile Interface
12.1.4.3.1. OBE Time History
For the purposes of the feasibility level design only one event was run for the OBE case. The
crustal motion, GIL, was scaled by 0.61 to match the 500 year return period from the
Probabilistic Seismic Hazard Analysis. The geometric mean of the horizontal components from
the Crustal GIL motion was also computed. Figure 12-1 plots the 500 year return period (OBE),
the geometric mean from the GIL horizontal components with a factor of 0.61 applied and the
crustal response spectrum scaled to 500 year return period event for comparison.
Figure 12-1. OBE Response Spectra and Scaled Crustal Event
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13. ADDITIONAL STUDIES IN PROGRESS
13.1. PSHA Sensitivity
A PSHA hazard sensitivity for the intraslab source was computed using the model described in
this section (Fugro, 2014a). The ground motion prediction equations (GMPEs) and their
weighting were the same as those used in the initial PSHA (Fugro, 2012). A total of 27 results
were computed: all combinations of three correlation distances (defined as the width of the
spatial smoothing kernel used for the smoothed seismicity) three slab positions, and three
Mmaxes. Hazard curves for peak horizontal acceleration (PHA) and 1.0 second spectral
acceleration response (5% damping) were computed. These are portrayed for the purpose of
comparing the three sets of input parameters to each other, in addition to determining the overall
difference in hazard from the initial (2012) results (Fugro, 2012). No weighting of the alternative
values were included in this analysis.
In addition, new IMASW site response studies were conducted in the summer of 2013 (Fugro,
2014a), which resulted in a higher VS30 estimate for the site (1080 m/s) than the 800 m/s value
used in the initial assessment (Fugro, 2012). The PSHA hazard sensitivity to this higher VS30
value was also evaluated.
13.2. PSHA Sensitivity Calculations Conclusions
This study resulted in a more physically realistic geometric model of the downgoing Pacific plate
in the vicinity of the site for use in PSHA and ground motion analyses, based on planes fitted to
well-located seismicity. These analyses confirm earlier suggestions (e.g. (Ratchkovski & Hansen,
2002) ) that the subducting plate beneath southern and central Alaska consists of several
segments or sections with distinctly differing dips and orientations. For each of the geometrically
distinct slab sections defined by seismicity, best fit planes with 1σ and 2σ bounding limits were
defined to characterize the upper slab surface. For the intraslab section most proximal to the dam
site, 1σ and 2σ limits of slab seismicity thickness are 9.0 and 12.3 km, respectively, with dip
calculations of 21 and 25 degrees. The +2σ uncertainty for the best fit plane has a closest
approach to the dam site of 51 km, and depth beneath the site is 59 km. The best fit plane has a
closest approach of 57 km and depth beneath the site of 63 km.
Slab thickness in central Alaska from tomographic studies (Zhao, Christensen, & Pulpan, 1995)
is estimated to be 45-55 km, and their slab location indicates that the zone of relatively small-
magnitude seismicity shown in this report is occurring near the top of the slab. Using Puget
Sound historic Mw 6 slab earthquakes as an analogue, future large intraslab earthquakes beneath
the site are likely to stop at the top of the slab, but extend deeper into the slab rather than be
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confined to the thin zone of the upper slab based on small magnitude seismicity data that
indicates otherwise as shown in this report.
Comparisons to the PSHA results, for this updated intraslab source model show small variations
at PHA and 1.0 second spectral acceleration response, indicating that the Wesson et al. (2007)
model gives similar results, at least for these response periods, as long as the same Mmax value
is used (Fugro, 2012).
The PSHA results indicate negligible sensitivity to the model parameters correlation distance
used to develop occurrence rate grids and +/- 2 sigma variations in depth measures from the site
to the slab. It is perhaps fortuitous that the Wesson et al. (2007) uniform slab depth of 60 km is
very close to the distance from the site to the more refined slab model presented here. The results
are also relatively insensitive to a change in VS30 to 1080 m/s (Fugro, 2014b)from the 800 m/s
used previously (Fugro, 2012).
