HomeMy WebLinkAboutEA1759_Induced_Seismicity_ReportFinal Report
Evaluation of the Environmental Impacts of
Induced Seismicity at the Naknek
Geothermal Energy Project, Naknek, Alaska
Prepared for
ASRC Energy Services Alaska, Inc.
Regulatory & Technical Services
2700 Gambell Street, Suite 200
Anchorage, AK 99503
12 May 2010
Prepared by
Michael Andrew Hasting
Seismic Consultant
648 La Paloma
Ridgecrest, CA 93555
Evaluation of the Environmental Impacts of
Induced Seismicity at the Naknek Geothermal Energy Project Naknek, Alaska
DOE/EA-1759 1 May 2010
Table of Contents
SECTION ONE Introduction ............................................................................................................ 4
1.1 Scope of Work ............................................................................................................................... 4
SECTION TWO Geological/Seismotectonic Setting and Faults ............................................................... 5
2.1 Geological/Seismotectonic Setting ............................................................................................... 5
2.2 Faults ............................................................................................................................................. 9
SECTION THREE Alaska Historical Seismicity ................................................................................ 11
3.1 Overview ..................................................................................................................................... 11
3.2 Large Historical Alaska Events ..................................................................................................... 12
3.3 Regional Events ........................................................................................................................... 13
3.4 Local Seismic Events .................................................................................................................... 17
SECTION FOUR Induced Seismicity ............................................................................................. 20
4.1 Causative Mechanisms ................................................................................................................ 20
4.2 Predicted Induced Seismicity at the NGEP Site ........................................................................... 20
4.3 Predicted the Maximum Earthquake at the NGEP Site .............................................................. 21
SECTION FIVE Estimating Ground Shaking From Induced Seismicity ....................................... 24
SECTION SIX Impact of the NGEP Operations to Local Community ....................................... 29
SECTION SEVEN References ......................................................................................................... 32
List of Tables: Page
Table 1, Historical and Preliminary Data from the USGS NEIC PDE for a 20km
circular search around the NGEP, 1973 to Present. 19
Table 2, Perceived Shaking vs. Instrumental Intensity as related to PGA 24
Table 3, MMI Scale 25
Table 4, Wills et al Soils Classifications 27
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List of Figures: Page
Figure 1, Geological Map of the Region (Ellis 2009) 6
Figure 2, Geological Map of Local Area (Ellis, 2009) 7
Figure 3, Cross section from Figure 2, A to A’ view looking north
east (Ellis, 2009) 7
Figure 4, Location of Geophysical Line, solid red lines, Interpreted Fault or
Dike, red, dashed Line 8
Figure 5, Plot of all seismic events occurring in Alaska since 1990 color coded
to depth. This plot is from the USGS National Earthquake Information Center
(NEIC) website in Golden CO 11
Figure 6, Magnitude 6.0 and Higher, 1899 to present, data from the USGS
NEIC event catalog 12
Figure 7, Generalized cross section of a volcanic arc system associated with
a subduction zone, image from USGS 13
Figure 8, 250km ring of events, data from the USGS NEIC and AEIC
Online Seismic Catalogues 14
Figure 9, Zoomed area from Figure 7 15
Figure 10, Oblique 3D view of Figure 9, view looking northeast 15
Figure 11, 100km ring events, data from the USGS NEIC and AEIC Online
Seismic Catalogues 16
Figure 12, Oblique 3D image of Figure 11, view looking northeast 16
Figure 13, 20km ring around the NGEP site, white circle, event locations are
the red circles, data from the USGS NEIC and AEIC Online Seismic
Catalogues 18
Figure 14, NEIC Stations in Alaska 18
Figure 15, Naknek MEQ Network Layout 21
Figure 16, Shake Map Results for a ML3.7 near Anchorage, 23 April 2010 24
Figure 17, Estimation of PGA for NGEP site using OpenSHA, Shaded area
show MMI values, red arrows and line show approximate distance to key points 27
Figure 18, Estimated MMI Zones relative to a ML3.5 earthquake based on
OpenSHA results for the NGEP Site 28
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Acronyms and Abbreviations
μ Rock Rigidity
AES AK ASRC Energy Services Alaska, Inc.
AEIC Alaska Earthquake Information Center
EGS Enhanced Geothermal System
gpm Gallons per minute
ka Thousand of years ago
Ma Million of years ago
Mb Body wave Magnitude
Md Coda Magnitude
ML Richter Local Earthquake Magnitude
MO Seismic Moment
MT Magnetotellurics
MW Moment Magnitude Scale
MEQ Micro-Earthquake
MMI Modified Mercalli Intensity Scale
NEA Naknek Energy Association
NEIC National Earthquake Information Center
NEPA National Environmental Policy Act
NGEP Naknek Geothermal Energy Project
PDE Preliminary Determination of Epicenters
PGA Peak Ground Acceleration
PGV Peak Ground Velocity
PSHA Probabilistic Seismic Hazard Analysis
USGS United States Geologic Survey
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1.0 Introduction
At the request of ASRC Energy Services Alaska, Inc, Regulatory & Technical Services (AES
AK), the following report presents the evaluation for the environmental impact due to induced
seismicity of an Enhanced Geothermal System (EGS) near King Salmon, Alaska. This study is
part of Environmental Assessment (DOE/EA-1759) being prepared by the U.S. Department of
Energy, in accordance with the National Environmental Policy Act (NEPA) of 1969. The
potential for induced seismicity at EGS sites has been identified as a possible environmental
impact and as such this report presents the Naknek Geothermal Energy Project (NGEP) area and
the possible impact of induced seismicity from EGS operations to the surrounding area.
