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Home My WebLink AboutSeismicity Study Bradley Lake 1980Woodward-Clyde Consultants SEISMICITY STUDY BRADLEY LAKE HYDROELECTRIC PROJECT Submitted to Department of the Army Alaska District, Corps of Engineers P. o. Box 7002 Anchorage, Alaska 99510 Contract No. DACW85-79-C-0045 Modification P-00001 Woodward-Clyde ConsuHants SEISMICITY STUDY BRADLEY LAKE HYDROELECTRIC PROJECT Submitted to Department of the Army Alaska District, Corps of Engineers P. o. Box 7002 Anchorage, Alaska 99510 Contract No. DACW85-79-C-0045 Modification P-00001 4 7~11 Bc:~iness Part<, Bou!evrlr.J Su:\e 1 28 March 1980 Project No. 41229A Department of Army Woodward-Clyde Consultants Alaska District, Corps of Engineers P. 0. Box 7002 Anchorage, Alaska 99510 Attention: NPAEN-PM-PE (NPASU) Gentlemen: SUBJECT: SEISMICITY STUDY, BRADLEY LAKE HYDROELECTRIC PROJECT CONTRACT NO. DACW85-79-C-0045 MODIFICATION POOOOl Attached is our final report on the regional tectonic and seismic setting of the Bradley Lake Project area. We believe this report will provide you with important data that will be useful in identifying detailed studies to be conducted in the near future. If you have any questions or comments, please contact us at your convenience. Rupert G. Tart, Jr. Geotechnical Manager RGT:STF/cw Consui;,n~; Enq1neers, Geologists ;mel Enwonrnentai Sc;entists Oil1cm. Dliwr Pr:nupal Cit:es S. Thomas Freeman Project Geologist Woodward-Clyde Consultants TABLE OF CONTENTS LETTER OF TRANSMITTAL 1.0 INTRODUCTION •••••••••.•••••.•••••••••••••••• 1.1 Background and Purpose •.•.••••..••.••.. 1.2 Scope and Methods •.••••••......••••.... 2.0 REGIONAL TECTONIC AND SEISMIC SETTING .••.••• 2.1 Tectonic Implications ..•.••••.••.••••.• 2.2 Regional Seismicity ••.•••.••••••.••..•• 3.0 POTENTIAL EARTHQUAKE SOURCE AND EVENT CHARACTER! ZATION ...••••.•••••••••••••• 3.1 General Background •.•••...••.•.••.•.•.• 3.2 Regional Sources •••••...•.•••..•...•••• 3.3 Local Sources .••..•.••••••.••..••••••.• 3.4 Maximum Earthquake Magnitude Evaluation .........................•.. 4. 0 RECURRENCE OF EARTHQUAKES •••••..•••••••••••. 5.0 PRELIMINARY EVALUATION OF GROUND- MOTION PARAMETERS •••••.•.••••••••.••.•.•••.• 5.1 Background ............................ . 5.2 Factors Influencing Attenuation of Seismic Energy ..••••••••••••••••••.. 5.3 Ground Motion Parameters for the Site .............................. . 6.0 DISCUSSION ...••••.•..•••.••.•••••••••••••••• 7. 0 RECOMMENDATIONS .•••.•.•..•.••••••••••••••••• 7.1 Geologic Studies ...••••.•.•.••••••••••• 7.2 Seismicity Studies •••••••••••••••••••.. REFERENCES CITED Page 1 1 1 3 3 5 14 14 14 20 22 23 26 26 27 28 30 32 32 35 Woodward-Clyde Consultants Tables No. 1 2 3 Figures 1 2 3 4 5 6 7 8 LIST OF TABLES AND FIGURES Title Great Earthquakes in Southern Alaska pnd A leu ti an Reg ions ••••.•••.•••••••••. Summary of Larger Earthquakes Occurring Within About 100 Miles of the Homer Area in the Period 1788-1961 .•••.••..•• Earthquake Source Characterization and Site Ground-Motion Parameters ••.•••••.. Southern Alaska Regional Faults •..••.•• Lower Cook Inlet Region Section LA-LA Schematic Tectonic Model ••••••••.•••••• Southern Alaska Historical Seismicity •• Composite Seismicity Map of the Gulf of Alaska, Lower Cook Inlet and Kodiak Regions, 1899-1975 ••••••••••..•• Seismicity Cross-Sections Showing Benioff Zone in Kodiak Region •••••••••• Distribution and Depth of Aftershocks (4.4<M>6.7) From March 27 to December 31, 1964 .•.•.•..•..•.••••••••• Tectonic Uplift and Subsidence 1964 Great Alaskan Earthquake .••••••••.••••• Aftershock Zones of Earthquakes of Magnitude 7.3 or Greater Since 1938 •••• 9 Attenuation Relationships for Trans- mission Paths for Shallow and Deep Events-Rock Sites .••••••••.•••••••••••• Following Page 7 9 14 4 4 5 5 5 5 10 24 28 Woodward-Clyde Consultants -1- l.O INTRODUCTION l.l Background and Purpose Southern Alaska is one of the most active seismic regions of the world. Thus, when designing and constructing hydro- electric facilities in this region, the potential earth- quake-related hazards demand particular attention. Rec- ognizing this, the Alaska District of the Army Corps of Engineers retained Woodward-Clyde Consultants to conduct a study of the seismic setting of the Kenai Peninsula region, as part of the Army's continuing investigations for the Bradley Lake Hydroelectric Project. The purposes of this seismicity study are to provide the Corps of Engineers with background data for their selection of the project design earthquake{s) and to provide a basis from which future, more-detailed studies can be planned. A report giving a general description of the Bradley Lake Hydroelectric Project and the results of reconnaissance geologic investigations in the site area was submitted to the Army Corps of Engineers by Woodward-Clyde Consultants 1n December 1979. 1.2 Scope and Methods This seismicity study is based on the results of recent research described in the published and unpublished liter- ature. The results of the Offshore Alaska Seismic Exposure Study {OASES) done by Woodward-Clyde Consultants (1978) are applicable to the Bradley Lake area and are used extensively in this report. Data presented in the previous Woodward- Clyde report on the Bradley Lake Hydroelectric Project have also been incorporated. Woodward-Clyde Consultants -2- This report summarizes the regional tectonic and seismic setting in the Bradley Lake area and includes a regional fault map, seismicity maps, and appropriate cross-sections. Past earthquake activity in the region is discussed along with the implications for the project from future earth- quakes. Known faults capable of generating earthquakes significant to the project are identified, and character- istics of these faults which are important to an evaluation of potential earthquake hazards are also noted. In this report, the correlation of earthquakes with faults, based on knowledge of historical seismicity and geology, provides the means by which estimates of the maximum design earthquakes are developed for each specific fault. The estimated maximum magnitude of each source fault, the distance from the source fault to the site, and the site foundation characteristics, provide the input for making preliminary estimates of a range of values for ground-motion parameters that may be expected in the project area. Peak ground accelerations and velocities presented in this report are derived from relationships developed by Schnable and Seed (1973}, Idriss and Power (1978}, and Woodward-Clyde Con- sultants (1978). The range of estimated ground-motion parameters are provided for both shallow focus and deep earthquakes. The difference between ground motions generated from deep and shallow focus earthquakes are discussed. Results of analysis in this study are compared with pro- babilistic evaluations of ground-motion parameters made . for the region containing the project site by Woodward-Clyde Consultants ( 197 8) • Finally, recommendations are outlined for further work that may improve the level of confidence with which the seismic parameters can be predicted. -3-Woodward-Clyde Consultants From the range of estimated ground-motion parameters given in this report, the Army Corps of Engineers may select a design earthquake(s) based on the level of ground shaking that the proposed facility can be economically designed to accom- modate. 