Hazard variations due to Mmax choices of 7.8 and 8.1 are significant. Sensitivity of the new
intraslab source model to the three Mmax estimates are shown in Figure 13-1 and Figure 13-2. A
VS30 of 800 m/s (Fugro, 2012) was used in this comparison. The first observation is that hazard
curves are close to the 2012 results, indicating that the Wesson et al. (2007) geometric slab
model gives similar results for this site and these response periods, for a Mmax of 7.5. The
second observation is that the results are very sensitive to choice of Mmax. For PHA, at a
1/10,000 annual frequency of exceedance (AFE) the ground motion increases from about 0.8 to
1.2 g as Mmax increases from M7.5 to M8.1, an increase of 50% in ground motion. For 1.0
second response the increase is comparable.
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Figure 13-1. Mmax Sensitivity for Peak Ground Acceleration
Figure 13-2. Mmax Sensitivity for 1.0 s Spectral Acceleration
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In contrast, changes in the slab position, correlation distance, and VS30 do not appreciably alter
the PSHA hazard results. Specifically, sensitivity analysis of the PSHA hazard results to the three
slab positions for the new model shows that there is essentially no sensitivity to this parameter in
the computed PSHA hazard when Mmax was set to 7.5, and correlation distance to 25 km.
Similarly, sensitivity of the PSHA hazard results to the three correlation distances for the new
model shows that there is negligible sensitivity to this parameter when Mmax was set to 7.5, and
slab depth to the mean position. In the case of VS30, sensitivity of the PSHA hazard results
showed very little change when an Mmax of 7.5, a correlation distance of 25 km, and the mean
slab depths were used with site characterization inputs of VS30 = 800 m/s as compared to use of
VS30= 1080 m/s.
For the two response periods, at a 10,000 year return period, ground motion increases of about
25% and 50% are indicated, respectively. However, due to the paucity of ground motion records
for magnitudes above 7.5, magnitude scaling of ground motions in the GMPEs above Mw 7.5 is
highly uncertain, and warrants further investigation.
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14. OTHER EARTHQUAKE RELATED HAZARDS
14.1. Reservoir Triggered Seismicity
Reservoir-triggered seismicity has been described as earthquake events that are triggered by the
filling of a reservoir, or by water-level changes or fluctuations during operation of the reservoir.
It is believed that reservoir triggered seismicity (RTS) primarily represents the release of pre-
existing tectonic strain, with the reservoir being a perturbing influence ( (Yeats et al, 1997); (US
Committee on Large Dams (USCOLD), 1997); (ICOLD, 2011) ). Thus, the reservoir does not
cause or induce the seismicity, it merely triggers the release of the accumulated, naturally
occurring tectonic strain that already existed.
The potential for reservoir triggered seismicity (RTS) to occur during and after filling of the
reservoir has been evaluated. An assessment of the potential for the future occurrence of RTS to
occur in the vicinity of the proposed reservoir was made expanding upon the earlier study
prepared by Woodward Clyde Consultants in the 1980s (Woodward-Clyde Consultants (WCC),
1980). The preliminary assessment and analysis of RTS is a work in progress as additional
background information from the seismic hazard studies (see Section 9.3) and long-term
earthquake monitoring data (see Section 2.5) are essential to this study.
The attributes that were considered in evaluating the probability of RTS include reservoir depth;
reservoir volume; the tectonic stress state; and the rock type and structure underlying the
reservoir. The probabilities that are considered are conditional and represent the total chance for
RTS to occur as a result of reservoir filling and operation. Conditional probabilities were also
developed for each attribute, as well as for all attributes combined. For the multi-attribute
analysis, each attribute is considered independently and also in a discrete-dependent model
focusing on depth and volume.
Additionally, a literature review, case study, and numerical analysis was performed of RTS based
on other projects with large, deep reservoirs in order to develop an understanding of the potential
of RTS at the Susitna-Watana site.
Data from the long-term earthquake monitoring system provides a baseline of the rates and
seismological characteristics of local seismic events prior to the impoundment of the reservoir.