The NGEP site is a new geothermal prospect and as such little is known about the induced
seismicity in the region and the potential risks associated with an EGS project in this region.
Also, due to the poor seismic station coverage of the area the seismic catalogues for the region
around the NGEP site are incomplete, and as such make it hard to determine the true local
seismicity and seismic risk for the region.
For this report to determine the potential for induced seismicity due to injection testing of the
Naknek G1, G2 or G3 wells, here after referred to as “the NGEP site”, the geological setting of
the area, known faults, and the seismicity catalogues from both the USGS, and Alaska
Earthquake Information Center (AEIC) event catalogues were collected. This report applies
known equations to calculate the maximum expected event during the injections testing at the
NGEP site and related this to ground shaking in the region and their impact to local residents and
key structures in the area.
1.1 Scope of Work
The following tasks have been performed as part of this study:
Task 1. Review of available geological and seismic data relevant to the NGEP site.
Task 2. Evaluation of the historic and regional seismicity of the region at various distance
and magnitude ranges.
Task 3. Estimate Attenuation Models for the NGEP Site
Task 4. Assess the potential for local resident disturbance and property damage from ground
shaking as a result of induced seismicity.
Task 5. Prepare a final report describing the results of these analyses.
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2.0 Geological/Seismotectonic Setting and Faults
The following section describes the geological/seismotectonic setting of the region and known
faults near the well sites and surrounding region. Much of the geological information is from the
Ellis (2009) report provided to NEA.
2.1 Geological/Seismotectonic Setting
There are several tectonic elements present in this region that form the basis of the geologic
framework around the NGEP site. The Bristol Bay basin lies in a back-arc tectonic setting
bounded on the south by the Alaska Peninsula, and associated active volcanic arc, the result of
the active subduction along the northeast-trending Aleutian trench (Ellis, 2009), Figure 1. In the
Naknek lake area, the geology mapped immediately north and east of the moraine covering the
project area are Meshik age mafic volcanics. Along the east side of Naknek Lake, they are sitting
unconformably on lower Jurassic Talkeetna formation sediments, and volcaniclastics. The
Talkeetna is considered the basement along with some older Triassic metamorphics. The
basement has been intruded extensively by mid-Jurassic granitic bodies and locally by mid-
Tertiary granitic bodies.
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Figure 1, Geological Map of the Region (Ellis 2009)
In the Ugashik Lake area, ~150km south of Naknek Lake area, various seismic studies and
boreholes document an approximate 3 km thick sub-basin that is flanked on the south by a thick
Meshik volcanic center (Decker et al, 2008). The sub-basin is filled with marine to non-marine
locally coaly sediments of the Stepovak Formation (Meshik volcanics age-equivalent) sitting on
Jurassic or older crystalline basement overlain by younger Bear Lake and Milky River
Formation, Figures 2 and 3.
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Figure 2, Geological Map of Local Area (Ellis, 2009)
Figure 3, Cross section from Figure 2, A to A’ view looking northeast (Ellis, 2009)
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North-vectored convergence throughout the Tertiary has created a series of northwest fault zones
that are perpendicular to the arc that have accommodated differential subsidence. These zones
have created sub-basins and Eocene-Oligicene (Meshik age) volcanic highs. A series of
northwest trending fault structures and very young Holocene dikes are present south of a sharp
bend in the Bruin Bay Fault in the Valley of Ten Thousand Smokes (the center of the famous
eruptive event at the beginning of the last century). This northwest trend strikes into the Pikes
Ridge study area and is strikingly analogous to the Ugashik Lake study area that predicted the
basinal development to the south and uplift to the north at a northwest bend in the Bruin Bay
fault complete with a recent volcano. Ellis (2009) argues that a northwest trending bounding
structure has an uplifted basement on the south and down-dropped and preserved the Meshik
volcanics to the north. That interpretation is corroborated by the apparent break in the
geophysical signature along MT line 4 at ~ 9500N which can be interpreted as bringing the
basement up on the south end of the line, Figure 4.
Figure 4, Location of Geophysical Line, solid red lines, Interpreted Fault or Dike, red, dashed
Line.
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The Meshek volcanics exhibit a noisy magnetic signature and have variable conductivity. They
appear to thicken from 0 on line 3 & 4 to ~ 1 km thick and dip gently to the south and east.
Several conductive breaks are present on line 4 that could be interpreted as northwest trending
block faults with a thickening basin (Stepovak sediments below Meshik volcanics) developing to
the north of 9500 on line 4, Figure 4.
There is another distinctive north-northwest trending feature observed on several of the
geophysical lines, Figure 4. It is a distinctive 300-400m wide, magnetic, shallow (<200m)
resistive dike-like feature that extends through the entire survey area. At 5000W on line 5 it
domes the conductive unit, on line 2 it is at 5000NW, and on line 4 it is at 12,400N.