2.0 REGIONAL TECTONIC AND SEISMIC SETTING 2.1 Tectonic ications The active faulting, seismicity, and volcanism of southern Alaska are products of the regional tectonic setting. An overall view of this regional tectonic setting provides the basis for evaluating the styles of structural deforma- tion that weigh heavily on an evaluation of the seismic activity and the potential seismic hazards of the area. This understanding of the tectonic setting is especially useful in areas, such as Bradley Lake, where geologic data are lacking or where seismicity records are too short to define ade- quately the degree of seismic activity. The primary cause of seismic activity in southern Alaska is the stress imposed on the region by the relative motion of the Pacific and the North American lithospheric plates at their common boundary. The Pacific plate is moving northward relative to the North American plate at a rate of about 6 mm/yr. The relative motion between the plates is expressed in two different styles of deformation in southern Alaska. Along the Alaskan panhandle and eastern margins of the Gulf of Alaska, the movement between the plates is expressed primarily by high-angle strike-slip faults. Along the northern margins of the Gulf of Alaska, including the Kenai Peninsula and extending westward parallel with the Aleutian Woodward-Clyde Consultants -4- Islands, the relative motion between the plates is causing the underthrusting of the Pacific plate beneath the North American plate. This underthrusting results primarily in compressional deformation which causes folds, high-angle reverse faults, and thrust faults to develop in the overlying crust. The boundary between the plates where the underthrusting occurs is a northwestward-dipping megathrust fault or sub- duction zone. The Aleutian trench marks the surface expres- sion of this subduction zone. The trench is located on the ocean floor approximately 185 miles (295 km) south of Bradley Lake (see Fig. 1 and 2). The orientation of the subduction zone at depth is inferred to be along a broad inclined band of seismicity, ref erred to as the Benioff zone, that dips northwest from the Aleutian Trench, and is approximately 30 miles (50 km) beneath the Bradley Lake site. The s ubd uc t ion of the Pacific plate not only dominates the present tectonic setting in southern Alaska, but it has controlled the development of this region at least since the Mesozoic Era. Geologic evidence indicates that subduc- tion has progressively shifted from its ancient position at the Border Ranges fault (discussed in Section 3. 0) , south- eastward to its present position (MacKevett and Plafker, 1974 and Tysdal and Case, 1979). This southeastward shift ap- parently resulted from the scraping off of the oceanic materials on top of the lower subducting plate and from the accretion of them to the leading edge of the overriding plate (MacKevett and Plafker, 1974 and Tysdal and Case, 1979). These accreted sediments are the highly deformed and faulted rocks we see today in the Bradley Lake area and throughout the Kenai Mountains (Magoon and others, 1976 and Woodward- ' ' "" "" 148° \ I I ]/·-.=-.u_ ~~)! --?/ l!' . _..--~~ iiORDER 142° ~ ~-~~0 * \'~~----.. '~~:~~.., 0~ ,..s ', '... , ', l.. 1 "'-"' C';.,u ', ··\, ',' '"'~ lito ',' '' -N- ··~--4u( r-', ..... , ', EXPLANATION A.clt~ faults -"""r" """r" -r-··-·-f.uiL ~where ~roxtm.ately IIXitted, dottea ..wf\efe conceM'd Of ~~~Of"~. H<Kh'-'''" tnd•c.ate rt:litlt~l'f' down droppe-d Stele of lauh. -....&... ~ ~ •• •••• Th"'st F AoUit, dashe-d where aPOroxtmately ,.,. .... ~-n...l Spurr , 1--" ,.. "" '"'"" D ~~ ;;1~, ~ .. ~·0----,~·/;~;-:.:::::::..":.---<--=:.;., * * '··-'"::.·~. 4 t:" ..... ~---~'::-,:',..c--41(1 ~ * * ··· No1 ' ..................... ..... ··-·--·. _,v n ............. _ '-'( ;--ff ~ ... __ * Mopped ', located. dotted .... ~re conoeo~le-d or q~tu:>oable BarD& mo•ute relal•~lv upthrown LICie ot fault / I I ....... -:Y. I / / / '\_~- ~ \" '( ,/ ~.[.~,. ... -.- ¥ "\ ... \ ( ( / ~ .--~ ~ \)~ / \_'"--.__; ,'( ,~".r ,.J "" "'0 __ .,..·· . chor"fle!.[; •--' / tf" .. ··· f [)~' .,_,.._. , ' I -if'' /,./.... _ ... / .... «..~ '\. / G~'-~·-··,.. ' r * roun __ , .. ·,~ : 1 ', <J: Gracki one .. ( •. ' _: ~ I c,~v~/ • • ( ;_,· ~-..,'i G~ / o:!<c<Jo~.-1 / .'. .:.:::,.> -· f:)'_: ~ Kenai J4.. r // V-.lc:.-.o , _o. Kenat ....... ::.;;_;;_,_::: -~....._ .... sr.·<S" ~ Lake Ill: /k/ir ~ .J_,;7 .;1-/ ~-If ·••'•'!'''• . ..., .· '< _. . ;; I OJ (4' "if. ~ " ?~?-'/ ,>\,.>,-Js ~" }:)'~.._.,." ~l '\:£~ ~~ I " '';';cij./ ~•' ,'-' '>~~ .··· ~-/ IJ.q; Kenoil 11:] / '(} C ~ (/ / ) V')lca"o ·I .}i "" · • V .-: 1 ~ ...J I '!R,(J ;.." ..\.. / \SLr., _o ::./~ ;;~: ~ ~ .. :"' ,~ ,.J !a.. 1 o; ..,.. //-.... fr.,u1_1s __...-/-, /' • ·v ..::::= :· . -..r;:-I I ~ 1 1..::, ~ .,--.6--,' (fl / /-' / +'· qf__,_ if·/ \v~~ -:Jd; d'MONrAGUC -"'~"-~-~' ',~(Qo / ~ G .42 ,.,<lc __ .· g-_ 'Bf'ad~sy· 0 .-_,. ._ 1SLANO ,' ;-· ~~ ,. / '"· ~ ~...., //Y"'-"""' G ~....,_. Q)· • ":>. ......... ,."" " \o' ,. / r '-.../ / .. c., ,c.: _.· .... Lake Sl e . · // ,. 1 ./ .-~-..... rb, " " /~/ .:) .-· , . ,. ""J'(>-/ -~ ..,......,4"'• ,.. /f HCI / .--"(_/ v 4.'\f" .· Homer .· 1 ·.: , :~/, ~~ ,' -,..,...;, /",l ~ . I ( t """"~ I ,_..,. y " 0~'~' -/~ Kachemak Bay ', ii'~. 4 '1-~ 1 ,, ::--1 _ __ C\ ~-?0 /'-.,y ~ AUGUSTiNE ·-c.~~ '\.,' 4>:ilf A' A' -r _,. I ""' '-.., /,. .. '(, ,"" \.__ _J) .._ __ ~'''"~_7'15L..;'ND ~(,..·"'~ ]\ ~~ ~ A'~ -... -... :_ IJ r / Yorcono./L.-:/ Seldovia . J 1-_[} II ~~ ... ..,..~............. ltiiODiETON ,.r ~ 61° ',,, ~--''-F 4 U L T ..... ___ ... , ,_ Y...... ,1~ ;J' ...... t ,/ ,...,... .............. --()""- ...... / .......-"' ,:~,.,.,-\ ,-'/" fr'• } r., ! ·,~··::;;y}, \._ ----······ Stnk.-Shp Fault, da-shed where iiPPfOKtmatel'f' located, dottrd wnere cona~Jed or Quest•onable Arrows tna•~te relattW dis.pl~ment . - - - - -Sub,.nne surface fault or tcarp recoqnul!'d hom QI!Ophysu:al data. Hachure-t on downthrown Stde of fault. ---·-·-·--•-·-linrarneot, inferre'd 101' thts STudy to bill' .-n earthQUolke-prod_ucu"9 structure. lnK'ti~ F aulu --.-----.---.-· Fault, ~ wh~e ~roximately loc.atl!'d, doned ..N-re conce~led Of QUHhonabte. H.churn tnd.ut• rriatiwely DeMon dropped Sl<k ol r..,Jt. -~ ........_ .......__ Thrus1 Fault. dalhed .twn IIPt)TOll.i~t•ly located, dotre'd whet'• conataled or questtOn~e. 8Mt. tndtute rtl.1t1't'e'ly upthrc-n sam of fault. ----·· Strik .. ~ip F.1ult, d.-hl!'d where M>PJ0111rNI1el.,.- louled, dotte-d w~re concealed 01 quesnon.able. Arra-s mdM:ate rel~liveo r;t;s.pl~ment. --- - - -Submanne surf~ fMJit Of sc.wp recoqnized from geophySical data. Hachu'" on downthrown Stde. * Vo:cantc Centers Ou•tern~ry -..olc.antc ce-ntl!r, vent or cone /( r~ ( NAI<'NE, ~ (,"" ~LA~E l~---._/; ,' ,-~~ t,J\. ~ ,C ,.,r.-,. \ ISLANO .,. .,. ./ ' ., ~.. ' j. c. .ti ,r -----..;-.... -' .,. ,. ,.· ' '\. J. 1,.0~" ~~--200177 .--"'/ } Dtloql 1 Vnl~ono / * * _...>' l:;;::,t:> ~ J ,~ A~/ x-----------, J. / 7 '>( 1 r, , '-,1-./ ~ II __ / ~--~... ~ ~ 't,J. !f / .... --·-------------- -' '' ·-,--·------------- Ostutated Shett 12:--------, t----------1 L•ne of crc~wction. NOTES: Active Fault - A subaerial or subma-ri,.... f.ult that bleaks or I! inferre-d to break Holocrne or uncon~hditte'd sedm·•er·•ts (Noleably young but without any ~ determtnalir•n) c~ , r f .... / ' I /, * ( ,. '[>-* <'( / * 0* ', H,lf_., 1;-\ 'J ·~1/.>CF * 1-1\ -)~ o:S '-"-( . / --~ I /A'A' •. ' ( ?J -y 11, /.-1-0 'v' C· ,.,-" ~,_,1 . '· .-'L "'/ v.i:. r· ;·/ ' . _.._ci"" 'o ~~ , I / I, : II,: ,, ''-·"' ~ ,J/ I ';,~//~ljf· "~ , . I '-• ---- " ~/" ,/)i.,_ I ~, ,/' ,o' 0G ')' t l>.-J ; I 0rf' "' <:/" ~"-"' "-' ! ·., i' .'~ o'?-/rotc->' ~ '," ) I ..b '< / ,~ ~" "' ~~· ~·/ ,, ,/ I y , j ' .,~., ~ ,.......--... , \ ~/ ~ \ / ~' e, -;--......~ / ' Jl' ,I ~ --;._..'-: /?., / ' / ,;I' ,t "" ' ,-> ,_, ~ ,__ ~ ·-' J' A' "' \ <c <c ;t;t AA' J"-v ,II(',;( "/ A....~<t-/ A o0 / ~ A ' ? A ,' J"/1 s ~(' I GULF~~ ,'~'·I ,)~ vr.,; ' 'I'" '"'/ ,/., ;;'" ;r"" / -<"' ,// zoo"' ;~ '-1'--9" LA' / ""' 146° A 0 F 1440 or a submarine fault IS exposed oo tt.. wa floor. lnactt¥e Fault -A subaerial or submaune f..ult that doe-s no1 break. 9e Structure (p------ age "' -, __ _ L A 5 K A , ...,.·4 .• 19Jsr-- • Holoc::e-ne nor unconsohdatl!'d sediments ( f'lloteablv voung but without ""'*' ~ determinatton) nor IS it exposed on the ~~ta fiOOf. BMI! Map Compiled from World AerOf•M.>tic.al 01~ . 25 0 25 50 Nauttcal Mdn 25 0 25 50 100 KtiOIUtlrl 25 0 25 50 Shtutl Mtlu 57° 138° Source: Woodward Clyde Consultants, 1978, Offshore Alaska Seismic Exposure Study WOODWARD-CLYDE CONSULTANTS SOUTHERN ALASKA REGIONAL FAULTS Project No. 41229A BRADLEY LAKE Fig. 1 LA v 0' ~"$.::! 4.. 4.."< ----N45W 'b c.."' ,~~ ~4..~ ·-~::::::::::::l~ljj~~ElW~n-~i~/====· -: 1:· :::::i~~Hliijiijj~)!:::: I i I I North American Lithospheric Plate EXPLANATION Tertiary and younger bedded sedimentary sequence. I ~_ ~ _ ·. 1 Highly folded and/or metamorphosed :~~c~c·";_~<-_>:;-;-: _ basement complex. 1:::::::::::::::::::::::::1 Basalt or undifferentiated volcanic ·. ·. ·. ·. ·. ·. ·. ·. ·.·. ·. ·. · rocks. ·.·.·.·.·.·.·.·.·.·.·.·.· I' "'lJl2l'"····----. ··············· : ::::::::::::::: Granitt'c Rocks . ··············· . ··············· . ··············· r> ?K.i > h I Magma or resorbed oceanic crust. LA 1 Cook Inlet I Kenai Kenai Lowland Mountains Continental Shelf Continental Slope Abyssal Plain ~ Kachemak rJ> Bay q; .:,0' .l.. I I BRADLEY~~ 0 ~ $' "< ,9;-~ LAKE q; ~ 0~/o.... ?