Seismicity data collected as part of the long-term monitoring system data collection was used to
perform seismological analyses to help define local seismotectonic characteristics. Such
analyses include development of local velocity models, focal mechanism and regional stress
analysis, analysis of spatial patterns, and relationship of seismicity to reservoir operation. The
preliminary study will account for possible RTS earthquakes in the development of seismic
design parameters.
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At reservoirs where RTS has been suspected, the maximum reported earthquake magnitudes for
RTS events are primarily less than M 6.0, and typically less than M 4.0, and often below the
range felt by the public.
The most significant aspect of the RTS record is that of the verified RTS cases large enough to be
potentially damaging. Of recorded instances of RTS, just four events have exceeded M 6.0 and
only 13 events were in the range M 5.0 to M 5.9 ( (US Committee on Large Dams (USCOLD),
1997); (Yeats et al, 1997)). The largest reported RTS earthquake was the 1967, magnitude
M 6.5, Koyna, India event. The other three events were Hsinfengkiang (China 1962) M 6.1,
Kariba (Zambia 1963) M 6.0, and Kremasta (Greece 1966) M 6.3.
For this Project, the reservoir depth, reservoir volume, existing tectonic stress state, rock type
underlying the reservoir, and the rate of filling were considered when evaluating the probability
of RTS. The Project reservoir will have characteristics that might make it somewhat susceptible
to RTS, in that the maximum reservoir depth is greater than 575 ft. (175 meters), and it is within
an active tectonic region.
As described above, the Talkeetna Block is bordered by the Denali Fault to the north, and the
Castle Mountain Fault to the south, and the Wadati-Benioff Zone (Intraslab) lies at a depth of
approximately 50 km below the site based upon the focal depth of recent earthquakes. These
distant sources do not lie within the zone potentially influenced by reservoir filling, and thus RTS
is unlikely to occur on them.
Studies performed in the 1980s estimated the probability of RTS for the Project to be between
30 percent and 95 percent, with an event up to M 6.0 (Woodward Clyde Consultants (WCC),
1982). Recalculations performed during the present studies indicate that the reservoir has a
potential for producing an RTS event up to M 6.5, but the probability of an RTS event is between
16 to 46 percent. Any event would most likely occur within 10 years of initial filling.
RTS has been considered in the derivation of the seismic design parameters for the Project, and
will be further updated during detailed design. However, triggered seismicity requires the
presence of a causative fault. A seismic hazard assessment requires that all faults be identified;
hence, any fault identified during the seismic hazard assessment would likely cover those with
the potential for RTS.
For completeness the present studies have also considered the potential effects of RTS on the
nearest populated area, the town of Talkeetna, which is about 62 miles (100 km) from the site.
Using the RTS event of M 6.5 and GMPE, deterministic methods were used to estimate the peak
ground accelerations (PGA). The calculation estimates a PGA in Talkeetna of 0.02g for the
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median and 0.04g for the 84th percentile (+1 standard deviation). The inputs to calculate this
hypothetical event are shown in Table 14-1.
Table 14-1. Deterministic Input Parameters
CASE Crustal
Magnitude 6.5
RRUP (km) 100 (RJB=100)
VS30 (m/s) 760
Type of faulting Strike-slip
Dip (degrees) 90
Seismogenic Depth (km) 20
Width (km) 20
PGA(g) [percentile] 0.02[50th]
0.04[84th]
Ground Motion Prediction Equation [weight] BA08 [0.25]
CY08 [0.25]
CB08 [0.25]
AS08 [0.25]
Acronyms:
BA08= Boore and Atkinson 2008 (Boore & Atkinson, 2008)
CY08=Chiou and Youngs 2008; (Chiou & Youngs, 2008)
CB08=Campbell and Borzorgnia 2008 (Campbell & Bozorgnia, 2008)
AS08=Abrahamson and Silva 2008 (Abrahamson & Silva, 2008)
For comparison, the Shake Map for the 2002 Denali earthquake (Figure 14-1; USGS) was
reviewed and indicates the peak ground acceleration in Talkeetna were light and ranged between
about 0.09g to 0.18g. Based on the above analysis it is considered that the maximum RTS event
would expose the nearest town of Talkeetna to ground shaking substantially less than that
experienced during the 2002 Denali event.