2.2 Faults
There are two major northeast-trending bounding fault zones in the region, the Bruin Bay and the
Lake Clark fault systems, Figure 1. The Bruin Bay fault is located along the crest of the Aleutian
Range to the southeast, and is the most seismically active, while the Lake Clark fault system is
located to the northwest. Both of these regional faults converge in upper Cook Inlet where they
are named the Castle Mountain fault system.
The Bruin Bay fault represents a major, long-lived, tectonic boundary that has accommodated
crustal-scale stresses generated by the subduction of oceanic lithosphere beneath the continental
North American plate since Middle to Late Jurassic time (Gillis, R. et al., 2008). The Bruin Bay
fault exerted a fundamental control of the forearc basin structure of the region (Gillis, R. et al.,
2008). This fault extends along the west side of the Cook Inlet from near Mount Susitna to the
northeast and extends southwest to Becharof Lake, about 515 km. The sense of displacement is
reverse and bedded rocks in the downthrown block which are commonly upturned against the
fault. Less than 10 km of sinistral displacement is inferred along several places of the fault
(Detterman and Hartsocks, 1966). A review of the seismic catalogues, to be discussed later in
this report, shows this is an active fault and produces thousands of earthquakes per year.
The Lake Clark fault is a graben-like extensional structure ~225 km long striking northeast. It is
predominately a reverse fault that is along the strike from the Castle Mountain fault and is
located on the northwest side of the Cook Inlet (Detterman et al., 1976a). Using aeromagnetic
data Haeussler and Saltus (2004) show up to 26 km of right-lateral displacement has occurred
along the Lake Clark fault in the past 34-39 Ma. The Bruin Bay fault system has as much as 3
km reverse motion (up-to-northwest) and 19 km of left-lateral displacement along it. The age of
displacement is mainly Late Tertiary based on an offset of the 38 Ma old quartz monzonite near
Lake Clark and juxtaposition of Miocene strata against Eocene beds on the Chuitna River;
however, there is no evidence of Holocene movement (Dettermann and Hartsocks, 1966).
Detterman et al. (1976b) estimated the northwest-side-up vertical offset on the fault as 500–1000
m on the basis of the juxtaposition of the late Miocene Beluga Formation with the early Eocene
West Foreland Formation west of Tyonek. Plafker et al. (1975) found no evidence for vertical
Quaternary movement, but found 561 km of net post–late Eocene dextral slip. In contrast,
Schmoll and Yehle (1987) reported some evidence for Pleistocene, but not Holocene, movement
on the fault. Schmoll and Yehle (1987) referred to a 6-km-long to 25- m-tall, south-facing scarp
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along the Lake Clark fault as the Lone Ridge fault. They found that it appears to offset three
Quaternary moraines in a northwest-side-up sense, but the youngest moraine, dated as ca. 15 ka,
was not offset. A review of the seismic catalogues, to be discussed later in this report, shows this
is a relatively inactive fault over the past few decades but still should be considered an active
fault.
Ellis (2009) also shows a possible northwest to southeast trending fault, or dike, that crosses the
local area just north of the NGEP site, approximately 500m, and terminating at the Lake Clark
Fault Zone, Figures 1 through 4. This possible fault is interpreted from magnetic data, and does
not show any surface manifestations. Since this fault does not cross the Lake Clark Fault Zone it
may be an old, non-active, fault that has been offset by the Lake Clark Fault but could be of
concern during the stimulation process due to its proximity to the NGEP.
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3.0 Alaska Historical Seismicity
This section covers the background seismicity for the area dating back to 1890’s up to the
present. This report compiled data from both the USGS National Earthquake Information Center
(NEIC) and the Alaska Earthquake Information Center (AEIC) catalogues and used Google
Earth Pro to plot these events.
3.1 Overview
Alaska is the most seismically active state in the USA and the site of the second-largest
earthquake ever recorded, the 1964 magnitude Mw9.2 earthquake in Prince William Sound. This
earthquake killed 132 people, more than the recent Loma Prieta and Northridge earthquakes
combined, and generated a tsunami that killed people as far south as California. The subduction
of the Pacific Plate created both the string of volcanoes known as the Aleutian mountain range
and the associated Aleutian trench. The collision of the Pacific plate with Alaska produces over
20,000 locatable earthquakes in Alaska each year that are recorded and located by the AEIC.
However the area around the NGEP site is poorly covered by the Alaska Seismic Network and as
such the catalog of seismic events for this area is minimal. The minimum magnitude threshold
for the area is about a ML2.5 with a location and accuracy of about 10 km. Figure 5 shows a
plot of all seismic activity along the Aleutian area centered on the NGEP site since 1990. Figure
5 shows that deeper events occur continuously to the north-west where the Pacific Plate is
subducting beneath Alaska.
Figure 5, Plot of all seismic events occurring in Alaska since 1990 color coded to depth. This
plot is from the USGS National Earthquake Information Center (NEIC) website in Golden CO.