§> ~"-{i!GI... ~ -e> "' R:' t-..."< ;; CQ r."' ~ 1 . t.v "-~ <:- v"<j.::; ~-0 ~ .::/ ~oq;t :::J" t.v"< ~" :f DEFORMED _/ ;f I \"< . _I . ~ ""< (<.'<(" \ I(( r Is.. Level ~ "'<(" ~"' ~m~~ @B~~xwm:~m I ~ / A{rs:&f:S'"'\c~· ~1 /_e;;c'(f_·L;;.i:Sv\:~·. SUBDUCTION ZONE \\ f-:,R',::,Y~C:Y \\ ,.?f-:1:7 I -----Lithologic contact, dashed where inferred --~ --Fault, dashed where inferred'. Single and double-sided arrows represent the relative component of displacement inferred for this cross-sectional orientation. Pacific Lithospheric Plate 25 0 25 50 100 Kilometers 25 0 25 50 Statute Miles ~-__... . I Vertical Scale Equals Horizontal Scale NOTE: Lithologies and structures that are not exposed at the surface are inferred. SOURCE: Woodward Clyde Consultants, 1978, Offshore Alaska Seismic Exposure Study WOODWARD-CLYDE CONSULTANTS LOWER COOK INLET REGION SECTION LA -LA' SCHEMATIC TECTONIC MODEL Project No. 41229A BRADLEY LAKE Fig. 2 Woodward-Clyde Consultants -5- Clyde Consultants, 1979). The southward shift in the plate margin has been recorded by at least two major thrust faults, and possibly by three or four. These thrust faults divide the accreted rocks of the crust into several distinct groups. The close relationship between the Benioff zone and the structures within the overlying crust introduces important implications regarding the affect of the tectonic setting on the Bradley Lake project. The Benioff zone represents a source of major earthquakes near the site. Faults at the surface may be subsidiary to the megathrust at depth, which, as mentined earlier, has been inferred from the earthquakes of the Benioff zone. These faults may be the sources of local earthquakes and they may also represent a potential hazard for surface fault rupture. This is of special concern because numerous faults, such as the Border Ranges, Eagle River, Bradley River, and the Bull Moose ults have been mapped near or through the locations of important structures in the proposed Bradley Lake Hydroelectric Project (Woodward- Clyde Consultants, 1979}. If future ruptures on these faults were likely, they may have a more profound affect on the seismic design of the project than the underlying Benioff zone because of their closer location. 2.2 ional Seismici Data Sources Figures 3 through 6 illustrate the historical seismicity of the region around Bradley Lake. The distribution of earth- quakes, the accuracy of their locations, their sizes, and the completeness of the earthquake records are all factors that should be considered when estimating the potential for future earthquake activity. 62°[;; "'"' K!Jsll:~ ~~ 148° D (_l -,----:__.-----,-----·-1_40° 146° 1440 ~ 1 (;(~ .. ,_.-~ ~ q ··"')'' ~ '~~ r ·-.. ·'~~7~ ,., ,,,.,-: ~~ o, -~ "'' -~~. C(t)'" 28 FEBRUARY 19 f1'1 131 ':_~ -117 69 ~) c=], rf .. . . .':~) (.;( . •ocTOBER ,900 .arr _ / ';!-fl; 4 SEPTEMBER ~~ (if':-~) ... t3[' 1899~ (.Rh'o' 9\'-' ' -. 2"''-'' -~ ·~ A ~-\ <.._ -ND~l('_-:: "'~"\'" :"'· I~ _.&=E£1-..,, ('X~, 1 /\: J ') • I ,f' '"' 'i . --~ -, ,)'"r~rr _"~,c .. cj ~!14 ... ~ • ~ ••.•. .. , 1) -1 ~~~ l I . @ ___.A-\ ~· '. 1Y , (T)'ol< , • ,· ,0 '---· - IDS 9S !) . ,'~\.... \._,.: \ 1r'~, ~" 19 CJ,,Ii'C-,;~@ .. , "'~" (1)(1~' .. :.: \__ " :~~ --~-,.---.. -.__. _::;::;~.! l_ ) c·r crJ' ..... ./ '" , ,/J 'l ) ·. I · •./ r;; 0 ) l '1 ~_)~ -ri)'" ·< '-"' . ' . ,, . ,•, \ "'"'. :;" , -.. ,-n .~--1 ' 1 '·"'0., •" [ . ,,? I ' R ([) }C)'' ' ' 0l'i ' '7 <yJ I' ,, r";/ ' ~-·~ :tJiili".r :{\'·Dr ) l'i!· ·•(.9'' ' . ' -~ ,., (Icy ; 1 ( )J t-0 ,, _ ~· , ,r-:C U rQ~~J~~·~t-.. r' . ·· · IAMVA ~ ... !}'~,>-/---~ [ "-""Nh ' /4!<,f' '\. . ---- b '~ t ~~~, ~--t1-~1 : C!J''' 1 I "J(~~~)cl' 1 0!J'!J',: ,I lU2i Ji~}l· · rf '-J ' 'l, I'M -uv ''\"T.:-;' I' I '• )( b s 0 'v· I •/1 { \' £w'1 • I ~ 1)11111 ~ ' \ .,. ' • • ,, ' ' ' 1._\_y ' c ~.f.' /-u_· ' . ' ' c . ' ' • " : ",-L_ , , ' ' ,"cc '" . , ; Lcij t)'J ~~ ~~~/~, ' 1' "'J'·f' ~ ' ,, n,n "J' ,, •••• 1560 ---L .. -, ''" _ _,..,._':::) '--'I IJ,~.9-li7.,,,.L:rl -~--jV!-'':-<:-<2: .----' • ,-,,I -.'"l .' ' ,c:--.; , 1" _,] ( ' I ' -"'i{'l ,,,"' Jnib''' 154o ~1I.i..J"' f I) ~I '" fj])J ,J?.., ' ' . . . . . I ( l ,' ·-• 152° ---) I 150° I 148° ALAsKA G U L F oF o···. o·· 146° / ~ -N- ~ 61° fXPlANAIION R!PORTEO MAGHITUDf 8.0 1.0 C) 6.0 (['; u ~.0 4.0 (') 3.0 0 2.0 LO ~ Ho nprfltd lftlQilltud• Magnitude symbol su:n ~tl! 'Sho~ Oft I UHHHIUOII'S nonhn.it sta!• . NOTES: 1) Minimum magnitude 5.0 2) Events are numbered chronologically. Numbers refer to entries in the Earthquake Data Bilnk catak>g under The Lower Cook Inlet and Gulf of Alaska areas, which is included in WCC, 1978, Offshore Alaska Seismic Exposure Study Report. ~; 3 l Selected earthquakes have been added and are discussed in the text. 58° 25 0 2~ ~0 N•utu:el Mdu ~ 25 0 25 50 !00 K!fcmthn ----.--- 57° 139° 25 . 0 __ ---~-25 50 :;tatut• Mdu ~~ Source: Woodward Clyde Consultants, 1978, Offshore Alaska Seismic Exposure Study and NOAA Hypocenter Data File 1638-1975 WOODWARD-CLYDE CONSULTANTS SOUTHERN ALASKA HISTORICAL SEISMICITY Project No. 41229A BRADLEY LAKE Fig. 3 s z~ i I r--......... ~ / ///;~~ ;.,./ I ! i €2" ~--·; i ,-- i ( , ~'<-'-" / ·~~-~~ // _..../' [']['] ['] [T] &.£l.'?ir.c·F['] /~';L4/Jl) ['] [']['] [~ ['] l'p.\l'-' // -----t ['] 1-, ., PAC!F;c OCEAN ) /' ['] ~ ['] "-\'' ~ )' r-h. 0 0;-:b.~< ~1-~ \"""" "'""''~< "'"' "'"' ' ' 1!1_... ['] {:}'""' \, "-. b.\.,_ ' ~ \'~'-\)\..~./' -~r '"""1, '-....~ ~ 1:!}--1!1 '\. ' "· 4& \ ~""'-[I] ~ E!r-...,.._ ~~ '( r ~ .---·\ (!] ....... \ 6~ ['] . ['] ['] '· ~ \ ~ ..... ~ ----~ >:;V 1!1 I~ 9~ ./ 1!1 ~ ,P ~"~!?j 1'1"" ~:: r~ 1!1 llll I 1'1 Bt / / 1!1 1!1 1!1 t- GULF OF [']ALASKA t~s .. ['] ['] :::.~j» '}(,"' ~,:::~ EXPLANATION FOCAL DEPTH (kilometers) ['] 0-20 or no depth given ~~~ 21-70 ['] 71-170 ----Generalized major tectonic structures * Volcanic centers L L' 1 1 Line of seismicity cross--section NOTE: Minimum magnitude = 5.0 SOURCE: NOAA Hypocenter Data File, 1638-1975. BASE MAP: U.S.A.F. Jet Navigation C'1arts 50 0 50 100 Nautical Miles ~---!- 50 0 50 100 150 Kilometers 1""5Wl'-i--t 50 0 50 100 Statute Miles SOURCE: Woodward Clyde Consultants, 1978, Offshore Alaska Seismic Exposure Study. WOODWARD-CLYDE CONSULTANTS COMPOSITE SEISMICITY MAP OF THE GULF OF ALASKA, LOWER COOK INLET AND KODIAK REGIONS, 1899 -1975 Project No. 41229A BRADLEY LAKE Fig. 4 cL r f- 0.. w 0 -"' I f- 0.. w 0 DISTANCE ALONG SECTION, km Approximate Bradley Lake Site L 1 • o~~~----------~----------~~--------~--------~~--------~----------21~------~~----------~~j_----~~=-------~~----~~--r---~~~~ 0 0 k7~"--<J.,) nr'....,J:;!r-r--___ o ) '0 'c'O ---------- () 50 100 0 150 K2 0 ~00 0 0 I so 00 1 [,n (l 200 Approximate Bradley Lake Site 0 0 u 0 T D-~1\00 500 --=-cW_tr; ______ _Q ___ _ 0 0 0 K/ 600 EXPLANATION MAGNITUDE RANGE () 8,0-8,9 /-"', 7.0 7.9 6.0 6.9 "..___/ 0 5.0-5.9 /-, 4.0 4.9 3.0-3.9 0 2,0 2.9 V = Volcanic Line D = Deformed Zone on Continental Shelf T = Aleutian Trench Axis NOTE: Includes earthquakes located within 1 00 kilometers of the cross-section line. For location of cross section lines see Figure 4. SOURCE: NOAA Hypocenter Data File. 1638-1975. Woodward Clyde Consultants, 1978, Offshore Alaska Seismic Exposure Study WOODWARD-CLYDE CONSULTANTS SEISMICITY CROSS SECTIONS SHOWING BENIOFF ZONE IN KODIAK REGION 1964-1975 Project No. 41229A BRADLEY LAKE F 5 i I I i I I I •i I L Project: Project No. " L _... .. S\JBMARIN[ CONTOURS IN FEET DISTANCE PERPENDICULAR TO ZERO ISO BASE EX PLANATION + )( • • • ,20 2!