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Figure 14-1. USGS Shake Map for 2002 Denali Earthquake (USGS)
14.1.1. Summary
The location and magnitude of any future RTS event associated with the Watana Reservoir are
highly uncertain. However, empirical data suggest that most RTS events will have relatively
small magnitudes and would most likely occur within 10 years of initial reservoir filling. From
these types of observations, ICOLD (2011) and Allen (1982) suggest that maximum RTS
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magnitudes may be on the order of 6.3 and 6.5, respectively. Others (USGS) have suggested
potentially a higher magnitude.
A more quantitative evaluation would require information on the fracture characteristics of
reservoir bedrock (density, orientation, length, etc.); more information on the state of stress with
respect to the orientation of existing fractures, as informed by local earthquake focal
mechanisms; and utilize a Mohr Coulomb approach to evaluating changes in crustal stress
associated with reservoir filling.
Additional details regarding RTS can be found in the technical memorandum on Preliminary
Reservoir Triggered Seismicity (MWH, 2013a).
14.2. Seismic Induced Landslide Potential
The consideration of seismically induced landslide hazard within the reservoir relates to the
potential for a large slide to occur and generate an impulse wave with the potential to overtop the
dam, and is considered to be very low, as wave generation of such a scale requires both a large
volume of material at, above, or within the reservoir, and rapid failure. For this to occur, a very
high, steep slope, with a potentially unstable block of large volume would need to exist adjacent
to or within the reservoir. High steep slopes above the reservoir are not present downstream of
PRM 200, approximately 16 miles upstream of the dam site. Upstream of PRM 200, large active
or historical landslides with the potential volume of material required to generate a large wave
were not observed. Upstream of approximately PRM 214, or about 30 miles upstream of the dam
site, the reservoir will be relatively shallow, narrow, and meandering within the confines of the
incised river valley. Waves generated by large slope movements along this reach would displace
significantly less water, and likely dissipate much of its energy before reaching the dam.
Additionally, most observed slope movements in the area are typically of small volume, and
occur as somewhat slower moving flows, which would not generate large impulse waves.
Submerged slopes comprised of fine grained sand and coarse silt materials may be prone to
liquefaction during earthquakes. Although these soil materials are not extensive in the reservoir,
liquefaction would likely be associated with shallow slides or individual slides of limited aerial
extent. In addition, with reservoir impoundment, some sloughing and shallow slides will occur
as the slopes adjust to the higher groundwater table, fluctuating reservoir levels, and thawing of
permafrost.
There exists a potential failure mechanism whereby a seismically induced landslide blocks the
river upstream of the reservoir, interrupting the flow of the river until the landslide debris is
overtopped and fails. However, examination of the river basin upstream of the reservoir
indicates few locations where significant blocking could occur, and in the event of a major
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seismic event, it would be important for any such blockage to be quickly breached to prevent a
damaging discharge.
No geomorphic evidence was found to indicate that large-scale, rapid movement landslides or
slope failures, either alone or in aggregate, that would cause a significant environmental or
operational impact post-impoundment of the reservoir to the Project. Additional details regarding
the reservoir slope stability can be found in the technical memorandum on Preliminary Reservoir
Slope Stability Assessment (MWH, 2014b)
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15. CONCLUSIONS AND RECOMMENDATIONS
This report presents the activities undertaken to quantify the seismic hazard at the proposed -
Watana Dam site. The results of several studies were used to compile this overall summary
report.
Seismic Hazard
The preliminary PSHA and seismic source model considers several seismogenic and potentially
seismogenic structures. These are: subduction-related sources (plate interface [megathrust], and
plate intraslab), the Denali fault, Castle Mountain fault, Pass Creek-Dutch Creek fault, Sonona
Creek fault, zones of distributed deformation north and south of the Denali fault, and Talkeetna
block structures (Fog Lake graben).
Four critical seismic sources are identified: (1) Subduction interface, (2) Intraslab, (3) Denali
fault, and, (4) Fog Lake graben. A deterministic evaluation for the southern Alaskan block
(SAB) central zone seismic source zone is derived from the 10,000-yr return period
deaggregation results from the PSHA.