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3.2 Large Historical Alaska Events
Besides the 1964 Prince William Sound event there have been over 280 earthquakes registering a
Magnitude 6 or higher since 1899 recorded within 1000 km of the NGEP site. Figure 6 is a plot
of these events and shows that most of these events are occurring along the subduction zone;
those events that occur inland take place at great depth along the subducting slab. Figure 7
shows a generalized cross section view of a volcanic arc system associated with a subduction
zone and the development of the various features associated with such plate boundaries.
Figure 6, Magnitude 6.0 and Higher, 1899 to present, data from the USGS NEIC event catalog.
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Figure 7, Generalized cross section of a volcanic arc system associated with a subduction zone,
image from USGS, http://earthquake.usgs.gov/learn/glossary/?term=forearc
3.3 Regional Events
Regional events examined in this study were collected using a circular search pattern out to 250
km from the NGEP site; over 5500 events of ML >2.5 occurred since 1970 with over 300 events
taking place within 100 km of the NGEP site. Most, if not all, of the events occurring within the
100 km radius of the NGEP site have hypocentral depths of over 150 km, indicating they are
occurring in the subduction zone. Figures 8, 9 and 11show plots of these events as located by
various seismic networks and reported to the USGS. Also provided are 3D views, Figures 10
and 12 are 3D views of the data showing the depth of seismic activity under the NGEP study
area. Views are looking obliquely towards the northeast along strike of the subduction zone.
Many of the events in Figures 8, 9 and 11 occur south of the NGEP site under Becharof Lake,
approximately 90 km away. The largest seismic event in the Becharof Lake area was a ML5.4
that occurred in 1998; there were over 18 events with a magnitude ML4.0 or larger in this area
between May 9th and June 4th. Figures 10 and 12 shows that many of these events have a
reported depth 0 to 1 km depth, however the location quality for these events is reported in the C
and D categories where A is the best location error, i.e. less than ~500m location error. As such
while the events did occur the depths and locations can be off by several kilometers. This 1998
swarm was associated with magma intrusion beneath Mount Peulik, detected via InSAR, which
may have triggered the swarm (Lu et al., 2002).
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Figure 8, 250km ring of events, data from the USGS NEIC and AEIC Online Seismic
Catalogues. Blue Dots = NGEP, King Salmon and Naknek
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Figure 9, Zoomed area from Figure 8.
Figure 10, Oblique 3D view of Figure 9, view looking northeast, along strike of plate boundary.
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Figure 11, 100km ring events, data from the USGS NEIC and AEIC Online Seismic Catalogues.
Figure 12, Oblique 3D image of Figure 11, view looking northeast along strike of plate
boundary.
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3.4 Local Seismic Events
Between 1973 and April 20th 2010 only 12 events occurred within 20km of the NGEP site,
Figure 13. All of these events had a focal depth of over 150km and a maximum magnitude of
ML4.6 which occurred on the 19th of November 1984. Table 1 is a list of these events as
catalogued by the NEIC Preliminary Determination of Epicenters (PDE) database. Most of these
events were felt by the local communities of Naknek, South Naknek and King Salmon and
according to local people caused little to no damage (personal conversation with locals). Again,
it should be noted that due to the poor seismic station coverage in this region the locations of the
events plotted in Figure 13 can be off by tens of kilometers. This location uncertainty is
primarily due to having only a few seismic phase stations to locate the event since most of the
stations being used to locate the events are east of the NGEP site.
Figure 14 shows the location of all AEIC seismic stations in Alaska. The closest seismic stations
are located approximately 70km east of the NGEP site in the Katmai National Park along the
Bruin Bay fault. Because of the large station spacing the program uses to locate earthquakes,
large correction factors are applied and the system can only locate seismic events in the NGEP
area to within tens of kilometers with any certainty. Depending on the velocity model used by
the program, events can also have a large station residual, or error, due to the uncertainty in the
velocity structure of the region and distance between the stations. Since there are few stations in
the region, and seismic signals decay as a function of 1/r3, more events less than ML3.0 may have
occurred in the region that were not detected by the AEIC seismic network and as such are not in
the catalogue. Based on a review of the catalogues of the seismic data set for the region around
the NGEP site the “completeness” for earthquakes greater than aML3.0 in size, can be seen in
Table 1, as compared to ML2.0 near the Bruin Bay fault in Katmai National Park. Therefore, the
events as plotted in Figure 13 may not represent a complete list of the actual seismicity rate for
events less than ML3.0 in the area around the NGEP site.
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Figure 13, 20km ring around the NGEP site, white circle, event locations are the red circles, data
from the USGS NEIC and AEIC Online Seismic Catalogues.