-35 36·45 46-60 61·90 EpicentPr of aftershoek showing depth of focus, in kilometers e ________ (!) _______ _ Approximate zero isobase be- tween major zones of tectonic uplift and subsidence • Active or dormant volrano Aftershock data after R. A. Page, Jr., Lamont Geological Observatory of Columb1a UnivNsity (written commun .. May 1966) Source: Plafker (1971}. BRADLEY LAKE 41229A 01 TRIBUTION AND DEPTH OF AFTERSHOCKS (4A...;; M ~ 6.7) FROM MARCH 27 TO DECEMBER 31 1964 r 70 Fio. 6 WOODWARD -CLYDE CONSULTANTS Woodward· Clyde Consultants -6- The earthquakes shown in Figure 3 are of magnitude 5 or greater and are from the National Oceanic and Atmospheric Administration (NOAA) hypocenter data file for the time period 16 38 through 197 5. The lower cutoff of magnitude 5 was estimated as the minimum magnitude for reliable earth- quake locations using the NOAA data (Woodward-Clyde Con- sultants, 1978). Seismicity relevant to the Bradley Lake Project since 197 5 has not changed the general interpret- ations regarding the seismicity of the Kenai-Lower Cook Inlet area of southern Alaska that were previously developed by Woodward-Clyde Consultants (1978). One major earthquake, which occurred on 28 February 1979 in the eastern Gulf of Alaska region, has been added to Figure 3. This earthquake, which is discussed in Section 4, did not affect the Bradley Lake area. Estimates regarding the completeness of earthquake data are important when using these data to evaluate the level of seismicity for a region. Lahr and Page (1977) concluded that the NOAA data are complete for magnitudes 7-3/4 or greater since 1899, for magnitude 6 or greater since the 1930s, and for magnitude 5 or greater since 1964. Thus, it can be seen that the time interval over which the seismic record extends is very short, especially for the smaller magnitude earth- quakes. The earthquake epicenters shown 1n Figures 3, 4, 5, and 6 have been located at teleseismic distances by seismographic instruments and, as such, may have errors in focal depths of as much as 30 miles (50 km) and errors in latitude and longitude of 15 to 30 miles (25 to 50 km}. Nonetheless, most of the larger earthquakes generally can be associated with principal geologic structures in southern Alaska. Woodward-Clyde Consultants -7- Distribution of Seismici As indicated in Section 2.1, the majority of the seismicity illustrated in Figure 3 is due to the release of strain energy along the interface between the North American and Pacific lithospheric plates. Earthquakes associated with the Benioff zone at this interface can be large; many have been of magnitude 8 or larger. These earthquakes are commonly called great earthquakes and they result from rupture on fault planes with square kilometers. have occurred along list of the great some of these are areas as large as several thousands of Historically, several great earthquakes the plate boundary in southern Alaska. A earthquakes is provided in Table l, and discussed in the following sections. A great earthquake is typically accompanied by hundreds of small magnitude aftershocks that have been used to define the plane of rupture associated with the earthquake. Seismicity cross-sections showing the Benioff zone beneath the site (Fig. 5) were constructed by plotting the hypo- centers of more reliably located earthquakes, for the time period 1964 through 1975, within 62 miles (100 km) of the cross-section lines (Fig. 4) (Woodward-Clyde Consultants, 1978). Earthquakes shallower than 6 to 9 miles (10 to 15 km) are associated mainly with crustal sources, Volcanic sources occur along the eastern edge of the Alaskan peninsula. Deeper earthquakes are associated with the Benioff zone. The Benioff zone in the Bradley Lake area appears to have a progressively steeper dip toward the northwest. The general inclination of the zone of seismicity is represented by color coded symbols in Figure 4. The cross-sections (in Woodward-Clyde Consultants 'JABLE 1 Qill\T Ei\Rl'HQU\KES IN SOUTHERN AlASKA AND ALEUTIAN REGIONS Epicenter D2pth Date Coordinates Magnitude (km) Location Panhandle Region: 1899 Sep. 4 60N 142W 8.5a Near Cape Yakataga 1899 Sep. 10 60 140 8.4a Yakutat Bay 1900 Oct. 9 60 142 8.la Near Cape Yakataga 1949 Aug. 22 53 133 8.1 25 Queen Charlotte Islands 1958 Jul. 10 58.6 137.1 7.9 Lituya Bay Prince William Sound to the Aleutian Islands: 1902 Jan. 1 55 165 7.8 25 Unimak Island 1903 Jan. 2 57 156 8.3 100 Alaska Peninsula 1904 Aug. 27 64 151 8.3 25 Alaska Range 1905 Feb. 14 53 178 7.9 Andreanof Islands 1906 Aug. 17 51 179E 8.3 Rat Islands 1907 Sep. 2 52 173E 7.75 Near Islands 1929 Mar. 7 51 170 8.6 so Fox Islands 1938 Nov. 10 55.5 158 8.7 25 Alaska Peninsula 1940 Jul. 14 51.75 177.5 7.75 70 Rat Islands 1957 Mar. 9 51.3 175.8 8.3 Andreanof Islands 1964 Mar. 28 61 147.7 8.5 33 Prince William Sound 1965 Feb. 4 51.3 178.6E 7.9 40 Rat Islands a Revised magnitudes are from Thatcher and Plafker (1977). Woodward-Clyde Consultants -8- Fig. 5} show a nearly horizontal Benioff zone beneath the continental shelf, a 15° northwest dip inferred beneath the Bradley Lake area, and a 35° dip beneath the volcanic line. Using the interpretations in Figure 5, it appears that the Benioff zone may be as close as 25 to 30 miles (40 to 50 km} beneath Bradley Lake. Since the publication of the Woodward-Clyde Consultants (1978) OASES report, significant improvements have been made in the seismograph coverage around the Lower Cook Inlet. This has increased our ability to detect and accurately locate earthquakes in the Lower Cook Inlet area. The University of Alaska and the u.s. Geological Survey now operate seismographic networks that have reduced the mag- nitude detection threshold in the project area from magnitude 5 to magnitude 2 (Pulpan and Kienle, 1978}. Data from the University of Alaska stations for the period January through December 1978 show diffused shallow earthquake activity (generally smaller than magnitude 3-1/2) on the Kenai Peninsula, with no apparent alignments or obvious relation- ships to known structural features. Clusters of shallow seismicity in the Lower Cook Inlet area have been associated with the active volcanism along the Aleutian Peninsula (Peterson, 1979). Historical Earthquakes In this seismicity study, review the characteristics as of in others, historical it is useful to earthquakes that have occurred in the region of study and to review the styles and magnitudes of associated deformation and the kinds of resulting damage. This type of review provides insight when evaluating the styles and amounts of surface deformation and Woodward-Clyde Consultants -9- damage due to ground shaking from future earthquakes. Because of the remote location of Bradley Lake, little or no site specific data are available on ground shaking effects during historic earthquakes. Table 2 summarizes the known earthquakes from 1788 to 1961 within about 100 miles of Homer (the closest inhabited area to Bradley Lake) (Waller, 1971) • One earthquake identified on Table 2 had an epicenter located near the Bradley Lake area. The earthquake had a focal depth of 26 miles (43 km) and an intensity of I to III was reported 5 miles (8 km) northwest of Homer. The 1964 Prince William Sound earthquake (magnitude 8-1/2) is the largest earthquake known to have affected the Bradley Lake area. The 1964 epicenter was located in northern Prince William Sound approximately 150 miles (235 km) from Bradley Lake. The focal depth of the event was between 12 to 31 miles (20 to 50 km) (Plafker, 1971). The locations of nearly 600 aftershocks defined a rupture surface in the Benioff zone that extended from the eastern Gulf of Alaska to Kodiak Island, a distance of 500 miles (800 km). Stauder and Bollinger (1966) showed that the aftershocks in the Kodiak Island area all had focal mechanisms that corresponded to a low-angle thrust in a northwesterly direction associated with subduction of the Pacific plate. The concentration of the aftershocks in the offshore continental shelf (Fig. 6) suggest that most of the ruptured surface for the 1964 event was restricted to the very shallow southernmost section of the Benioff zone. Some secondary effects of the 1964 earthquake, probably due to long-period ground motion, have been reported in the Bradley Lake area by Foster and Karlstrom (1967). These secondary effects included cracks, sand boils, and slumping of unconsolidated river bank sediments along the Fox River Woodward-Clyde Consultants TABLE 2 SUMMARY OF LARGER EARTHQUAKES OCCURRING WITHIN ABOUT 100 MILES OF THE HOMER AREA IN THE PERIOD 1788-1961 Local Approximate location Date time (lat. N .; long. W .) Remarks Oct. 6, 1883 0800 Augustine Island (59°, Volcanic activity; 154 °). seismic wave struck Port --------1 Graham. Aug. 1898 South-central Alaska ____ Trees swayed vio- i- lently at Susitna Station. July 11, 1899 =I Severe. Oct. 7, 1900 ' Severe. Probably is 1-_J same as following l one. Oct. 9, 1900 I 0216 South-central Alaska Severe, felt at I Seldovia. Dec. 30-31, r Volcanic eruption and 1901. several sea waves. Sept. 19, 1909 1000 Kenai Peninsula. Strong at Seward. Sept. 21, 1911 i 0701 Prince William Sou~d---6. 