In the current study, the deterministic evaluation finds that the intraslab source produces the
largest peak ground acceleration (PGA) at the site. The deterministic evaluation indicates that
the largest values of ground motions at the site are associated with the subduction interface and
intraslab sources, because of their large magnitude, relatively short distance, and GMPEs used
for these sources. The deterministic results for the crustal sources (e.g. Denali fault, Fog Lakes
graben, Castle Mountain fault, and 10,000-year crustal seismicity) indicate that these sources are
relatively less significant, as compared to subduction megathrust and intraslab seismic sources.
The results of detailed evaluations of new imagery data, evaluations of local and regional scale
mapping, and field investigations did not identify any specific features with evidence of late
Quaternary faulting within at least 40 km (~25 mi) of the Watana dam site. For most of this area,
the time and detection limits of the imagery and field investigations imply post-glacial time
limits of about 12,000 to 15,000 years, and detection of surface offsets of more than about 1 m
extending over several kilometers.
For the area near Watana dam site where detailed LiDAR data was the basis for this evaluation,
potential detection limits of surface fault displacements are much lower (about 0.5 m over
several hundred meters). This is consistent with the observations that the dam area is structurally
coherent with lack of pervasive penetrative deformation. Further, these data strongly suggest that
potential sources of primary or secondary, surface fault rupture at the dam site are absent.
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Geomorphic evaluations based on the detailed LiDAR data within the dam site area have not
identified any expression or continuity of potential faults or specific geologic features extending
from the site area that would be indicative of deformation of Quaternary deposits. This indicates
that although shear features may be present in the foundation, there is evidence to support a lack
of surface displacement along these features in the last 12,000 to 15,000 years. Furthermore, the
potential for any reactivation of the geologic features that might transect the dam footprint must
be considered extremely low given the following:
• The apparent lack of continuity and small scale of structural geologic features at the site
(shear zones) upon which surface fault rupture could conceivably take place;
• The dominant northwest-southeast trend of geologic features is unfavorably oriented with
respect to the contemporary tectonic stress regime, as the primary mode of tectonic
deformation appear to involve right-lateral strike slip structures with east-northeast
strikes;
• The absence of any nearby crustal scale fault structures and any neotectonic or
paleoseismic evidence of Quaternary faulting; and,
• The absence of Quaternary faults mapped with about 15 mi of the dam site
Shallow crustal deformation in the nearby region of Watana dam site appears to be characterized
by near-horizontal maximum compressive stresses oriented northwest-southeast, based on nearby
GPS vectors and earthquake focal mechanisms. Strain ellipse deformation concepts suggest that
the likelihood of reactivating northwest-oriented features under existing conditions is low
because of their near parallelism with compressive stress. Additional analysis would be needed
to evaluate if and how these features might respond to reservoir loading, fully-loaded reservoir
conditions, or fluctuating reservoir conditions.
With the installation of the Susitna-Watana Seismic Network, the seismic station density in the
region has increased. This has led to greater magnitude detection capabilities, a decrease in
magnitude of completeness, and greater location accuracy.
Focal mechanisms produced by the AEC in the Susitna-Watana project area indicate that the
crust around the proposed dam site is undergoing north-northwest south-southeast horizontal
compression, consistent with the relative Pacific – North America plate motion, with the
maximum horizontal stress rotating in a counterclockwise direction from east to west in the
network area. This appears to be consistent with what is known about the seismotectonic regime
in the project area (Haeussler, Saltus, Karl, & Ruppert, 2008).
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The seismicity analyses confirm earlier suggestions (e.g. (Ratchkovski, Wiemer, & Hansen,
2004)) that the subducting plate beneath southern and central Alaska consists of several segments
or sections with distinctly differing dips and orientations.
Source deaggregation plots are developed, one for each of the four spectral response periods
(PHA, 0.5 sec, 1.0 sec, 3.0 sec). Only sources contributing 5% or more at any ground motion
level are plotted on the de-aggregations. The peak horizontal acceleration (PHA) hazard is
dominated by the Alaskan subduction zone intraslab source at all return periods.