Figure 14, All NEIC seismic stations in Alaska
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NEIC: Earthquake Search Result
U. S. G E O L O G I C A L S U R V E Y
E A R T H Q U A K E D A T A B A S E
FILE CREATED: Fri Apr 23 23:03:20 2010
Circle Search Earthquakes= 12
Circle Center Point Latitude: 58.699N Longitude: 156.504W
Radius: 20.000 km
Catalog Used: PDE
Data Selection: Historical & Preliminary Data
CAT YEAR MO DA ORIG TIME LAT LONG DEP MAGNITUDE IEM DTSVNWG DIST
NFO km
TF
PDE 1984 11 19 004427.22 58.57 -156.70 205 4.6 mbGS 2F. ....... 18
PDE 1990 06 12 134717.11 58.71 -156.26 204 3.6 mbGS ... ....... 14
PDE 1995 01 08 025320.65 58.72 -156.25 201 ... ....... 14
PDE 1995 06 19 144432.35 58.57 -156.62 192 3.9 mbGS ... ....... 15
PDE 1995 10 29 024009.76 58.68 -156.17 175 ... ....... 19
PDE 1996 07 11 145427.50 58.67 -156.53 180 ... ....... 3
PDE 1998 11 12 200934.94 58.69 -156.72 202 ... ....... 12
PDE 2001 03 24 033147.53 58.57 -156.34 172 3.0 mbGS ... ....... 17
PDE 2004 01 13 215554.69 58.66 -156.66 198 4.2 mbGS ... ....... 10
PDE 2004 10 19 064129.07 58.69 -156.45 195 ... ....... 3
PDE 2006 03 25 093810.78 58.76 -156.42 191 4.2 mbGS ... ....... 8
PDE 2007 10 31 103758.37 58.68 -156.39 212 3.2 UKAEIC ... ....... 6
Table 1, Historical and Preliminary Data from the USGS NEIC PDE for a 20km circular search
around the NGEP site, 1973 to Present. Note that not all events have a Magnitude assigned,
indicating these were poorly located. Far Right Column is the approximate epicentral distance
from the actual NGEP location.
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4.0 Induced Seismicity
As noted in the Introduction, the Naknek Geothermal Project is the first geothermal well to be
drilled in this part of the World. As such there are no records of Induced Seismicity in this
region. No history can be presented at this time, therefore this report reviews Causative
Mechanisms of induced seismicity, as they relate to geothermal wells, the “Predicted” induced
seismicity, and the maximum earthquake size for the Naknek Geothermal Energy Project.
4.1 Causative Mechanisms
Greensfelder and Parsons (1996) concluded that there are multiple causes of induced seismicity
in a geothermal field. They involve both increases and decreases in the reservoir rock strength
caused by the changes in confining pressure, i.e. the normal stress across cracks, or in the
coefficient of friction. These increases and decreases would be due to the injection of fluids into
the fracture system under pressure that can weaken the rock and apply differential stress in local
areas.
According to Greensfelder and Parson (1996), during steam withdrawal, induced seismicity may
be caused by an increase in rock strength and then a decrease in rock strength during water
injection. These seem contradictory but appear to operate independently over distinct reservoir
volumes out to about 1km from a well (Greensfelder and Parson, 1996). One can assume that
during fluid withdrawal fractures are closed and the rock is strengthened due to the increase in
the friction along preexisting fractures. These fractures can support higher stress levels which
would then lead to a seismic event upon failure or slip. During fluid injection fractures can be
lubricated and as such cannot support high stress levels and slip much easier, which may not be
recorded by the seismic network or may even slip aseismically. However, Rutquist and
Oldenburg (2007) point out that the most important cause for injection-induced seismicity is due
to the cooling and the associated thermal-elastic shrinkage that alters the rock’s stress state
causing mechanical failure of the rock. The cooling of the local rock causes shrinkage due to the
thermal coefficient of expansion of the rock, thus inducing an unloading and a loss of shear
strength of the rock. Rutquist and Oldenburt (2007) modeling for The Geysers shows an
agreement with observations that most of the injection seismicity occurs near injection and
production wells and can spread several kilometers below injection wells. The deeper seismicity
may be due to both thermal-elastic cooling and increased pore pressure in the rock at depth.
4.2 Predicted Induced Seismicity at the NGEP Site
If injection or stimulation is conducted at the NGEP site, it would be difficult to estimate the
level of induced seismicity since the physical properties and the state of current stress levels in
the rock at depth are unknown. Currently a temporary ten station micro-earthquake (MEQ)
network has been deployed around the NGEP site to record the “background” seismicity in the
region with much greater resolution than the AEIC network currently provides. Figure 15 shows
the layout of this temporary network around the NGEP site. The MEQ network should provide a
detection threshold level of about ML0.5 for the NGEP area. This network will be deployed for
up to six months to evaluate the baseline level of seismic activity around the NGEP site, and to
measure the naturally occurring background seismicity prior to any injection testing. This
network would also help to determine if any induced seismic events are occurring during the
drilling and/or cementing process of the casing, as was seen in Basel, and most recently at the
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Paralana-2 borehole in South Australia. Based on the results of the background study, estimating
the probability of induced seismic activity during well stimulation would be more accurate.
Other factors that can influence the rate of induced seismic activity are the injections rates and
pressures. Greensfelder (2003) estimated that injection rates at 480 to 950 gallons per min (gpm)
would induce earthquake activity in The Geysers. Since the G1 well has just been completed,
and G2 and G3 wells still need to be drilled, NEA is not able to provide estimations as to the
injection rates at the writing of this report but initial indications may be that injections rates
could be as high as 500 to 750 gpm. While the NGEP site is in a different tectonic setting and
the rock type are different it would still be expected that the NGEP site will induce seismic
events during injection tests at any of the wells but the rate of seismicity would be at a different
rate as that of The Geysers.