9 Severe, felt at and Kenai Peninsula Kenai Lake. (60.5°, 149°). Dec. 9, 1927 South-central Alaska ___ Kenai Lake severely shaken by three Jan. 27, 1931 0429 ____ do. quakes. ~-------5-6 Cracked walls in Seward. Oct. 6, 1932 0705 Homer ______ A wakened all. Apr. 26, 1933 1703 Susitna Flats (61.2.')0 , At Homer, worst 150.5°). shock in 15 years. May 13, 1933 Old Tyonek._ .. _____ Severe, some damage. June 13, 1933 0219 Northeast of 1\ikishki 6. 25 (61°, 151°). June 17, 1934 2314 Soldatna (60.5°, 151°) ____ 6. 75 Oct. 10, 1940 2153 Mouth of Kachemak Bay 6 (59.5°, 152°). July 29, 1941 1551 Northeast of Xikishki 6. 25 Damage at Anchor- (61°, 151°). age. Dec. 5, 1942 0428 Mouth of Kachemak Bay 6. 25 (59.5°, 152°). Sept.27, 1949 0530 Blying Sound (59. 75°, 7 Strong aftershock 149°). also. Damage at Seward and June 25, 1951 0612 Chickaloon Bay (61 °, 6. 25 Anchorage. Damage at Anchor- 150°). age. Oct. 3, 1954 0118 Caribou Hills (60°, 151 °) 6. 5-7 Damage at Homer. Jan. 24, 1958 1317 North-northwest of 6. 25-(.i. 5 Anchor Point (60°, 152°). Mar. 19, 1959 0503 South of Perl Island 6. 25 (58.6°, 152°). June 4, 1959 0231 50 miles W of Homer 5. 5 (59.5°, 153°). Dec. 26, 1 959 0819 Stariski Creek (59.9°, 6. 2.':i 151.7°). Sept. 5, 1961 0134 Bradley Lake (59.8°, 6-6. 25 Felt. Anchorage 150.6°). rocked. Sept. 24, 1 Y61 1627 Xorth of Mount Iliamna 5. 75-6 (60.3°, 153°). Source: Waller, 1971 Woodward-Clyde Consultants -10- and on the stream banks southeast from the inlet to Bradley Lake. Waller (1971) described the damage in Homer as relatively light, in comparison to affected areas closer to the main shock. Waller also reported Modified Mercalli intensity estimates of VII and VIII for the 1964 earthquake in Horner. Reported ground effects occurred principally along the edges of the sea bluff and on the sand spit at Homer. These effects included earthflows, several terrestrial and submarine lands! ides, and fissuring of the ground (Waller, 1971). Reports also suggest that seiche waves, and possibly tsunamis, entered the mouth of Kachemak Bay (Waller, 1971) • The most pronounced effect of the 1964 earthquake on the Kenai Peninsula was subsidence and widespread warping of the ground surface. Figure 7 shows the distribution and amount of uplift and subsidence in 1964 (Plafker, 1971}. From this figure, it can be seen that the Bradley Lake area lies within the contour that represents about 4 feet of sub- sidence, whereas the maximum subsidence of 7 1/2 feet occur- red along the southwestern coast of the Kenai Peninsula (Plafker, 1971). Crustal warping associated with the earth- quake tilted the basin of Kenai Lake (Fig. 1); the western end of the basin sank three feet with respect to the eastern end (Foster and Karlstrom, 1967; Plafker, 1971). Little or no tilting occurred at Bradley Lake (Foster and Karlstrom, 1967). Investigations following the 1964 Prince William Sound earthquake located several features suggestive of secondary faulting in the crust over the Benioff zone. The features closest to the Brad ley Lake site were: ( 1} a broad north- south-trending topographic expression referred to as the Kenai lineament (Plafker, 1971}; and (2} a zone of extensive ... - .... - EXPLANATlON -s--··~ ............ . lsobase contour, showing uplift ( +) or ~uhsidence (-) in feet Da.,hed where ttppro.rimalel11 lo- cn!Hf: dolt(;(} where iHferred ------r-- ~ pproximate axis of maximum subsidence ~ . ',,\\\\\\\\1\\\ 62° Po,;,!ble zone of slight uplift lless than 2 ftJ I lJ .S. Coast and Geodetic Survey first order level net · Acti\·e or dormant volcano ·· ... Source: Plafker (1971). Project: Project No. BRADLEY LAKE 41229A . ..... -~ ....... ····· .... ··· SUFH-1Ai?INE rDNTCUR~ L"< F[Ef TECTONIC UPLIFT AND SUBSIDENCE 1964 GREAT ALASKAN EARTHQUAKE ..,., Fig. 7 WOODWARD -CLYDE CONSULTANTS Woodward-Clyde Consultants -11- ground cracking in the Kenai lowlands (Foster and Karlstrom, 1967). These two occurrences are important because their structural trends parallel structural trends in the Bradley Lake area. The Kenai lineament (Plafker, 1971) extends northward from Resurrection Bay and is located approximately 52 miles ( 79 km) east of the site (Fig. l). The evidence suggestive of faulting included: {1) a concentration of fissures, ap- parently unrelated to the seismic shaking; (2) up to five feet of left-lateral displacement across the lineament determined by pre-and post-earthquake triangulation surveys; and ( 3) distinct changes in the trend of isobase contours (contours of equal uplift or depress ion) across the linea- ment. In summary, the evidence suggests either slight movement on a concealed north-south-trending fault or crustal warping localized along the lineament (Plafker, 1971). The Kenai lineament is located in the same structural and lithologic terrain, the "Chugach terrain," as the Bradley Lake site and has a similar north-south trend as the Bradley River and Bull Moose faults which cross the site (see Section 3.3). The zone of extensive ground cracking in the Kenai lowlands is the second feature near Bradley lake suggestive of secondary faulting during the 1964 earthquake. This zone of cracking is approximately 40 miles (64 km) northwest of Bradley Lake (Fig. 1). The zone is important to the Bradley Lake site because it suggests the poss ib il i ty of faulting, associated with the 1964 earthquake, at distances farther north of the Aleutian Trench than the site. It has also been suggested that the ground cracking may have been the -12-Woodward-Clyde Consultants surface expression of fault movement at depth (Waller, 1971), possibly along the Border Ranges fault (Section 3.3), one of the closest major fault to the site. The ground cracks extended from near the town of Kenai northeast to Turnagain Arm. Foster and Karlstrom ( 1967} suggested that the ground cracks may have been produced by: (1) movement along a buried fault; (2) differential compaction of Quaternary sediments along a buried bedrock ridge; ( 3) gravity sliding along the interface between unconsolidated Quaternary sedi- ments and the Tertiary bedrock; or (4) a combination of all these processes. The absence of aftershock activity in or near the zone of ground cracking and the inability to detect vertical dis- placements across the zone with geodetic surveys led Plafker (1971) to suggest the ground cracks were not due to dis- placement along a buried fault. The data were insufficient to resolve which process caused the ground cracks. The Bradley Lake site lies in the same tectonic environment as the epicenter of the 1964 earthquake. With the Benioff zone located directly beneath the site, future great earth- quakes can be expected to occur on the zone at minimum distances beneath the site of about 25 to 30 miles (40 to 50 km). If future great earthquakes occur this close to the Bradley Lake site, the affects will be significantly more severe than those recorded in the area in 1964. Some of the more pronounced of these effects that could influence the Bradley Lake Project include tectonic subsidence, which may influence the tailrace tunnel flow, seiche waves on the lake, general ground instabilities due to shaking, and possibly even ground ruptures along faults. Woodward-Clyde Consultants -13- Correlation of Ear with Faults The historical earthquake data and procedures used in the OASES report (Woodward-Clyde Consultants, 1978) to correlate earthquakes with faults were adequate for the objectives of this study. Briefly, the correlation procedure consisted of the following: 1. Earthquakes in the historical record as shown on epi- center maps were used as overlays to geologic structure maps. 2. Consideration of earthquake location inaccuracies were accounted for in the assignment of earthquakes to particular geologic structures. 3. Focal depths of the earthquakes provided an additional means of subdividing the seismicity for assignment to appro- priate sources. Due to artifacts in the reporting procedures for focal depth in the region, some earthquakes were statist- ically weighted for assignment with shallow or deep sources. These correlations were used both in the assessment of maximum earthquakes (see Section 3.4) and to develop recur- rence estimates for given sources (see Section 4.0). Detailed discussions of these procedures are provided in the OASES report (Woodward-Clyde Consultants, 1978). As in the OASES work, earthquakes that could not be correlated with a known geologic feature were presumed to have occurred on random sources that have the potential for occurring anywhere in the region. Woodward-Clyde Consultants -14- 3. 