Sensitivity studies indicate hazard variations due to Mmax choices of 7.8 and 8.1 are significant.
For the two response periods, at a 10,000 year return period, ground motion increases of about
25% and 50% are indicated, respectively. However, due to the paucity of ground motion records
for magnitudes above 7.5, magnitude scaling of ground motions in the GMPEs above Mw 7.5 is
highly uncertain, and warrants further investigation.
Preliminary investigations into historical occurrence of the largest earthquake magnitudes for
worldwide subduction zones indicate that an upper bound value for future slab Mmax
distributions used in the final PSHA analysis is likely to lie above 7.5.
For final PSHA hazard calculations at the Watana dam site, further evaluation of the ASZ and
worldwide subduction zone data will need to be conducted to develop appropriate weighting of
uncertain parameters such as Mmax. The present evaluations confirm that the Mmax value of
Mw 7.5 used in the preliminary PSHA analyses must likely be considered a minimum estimate
for a Mmax distribution. Further evaluations are needed to assess the full range and weights for
larger Mmax estimates and to develop a basis for estimation of an appropriate characterization of
Mmax for the intraslab source for use in deterministic evaluations.
Ongoing study results such as the seismic network data readings, updates to the GMPEs, and
subduction zone intraslab source characterization, will (when completed) provide additional
information and should be incorporated into this report. In the interim a deterministic approach
was followed and design response spectra were recommended as follows:
MCE
• Interface 88th percentile, M9.2 at a rupture distance of 78 km, PGA=0.58g
• Intraslab 84th percentile, M7.5 at a hypocentral distance of 50km, PGA=0.69g
• Intraslab 69th percentile M8.0 at a hypocentral distance of 50km, PGA=0.81g
• Crustal 84th percentile M7.0 at a rupture distance of 3.5km, PGA=0.49g
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OBE
The development of the OBE followed a probabilistic approach and was assigned a return period
of 500 years, which has resulted in a projected PGA of 0.27g.
In addition to the seismic design criteria regarding earthquake events, other earthquake related
hazards such as fault rupture in the immediate vicinity of the dam; slope stability at the dam site
and within the project boundary; and RTS have been addressed.
Based on the lineament studies and the structural geological mapping – particularly of features
that transect the dam foundation - it is considered that the crustal stress pattern and feature
characteristics are not conducive to co-seismic movement on existing identified features in the
foundation or in the immediate area of the dam.
The potential for a large slide to occur and generate an impulse wave with the likelihood of
overtopping the dam is generally considered to be very low. RTS has been considered in the
derivation of the seismic design parameters for the Project, and will be further updated.
However, reservoir triggered seismicity requires the presence of a causative fault. A seismic
hazard assessment requires that all faults be identified; hence, any fault identified during the
seismic hazard assessment would likely cover those with the potential for RTS. In addition, the
Susitna-Watana micro seismic network will provide a baseline for RTS.
15.1. Recommendations to Update Existing Reports and Advance
Studies
The seismic hazard analysis (Fugro, 2012) and RTS assessment (MWH, 2013a) should be
updated with the results of the crustal seismic source assessment and use of most recent GMPEs.
The source characterization for the intraslab should be refined to establish a Mmax and
associated weighting, as the seismic hazard results are very sensitive to this parameter. Another
factor specific to the intraslab is that the dam site lies above the McKinley Block which may be
detached and moving independently placing limits on the available physical volume and in turn
limiting the maximum size earthquake it can support. The seismic hazard analysis (Fugro, 2012)
should also include these updates to the interface characterization.
Additional field work could improve preliminary studies, such as the crustal seismic source
evaluation by expanding the exposures of rock conditions across the foundation footprint. A
shear wave velocity study based on the alignment of the dam should be performed to refine the
input VS30 parameter used in the ground motion calculations for the seismic hazard analysis.
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The ongoing monitoring of the Susitna-Watana network should be continued to help clarify the
recurrence relationships, subduction zone geometry, stress field, and the background seismic
level to help judge RTS during reservoir filling.
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