Figure 15, Naknek MEQ Network Layout, station layout is based on access to the sites both
during the winter and summer months when access to the sites is very limited.
4.3 Predicted the Maximum Earthquake at the NGEP Site
A stimulation plan has yet to be worked out for the NGEP site, and neither the current in situ
stress state of the reservoir rock at depth nor the existence of faults, or micro-faults, at depth are
yet known. However, by applying Brune’s (1968) formula where the seismic moment released
during an earthquake must equal the rigidity of rock times the length, width and displacement
along a fault, an estimate of maximum earthquake size generated by EGS stimulation at the
NGEP site can be estimated as follows.
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MO = μ (L*W*D)
Assuming that the length, width and displacement must equal the injected volume of fluid then
we can replace the L * W * D with the injected Volume (VI):
MO = μ VI
If a fracture with a radius of 500m, similar to the Basel and Cooper Basin rupture areas, is
created with an average opening of 1cm along the entire fracture surface area a volume change of
about 7,900 cubic-meters, or about 2.1 million gallons of injection volume, would be required.
Assuming a rigidity of rock at 3x1011dyne/cm2, a typical value for a hard rock at 5 to 10 km
depth, a “Total” seismic moment release of 2.356 x 1021 dyne-cm would be needed. If we use
more conservative rigidity estimations, i.e. 2 x 1011dyne/cm2 or 1 x 1011dyne/cm2 which are more
typical for the rock at the depth of stimulation at the NGEP site, the seismic moment would be
much lower. However for this report the estimation on seismic moment is based on a worse case
calculation and as such the larger value is used for the calculations below.
To convert the seismic moment to a “Magnitude” scale there are several to choose from, ML, Mb,
Md, MS and MW and all vary in size but can be related to the seismic moment. The most
common scales used by seismologist today are the ML and MW scales, local Richter and moment
magnitude respectively. To relate the total seismic moment to a local Richter magnitude, ML,
developed in 1935 by Drs Charles Richter and Beno Gutenberg, and is good for local
earthquakes out to distances of about 150km we can use the Wyss and Brune (1968) formula:
log MO = 1.4 ML + 17.0
or
ML = ((log MO)-17.0)/1.4
This formula yields a local Richter magnitude of ML3.1 for the above estimated seismic moment.
Over the past few years, as instrumentation for recording earthquakes has advanced, the USGS
has adopted a newer relationship for reporting the magnitude of an earthquake that is more
applicable to estimating the magnitude and is based on the recorded seismic moment. This new
formula, MW =2/3 log(MO) -10.7, where MW is the “moment magnitude”, is based on work done
by Hanks and Kanamori (1979) and yields higher magnitude numbers than the ML scale for the
same seismic moment for the Brune formula. From the above estimation of seismic moment we
can estimate the moment magnitude, MW or M, as follows:
M = (log 2.356 x 1021 dyne-cm)/1.5-10.7
M = 3.7
It should be pointed out that this estimation is for the “entire” seismic moment released during
the fracturing process. It is highly unlikely that a single rupture of this size would take place and
that it is more likely, as has been seen in the past, that there would be thousands of micro-
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earthquakes to accommodate the total seismic moment that would equal the M3.7 estimated
above. Based on the Brune (1968) and Hanks and Kanamori (1979) formulas, even if the
injection volume is doubled, thereby doubling the rupture area or opening, the NGEP site would
only generate a MW4.1 size event.
As a single large event is not expected to occur but rather thousands of smaller ones, these MEQ
events are likely to occur over many days during the injection testing and even for some weeks
after the injection testing has stopped while the shut in pressure bleeds off. For example, the
Basel injection test recorded over 20,000 seismic events and over 35,000 for the Cooper Basin
injection testing. As discussed in Section 5, most of these events would be too small to be felt at
the surface or in the local communities of King Salmon, Naknek and South Naknek.
Given these assumptions and estimations above, a maximum “creditable” event size during any
injection testing at the NGEP site should be on the order of a M3.5 to M3.7 in size. Though it is
possible that a larger event could be triggered on an adjacent fault that is near failure already,
there is no conclusive evidence at this time that a fault exists near the site. The only indication of
a fault near the NGEP site is from the geophysical data which has not been conclusively
determined to be a fault or a fault under current stress. While drilling logs indicate fractures at
injection depth, faults or fault zones have not been identified as of the writing of this report. The
nearest known fault is the Lake Clark fault, which is over ten kilometers from the NGEP area,
and it would be unlikely that a M3.7 event could trigger an earthquake on this fault.
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5.0 Estimating Ground Shaking From Induced Seismicity
For any EGS system the most significant environmental impact from induced seismicity would
be from ground shaking. Ground motions are normally recorded by seismometer in either
velocity or acceleration units and can either be differentiated or integrated to go between units, or
even to displacement. For ground shaking studies the most common unit of measure by
engineers is the peak horizontal ground accelerations (PGA) but the peak ground velocity (PGV)
is also commonly used. Both the PGA and PGV can be correlated to shaking or intensity using
the classifications of Wald et al (1999). They have classified the following levels of ground
shaking, Table 2, and the levels are used as part of the USGS ShakeMaps generated for all
earthquakes recorded by the NEIC in the United States. Figure 16 shows an example shake map
result for a MW 3.7 event near Anchorage, Alaska, similar in size to the maximum expected event
size for the NGEP site.