0 POTENTIAL EARTHQUAKE SOURCE AND EVENT CHARACTERIZATION 3.1 General Background This section provides brief discussions of the known char:- acteristics of regional faults and the longest local faults around the Bradley Lake area that are considered potential earthquake generating sources. The faults and their esti- mated parameters are summarized in Table 3; their locations with respect to Bradley Lake are in Figure 1. Generally, Table 3 shows faults that have reported Quaternary activity and/or historic seismicity; however, for some of the faults 1 is ted in Table 3, such as the Border Ranges ul t or the local faults in the site area, evidence of Quaternary displacement has not been reported in the literature. There are several reasons for considering these faults: ( 1) there is a reasonable chance that such faults are subsidiary faults to the underlying subduction zone and may be active; (2) to illustrate their close location to, or even within, the study area; (3) to evalute the impact they would have on the design earthquake if they are shown to be potentially active in future studies; and (4) to help establish a priority of importance for future studies of faults in the project area. 3.2 Regional Sources Benioff Zone The Benioff zone underlies the entire Lower Cook Inlet and Kenai Peninsula region; its characteristics have been discussed in Section 2. 0. The seismicity in this reg ion is dominated by the activity of the Benioff zone. The subduct- ing plate passes beneath the Bradley Lake area at a minimum TABLE 3 EARrHQUAKE SOURCE CHARACTERIZATION -SITE GROUND l-'OTICN PARAMETERS Maxl.ffium Estma ted Est11Tia.ted Earthquake Estfinated Maxl.ffium Estimated Maxl.ffium Minimum Source Historic Maximum Recurrence Relationshipsd Accelerations (g) Velocity em/sec Name of Earthquake Distance to wngth Earthquake Earthquake Maximum Estimated Historic Maximum Historic Maximum Source Dam Site (km) Type of Source (Ms) (Ms) Historic Maximum Earthquake Earthquake Earthquake Earthquake Regional Source Benioff Zonea shallow 85-95 underlies 0.004 0.004 0.25 0.25 10 10 intermediate & deep 40-50 entire Megathrust fault 8-1/2 8-1/2 0.010 0.010 0.57 0.57 20 20 region Defonned Zone 120-130 20-soc Reverse & Thrust 6 7 1.394 0.444 .01 0.01-3 ( Includi119' outer-faults 0.03 shelf faults) Volcanic chain 130 -Volcanic eruptions 5-1/2 5-3/4 0.268 0.183 <0.01 0.01 <1 1 Castle Mountain Fault 160 lOOC Right-lateral fault 7 7 .009 .009 0.01-0.02 0.01-0.02 2 1-2 Bruin Bay Fault 120 woe Reverse fault 7-1/3 7-1/2 --0.02-0.03 0.02-0.04 2-4 2-4 Johnstone Bay Fault 125 soc N:::n:rnal ( ? ) 4-1/2 6-1/2 7.2 0.016 <0.01 0.01-0.02 <1 1 Contact Faultb 70 lOQC Reverse fault 7 ---0.05 3-5 Placer River Faultb 85-90 70 Reverse fault -6-1/2 -0.001--0.02-2 0.005 0.03 Local Sources Border Ranges Faul tb 6-10 lQOC Reverse fault -7 -0.00003 -0.47-30 thrust 0.62 Eagle River Faultb . lOOC Reverse to -7 - - -0.6-0.68 35 thrust fault Bradley River Faultb 1.2 19 Right lateral oblique -6-1/4 ---0.60-0.68 23 slip fault Bull Moose Faultb 3.5 11 High-angle oblique -6 ---0.43-0.58 16-27 slip fault Randan Source 5 underlies non-associated 5-3/4 5-3/4 0.58 0.58 -0.33-0.43 6-11 6-11 entire region earthquakes NOI'ES a b The Benioff zone has been divided into three sections based on seismological data, see Sections 2.2 , 3.2, and Figure 5 of text. Quaternary activity of fault not noted in literature, conservatively assigned to table because of tectonic environment, closeness to site, possible future research may disclose some activity. c Length assigned to source in OASES report or consistent with methods used in OASF~ (Woodward-clyde Consultants, 1978). d Recurrence values fran OASES (Woodward-Clyde Consultants, 1978). In terms of mean number of earthquake occurrences in 40 years. -15-Woodward· Clyde Consultants depth of approximately 25 to 30 miles (40 to 50 km). Although no great earthquakes in the Benioff zone are known to have occurred within the Kenai Peninsula region during historic time, the reg1on is just northwest of the area of aftershock activity that followed the 1964 (magnitude 8-l/2) Prince William Sound earthquake. The maximum magnitude of 8-l/2 is estimated for a future earthquake associated with the Benioff zone beneath the Bradley Lake area. The shallow section of the Benioff zone with a nearly horizontal dip (Section 2. 2 and Fig. 5) 1s pres en ted separately from the steeper dipping intermediate and deep sections of the zone on Table 3. This was done because the shallow section appears to have a markedly different spatial extent and seismicity distribution. Offshore Deformed Zone and Outer Shelf Fault The deformed zone of the Kodiak continental shelf extends from south of the Trinity Islands parallel to the continental shelf edge and projects northeastward onto Montague Island (see Fig. 1). Although no single major fault has been defined in this zone, the structure has been characterized as a continuous zone of intense deformation and faulting (von Huene and others, 1976). Correlations of individually recognized faults over appreciable lengths have not been possible from the available data (Bouma and Hampton, 1976). The deformed zone was first recognized as a result of the 1964 Alaska earthquake. Based on arrival times of seismic sea waves and aftershock activity, the deformed zone appears to have been the center of dislocation and major differential uplift 1n the crust overlying the Benioff zone during the 1964 earthquake (Plafker and Kachadoorian, 1966; von Huene and others, 1976). Woodward· Clyde Consultants -16- The maximum uplift according to von Huene and others (1976) was about 50 feet (15 m). Malloy and Merrill (1972) docu- mented significant displacements off the southwest end of Montague Island suggesting a broad 1 complex zone of fault- ing. On Montague Island, 39 feet (12 m) of vertical uplift was measured (Plafker, 1971). The Patton Bay fault and the Hanning Bay fault, both on the island and approximately 160 miles (257 km) east of Bradley Lake, were the only two mapped faults with surface displacement during the 1964 earthquake (Fig. 1). Plafker ( 1971) described these faults as north- ward-dipping high-angle reverse faults within the upper plate and subsidiary to the subduction zone. A maximum of 26 feet (8 m) of relative displacement was measured across these faults following the 1964 earthquake. Several active faults have been mapped between the deformed zone and the Aleutian Trench. Von Huene (1972) postulated that these faults are subsidiary thrusts branching upward through the upper 3 to 6 miles (5 to 10 km) of crust to the continental shelf slope. Volcanic-Induced Ear s Earthquake activity is typically associated with active volcanism. Either the movement of magma or adjustments of the earth's crust due to stress caused by typical volcanic processes can cause small-to-moderate earthquakes at moder- ate-to-shallow depths. Occasionally, more severe volcanic processes occur, such as ph rea tic explosions and explosive caldera collapses. These are rare events 1 but they may be as soc ia ted with significant earthquakes. The active vol- canoes near the Bradley Lake area are Augustine, Iliamna, Redoubt, Spurr, and Douglas, all more than 130 km from the Woodward· Clyde Consultants -17 site (Fig. 1). A tsunami associated with an Augustine eruption in 1883 shown in Table 2 (Waller, 1971) reportedly hit Port Graham on the western end of the Kenai Peninsula. The most Recent ~ruption of Augustine was in 1976. Castle Mountain Fault The Castle Mountain fault is a major tectonic structure approximately 99 miles (160 km) northwest of the Bradley Lake area (Fig. 1). It has been characterized by right-slip motion (Evans and others, 197 2). Although the fault is a very long structure, 310 miles (500 km) long (Beikman, 1979), only a small segment of it, about 80 miles (130 km) in length, near Anchorage shows indications of recent activity (Evans and others, 1972). This active segment is evidenced by lineaments and offsets in glacial deposits (Evans and others, 1972). If the Castle Mountain fault were to cause an earthquake affecting the Bradley Lake area, the event would probably be generated along that segment of the fault which shows the most recent activity. Bruin Bay Fault The Bruin Bay fault is located along the northwestern margin of the Lower