Table 2, Perceived Shaking vs. Instrumental Intensity as related to PGA, Caltech
Figure 16, Shake Map Results for a MW3.7 near Anchorage, 23 April 2010.
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While the Richter Magnitude, PGA and PGV values are good for seismologists to work with, the
Modified Mercalli Intensity (MMI) scale is a descriptive scale that tries to relate the level of
ground shaking to property damage and is more applicable to the general population. As the
effects of any one earthquake can vary greatly from place to place the MMI scale tries to
normalize these effects based on the following relatively subjective scale of descriptions:
Modified Mercalli Intensity Scale
I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall
buildings.
III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors
might not realize that an earthquake is occurring.
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors
rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel
movement. Parked cars rock.
V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close.
Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees
might shake. Liquids might spill out of open containers.
VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures
fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is
slight in poorly built buildings. No structural damage.
VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks.
Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings;
considerable in poorly built buildings.
VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their
foundations. Tall structures such as towers and chimneys might twist and fall. Well-built
buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches
break. Hillsides might crack if the ground is wet. Water levels in wells might change.
IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off
their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer
serious damage.
X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are
seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers,
lakes. The ground cracks in large areas. Railroad tracks are bent slightly.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground.
Underground pipelines are destroyed. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves
or ripples. Large amounts of rock may move.
Table 3, MMI Scale
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5.1 Seismic Attenuation
The actual relationship between any single ground motion parameter and intensity are highly
uncertain, as there are many variables to consider. Some of the variables include the type of
structures, soil conditions, depth to bedrock, type and the height of the structure to name a few.
Where numerous strong ground motion data are available empirical attenuation relationships can
be developed to help model a site’s response to an earthquake. By applying statistical regression
methods to the strong motion data, an attenuations model can be developed for a given region,
though it is only applicable to that given region. As no strong ground motion sensors are located
in the study area, the values to assign to the region are hard to estimate.
Traditionally in predicting ground shaking at a site you would use the attenuation relationship
derived from the empirical relationship mentioned above. Attenuation is defined as the decrease
in amplitude, or intensity, of the seismic wave over a distance. The decrease in amplitude is the
result of numerous factors, such as geometrical spreading, damping by the earth, scattering and
reflecting off of structural layers as well as refraction, diffraction and wave conversion. Given
the soil conditions, moraine and saturated soils, and depth to bedrock, ~150 meters, an elevated
level of ground motion for the local area would be expected as compared to a hard rock/bedrock
site. Section 5.2 is presented to estimate the level of ground motion for the area around the
NGEP site.
To help record actual strong motion data, during the injection/stimulation study at the NGEP site
it has been proposed that at least two strong motion stations be set up in the region to help
develop the attenuation model for the area. Additionally, it may be possible after the six month
MEQ deployment to develop a model within the footprint of the MEQ array, which could be
used as estimations for the local communities, assuming enough natural seismic data is recorded.
5.2 Estimation of PGA and MMI for the NGEP Site
As mentioned in section 5.1 no strong motion data is available for the NGEP area. As such only
estimations of ground motions can be made at this time. While there are standard formulas for
estimating ground motion Field et al. (2003) have provided a JAVA based program for
estimating ground motion using several models and methods, e.g. OpenSHA. For this report we
look at only the USGS ShakeMap 2003 model for estimating PGA and the MMI from the NGEP
area out to a distance of 50 km. For the modeling parameter a worse case soil condition, C, is
used for the area based on Wills et al. (2000) soils classifications, Table 4. For these calculations
magnitudes between MW2.5 and MW4.0 were used to estimate the ground shaking in both PGA
and MMI units. The results from the OpenSHA program are plotted in Figures 17 and 18.
Figure 17 shows the PGA vs. Distance while Figure 18 overlays the results for a MW3.5 event
onto a Google Earth image for the area. Note in Figure 17 that even at the Lake Camp site, the
closest facility to the NGEP site, a PGA of only 3% of g is estimated for MW4.0 size event,
which equates to a MMI of only IV, by King Salmon the MMI is only III and at Naknek an II.
Table 4 is a description of the MMI scale as published by the U.S. Federal Emergency
Management Agency (FEMA).
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Table 4, Wills et al Soils Classifications
Figure 17, Estimation of PGA for NGEP site using OpenSHA, Shaded area show MMI values,
red arrows and line show approximate distance to key points.
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Figure 18, Estimated MMI Zones relative to a MW3.5 earthquake based on OpenSHA results for
the NGEP Site
While these results are only estimations they help provide a basic overview of what to expect
from induced seismicity at the NGEP site.
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6.0 Impact of the NGEP Operations to Local Community
Any injection testing, or stimulation, at the NGEP site may result in a large induced seismic
event that could be felt by the local communities around the area, as has been seen at other such
sites around the world. For example, a ML3.4 occurred in Basel (Switzerland), at the Cooper
Basin (Australia) events up to ML3.7 were recorded, at the Soultz-sous-Forêts (France) EGS
development a ML2.9 event was recorded and at Rosemanowes (UK) an observed magnitude of
ML3.1 was recorded though ML3.5 size events had been predicted (Bromley and Mongillo,
2008). All these event sizes are within the predicted estimation for the NGEP site.
The largest “Local” earthquakes that have been recorded in the region over the past 35 years
were the MW6.8, which occurred on the 28th of July 2001 at a distance of 85km from the NGEP
and the MW6.3 occurring on the 1st of May 1990 at a distance of only 25 km, neither of which
caused any significant damage to the local infrastructure. There were also several magnitude
MW5.0 to MW5.9 size events during this timeframe as well. It should be pointed out that while
the Richter Scale is a base 10 log scale for one order of magnitude increase there is an increase
of ~33.3 times in the energy released during an earthquake. Given this the MW6.8 and MW6.3
events each respectively released over 350,000 and 30,000 times more energy than is expected
for an induced seismic event at the G1 site of MW3.5 in size or that has been seen at other such
projects around the world.
Intensity decays with distance at a rate of 1/r3, where “r” is the radius. Comparing estimated
intensities levels from OpenSHA and ShakeMap results for the NGEP site; i.e. a MW3.1 to
MW3.5, a PGA on the order of about 0.7% g (MMI zone III) may be felt at King Salmon (~10
km) and 0.2% g (MMI zone I) at Naknek and South Naknek (~29 km), Figure 17. From Tables
2 and 3, these fall into the “Light to Not Felt” perceived shaking with “Little to No” potential for
damage to occur. Some homes close to the NGEP site, i.e. within 5km, could see PGAs on the
order of 1.5% to 2% of g which would still have only a “Light” perceived shaking but with
“Very Light to Light” potential of damage to occur. Given this a MW3.5 to MW3.7 or even up to
a MW4.1 size events occurring at the G1 site should do little to no damage to any of the local
infrastructure.
For the injection testing the Institute of Earth Science and Engineering (IESE) at the University
of Auckland in New Zealand has been contracted to deploy a ten station seismic monitoring
network around the NGEP site. Data from each station would be radioed back to a central site in
real-time where a data acquisition computer would acquire all data and process the data for event
detection. Location and magnitude calculations would occur within 30 to 60 seconds of any
detected seismic event. Other advanced analysis would be carried out in near real-time such as
stress drop calculations, moment tensor inversion, spectral density calculations to name a few.
In addition to the ten seismic stations at least two strong motion seismometers would be
deployed in the region to monitor key facilities. These strong motion stations may, or may not,
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be included into the real-time network but at a minimum after any felt event the data would be
extracted and analyzed to determine the exact PGA for the event.
During the injection testing, the Senior IESE field seismologist on site would interface with both
NEA and the Contractor performing the injection testing. From the real-time data, the
seismologist would observe for signs of an increased rate of larger seismic events in the region.
The seismologist would provide both NEA and the Contractor with hourly updates on the rate of
seismicity, pattern(s) of seismicity and any changes to the pattern(s) of seismicity. The
seismologist would immediately notify the Contractor and NEA if any events are occurring away
from the intended injection area, i.e. on other faults in the area, which could indicate the potential
for triggering of an event out of the intended study area. This real-time input should help
mitigate the risk potential for damage from induced seismic events by allowing NEA, and the
Contractor to modify their injection parameters before a large event is triggered, thus lessening
the impact on the local community (Cypser and Davis, 1994).
The following protocols have been developed for the monitoring of seismicity at that NGEP site.
1) Before any injection testing takes place the local communities will be informed of the
possibilities of induced seismicity and to take appropriate actions to reduce the potential
for damage at their homes and offices. This includes the removal of glass items from
shelving and possibility the anchoring of book cases to the walls. Given that the region is
a highly seismically active area, most residents should have already taken these
precautions, however reinforcing these and other precautions would be good practice.
2) IESE will provide real-time monitoring of seismic activity in and around the NGEP site
during any stimulation, including rates of seismicity in the area as well as estimating the
cumulative seismic moment.
3) In the event of a significant earthquake occurring in the area of study IESE will notify
NEA and the stimulation contractor immediately. Significant events are those MW2.5
and up, or events of MW3.0 and up that occur outside the area of study, i.e. on or near the
Clark Lake fault.
4) NEA and its contractor will take the following actions:
a. In the event of an earthquake of MW2.0 to MW3.4 the rate of injection will be
decreased by at least 50%, and injection will cease if additional earthquakes of
this magnitude are recorded, until earthquake activity subsides below MW2.0.
b. In the event of an earthquake of MW3.5 or higher, injection will cease until
earthquake activity subsides below MW2.0.
Since the NGEP site is still at an exploration level, the data collected will help characterize the
reservoir response to injection during the first test of the G1 well. Data collected during this first
test can be applied to other tests that may take place at a later date or for future wells. By
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monitoring induced seismicity during injection testing in real-time, NEA will be able to respond
quickly and mitigate the level of induced seismic events.
While the potential risk from induced seismicity is low, NEA is committed to taking every
precaution to manage and mitigate the effects an EGS stimulation could have on the region.
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7.0 References
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