Cook Inlet about 74 miles (120 km} northwest of the Bradley Lake site (Fig. 1). This high-angle reverse fault has a mapped length of over 134 miles (225 km} and it dips northward at 60°. About 2 miles (3 km} of displacement has been reported for the Bruin Bay fault by Magoon and others (1976). No surface geologic evidence has been located suggesting any activity of the fault since Tertiary time (Magoon and others, 1976). However, a small number of earthquakes have been associated with the Bruin Bay fault Woodward-Clyde Consultants -18- (Woodward-Clyde Consultants, 1978) suggesting the fault lS apparently still active. The largest of these earthquakes on November 3, 1943 had a magnitude of 7.3. Johnstone Bay Fault The Johnstone Bay fault is located east of Bradley Lake along the western margin of Prince William Sound (Fig. l) and is as close as approximately 76 miles (125 km) to the site. The fault trends N35°E. The fault was first mapped as a linea- ment on aerial photos by Condon and Cass (1968) who connected it with a similar structure near Icy Bay, a fault length of about 45 miles (70 km). On the basis of a fresh scarp the fault was later classified as an active normal fault by Brogan and others (1975). The scarp was five to ten feet (1.5 to 3 m) high, indicating that faulting was down on the northwest side. Based on the youthfulness of trees growing on the scarp Brogan and others (1975) suggest that the most recent displacement along the fault may have occurred within the past 100 to 200 years. There was no evidence of rupture along the fault during the 196 4 earthquake. Ty sd al and Case (1979) disputed the recent activity of the fault and claimed the fault was a thrust rather than a normal fault. The Contact Fault The Contact fault is a major structure that extends along the margin of the Gulf of Alaska. It extends from Kodiak Island, through the Prince William Sound area, to south- eastern Alaska--a length of over 6 20 miles ( 10 00 km) (Fig. 1). The fault has a shear zone about 1.2 to 2.5 miles (2 to 4 krn) wide and dips approximately 50° to the west in the Kenai Peninsula area (Tysdal and Case, 1979). The fault Woodward-Clyde Consultants -19- has been interpreted to be the Paleogene megathrust along which early Tertiary deep marine sediments were accreted to the overriding continental plate (Tysdal and Case, 1979; Magoon, personal communication, 1980; and Plafker, personal communication, 1980). No evidence of Quaternary activity has been reported on this fault. Placer River Fault The Placer River fault extends from Day Harbor on the Gulf of Alaska side of the Kenai Peninsula northward for 43 miles (70 km) beyond Turnagain Arm. The fault was mapped by Tysdal and Case ( 1979) as a 164 to 500 foot (50 to 150 m) wide shear zone, dipping about 65° westward, and marked by topographic notches, stream valleys, and benches. The amount of dis- placement on the fault has not been reported. Tysdal and Case (1979) reported no evidence of displacement of the recent sediments in Turnagain Arm, nor was any evidence of recent faulting reported on any other portion of the fault. The fault was included in Table 3 because of its parallel orientation with the Kenai lineament and the faults located in the Bradley Lake area and may be similar in nature. We also believe the evidence presented by Tysdal and Case (1979) supporting an inactive fault is inconclusive. The Kenai 1 ineamen t (discussed in Section 2. 2) is located just over a mile west of, and is parallel to, the trend of the Placer River fault. The lineament was not included on Table 3 because its affect on the design earthquake at Bradley Lake is overshadowed by the Placer River fault. Should future studies show the Placer River fault not active and prove the Kenai Lineament is active, the differences in the ground-motion parameters would not be significant because Woodward-Clyde Consultants -20- of the lineament's shorter length and its similar distance from Bradley Lake when compared with the Placer River fault. 3.3 Local Sources The Border es Fault The Border Ranges fault 1s a major structural zone 1n southern Alaska. The fault extends in an arcuate form from Kodiak Island for over 1056 miles (1700 km) to the eastern Gulf of Alaska. This high-angle reverse fault, which is locally a thrust fault, juxtaposes a sequence of Paleozoic and lower Mesozoic continental and shelf sedimentary and volcanic rocks on the north against a sequence of Upper Mesozoic deep water marine sedimentary rocks on the south. MacKevett and Plafker (1974) suggest the fault represents an ancient Mesozoic subduction zone. The activity of this ancient subduction zone diminished as the plate margin shifted southward to the present Aleutian Trench. No evidence has been reported that suggests post-Tertiary displacement along the Border Ranges fault, and no historic seismic activity has been clearly associated with the fault (Tysdal, 1976). Six or more aftershock epicenters of the 1964 Prince William Sound earthquake have been located near the Border Ranges fault and the Eagle River fault (Fig. 6) (Plafker, 1971). We do not really know whether or not these earthquakes are associated with these faults because of uncertainties in the dips of the faults and the inaccuracy of the earthquake locations. The main trace of the Border Ranges fault was mapped by MacKevett and Plafker (1973) along the southern shore of Kachemak Bay and nor the as tward beneath the Fox River flood ----- - "" Woodward-Clyde Consultants -21- plain, about 5 miles (8 km) north of the Bra.di.ey Lake dam site. The closest approach of this fault t.o the dam site could be as little as 3.7 miles (6 km). More recent maps by Magoon and others ( 1976) and Beikman ( 1979) n:~ta a thrust fault along the south shore of Kachemak Bay, 0ut have shifted the main trace of the Border Ranges fault to a fault trace that extends from near the town of Seldovia and projected beneath the Kenai Lowlands along the Sterling fa1.1l t (Tysdal, 1976). The location of the Sterling fault is based on subsurface oil well and geophysical data. This trace of the fault passes about 15 miles (25 km) northwest of the Bradley Lake site. Eagle River Fault The Eagle River fault has been mapped about 3 miles (4 km) south of the Bradley Lake dam site and, on the basis of its apparent dip, the fault passes about .5 to 2 miles (1 to 3 km} below the dam site. This thrust fault has been mapped parallel to the Border Ranges fault from Kodiak Island to the eastern Gulf of Alaska, a distance of 450 to 466 miles (725 to 750 km). No evidence of Quaternary activity has been reported along this fault; however, because of its location close to the project, we have included it in Table 3 in case later re- search indicates it is active. Woodward-Clyde Consultants (1979} has discussed the characteristics of the Eagle River fault in the Bradley Lake area. Bradley River Fault The Bradley River fault is a major high-angle fault which crosses the proposed tunnel alignment between the dam site Woodward-Clyde Consultants -22- and the underground power house site. The fault 1.s as close as .75 miles (1.2 km) from the dam site and has length of 12 miles (19 km). Detailed discussions of this fault are included in Woodward-Clyde Consultants report \ l 79). As indicated in that report, no direct evide11C2 f recent activity on the Bradley River fault has been found; however, the occurrence of a large landslide near the fault trace suggests that the activity of the Bradley River fault is not resolved. Bull Moose Fault The Bull Moose fault is another high-angle fault which crosses the project area near the proposed tailrace tunnel outlet portal. The fault is 7 miles (11 km) long, about 2 miles (3.5 km) from the dam site, and trends parallel with the Bradley River fault. The amount and direction of displacement on the Bull Moose fault have not been deter- mined. Like the Bradley River fault, the activity of the Bull Moose fault has not been fully evaluated. Again, more detailed discussions of the Bull Moose fault and the many shorter faults and suspected faults that cross the tunnel alignment and pass through or near the dam site are provided by Woodward-Clyde Consultants (1979). 3.4 Maximum Earthquake Magnitude Evaluation The approach used in this study and by Woodward-Clyde Con- sultants (1978) to estimate the upper magnitude for an earthquake source is to select a maximum rupture length and use the relationships developed by Tocher (1958), Patwardhan and others (1975), Bonilla and Buchanan (1970), or Slemmons (1977) to obtain the magnitude. Historical evidence from Woodward-Clyde Consultants -23 California suggests that the rupture length in any single earthquake 1s between 20 and 50 percent of the total length of the source fault, and rarely exceeds 50 percent (Albee and Smith, 1966). In some cases consideration of 20 to 50 percent of total fault length did not provid realistic values. In such cases we used the portions of the long faults considered to be potential earthquake sources. Thus, when available evidence suggests that activity is 1 imi ted only to a certain segment of the fault, upper magnitudes are based on the length of the active segment. In the case of the Benioff zone, the following considerations guided the choice of upper magnitudes: (l) historical seismicity, ( 2) recurrence estimates for large earthquakes based on space-time relationships and the presence of seismic gaps for earthquakes greater than or equal to Ms 7. 4 (see Section 4), and (3) slip rates and sizes of areas undergoing subduction. Additional discussions of the evaluations for the maximum magnitude of the Benioff zone are presented in the OASES report (Woodward-Clyde Consultants, 1978). 4.0 RECURRENCE OF EARTHQUAKES The recurrence of earthquakes that might af ct the Bradley Lake area can be estimated by using the Gutenberg and Richter ( 1954) frequency-magnitude relationship for appropriate earthquake sources and by considering the historical earth- quake record. Such an analysis was carried out in the OASES work (Woodward-Clyde Consultants, 1978) in which historical earthquakes associated with each source were plotted and an appropriate frequency-magnitude curve established; however, some sources significant to the Bradley Lake area were not considered by Woodward-Clyde Consultants (1978). To help Woodward-Clyde Consultants -24- understand the estimated recurrence of selected faults in the region, the mean number of earthquake occurrences within an arbitrary time period are listed in Table 3; these estimates are for the maximum historical and the estimated maximum earthquakes from sources that were included in the OASES report (Woodward-Clyde Consultants, 197 8) • The arbi- trary time period used by Woodward-Clyde Consultants (1978) was 40 years. This analysis 1s appropriate for the Lower Cook Inlet reg ion for earthquake sources with surface-wave magnitudes (Ms) between 5 and 7-1/2. However, for earthquakes of Ms>7-l/2 associated with the Benioff zone in the Bradley Lake area, the Gutenberg and Richter (1954) frequency- magnitude relationship was not appropriate, great earthquakes were predicted than the shows. since many more historic record The method adopted by Woodward-Clyde Consultants ( 1978) to evaluate the recurrence of the large earthquakes utilizes several spatial and temporal relationships between the occurrence of large earthquakes on the Benioff zone in southern Alaska (Kelleher, 1970; Sykes, 1971). Detailed discuss ions of this method are available in Woodward-Clyde Consultants ( 1978) • The following paragraphs provide brief discussions of the general ideas of the method. A nonrandom nature of large earthquakes in space and time in southern Alaska is suggested from an examination of Figure 8 and Table 1. Similar observations of the nonrandom occur- rence of large earthquakes have also been made at other subduction zones around the world (for example in Japan) (Morgi, 1973). Project: Project No. 1964 8.5 I 7.9 197 GULF 7. OF ALASKA LEGEND Note: Aftershock zones; dashed where extent of zone is uncertain Date and magnitude for respective aftershock zone The February 28, 1979 event is.the first to occur between the rupture zones of the 1958 Fairweather earthquake and the 1964 Prince William Sound earthquake since the 1899-1900 series of great earthquakes near Yakutat Bay. Approximate rupture zone of earthquake of 28 February 1979 is shown. Source: Lahr and others (1979) and Page (1975). BRADLEY LAKE 41229A AFTERSHOCK ZONES OF EARTHQUAKES OF MAGNITUDE 7.3 OR GREATER SINCE 1938. FiQ. 8 WOODWARD -CLYDE CONSULTANTS -25-Woodward-Clyde Consultants These studies of the distribution of large earthquakes along subduction zones have shown: 1. Large earthquakes occur at periodic intervals, with smaller earthquakes clustering around them. 2. The aftershock zones of great earthquakes (Ms>7-l/2) have very little overlap. Areas between aftershock zones form "seismic gaps" in which future great earthquakes may occur. 3. The size of the gap influences the size of the maximum earthquake that may occur in the gap. 4. There is no known case of a second great earthquake striking twice in less than a few decades in the same area (Sykes, 1971). 5. There is quiescence in earthquakes of all magnitudes in a gap, especially before a great earthquake. However, a number of smaller magnitude earthquakes may occur inde- pendently of a great earthquake without seriously affecting the potential for great earthquakes. Rupture zones for earthquakes of magnitude 7. 3 or greater since 1931 (Fig. 8) are inferred from the locations of aftershocks (Sykes, 1971; Lahr and others, 1979). Several areas along the Aleutian-Volcanic arc have been identified for which there have been no recent major earthquakes of record. These areas, which abut other areas where major earthquakes have occurred, are referred to as seismic gaps. The Sitka gap was filled in 1972 (Page, 1975), leaving the gap between Yakutat Bay and the Kayak Islands in the Gulf of Woodward· Clyde Consultants -26- Alaska. A small portion of this gap was filled by the 28 February 1979 St. Elias earthquake of Ms 7.7. Although the area of the plane of rupture for this earthquake was estimated to be about 3,000 km2, it is interpreted that a significant chance remains for a large earthquake in this gap. Since the Bradley Lake area is within the aftershock zone of the 1964 earthquake, another such event there is not expected for a long time; this is in contrast to the Yakutat Bay-Kayak Island area or another area further southwest along the Aleutian Peninsula where large events are more likely to occur. 5.0 PRELIMINARY EVALUATION OF GROUND-MOTION PARAMETERS 5.1 Background This section describes the development of the ground-motion parameters that provide the basis for selection of design values. Maximum accelerations and velocities have been estimated for two levels of earthquakes--the maximum historic event and the estimated maximum event on the faults con- sidered significant to the project (Table 3). In estimating the values for maximum ground accelerations and velocities, considerations were given to factors that in- fluence the attenuation of seismic energy from its source. These factors include the source conditions (in terms of the magnitudes of associated events), the transmission path, and the site conditions. These factors are described in the following paragraphs. -27- Woodward· Clyde ConsuHants 5.2 Factors Influencing Attenuation of Seismic Energy Source Condition Source conditions that influence the level of ground motion at a site may include dimensions of the rupture surface, stress available to generate seismic motion, and amount of displacement. Because the interrelationships among these parameters are not well understood, a single parameter, the earthquake magnitude, is commonly used to represent source conditions. In the literature, both local magnitude (ML) and surface wave magnitude ( Ms) have been used. In apply- ing the various available attenuation relationships, Wood- ward-Clyde Consultants (1978) used the appropriate magnitude consistent with the relationship. Transmission Path During their travel from the source to the site, the seismic waves are attenuated due to geometric spreading and internal damping within the material along the travel path. The significant transmission path factors that influence the attenuation of seismic waves are the path length (or source- to-site distance) and the character is tics of the rna ter ials along the path, such as soft or hard rock. We recognize two types of transmission paths for seismic waves in the project region: (1) paths for events with shallow focal depths (hypocenter distance <20 km); and (2) paths for events on the dipping Benioff zone (with focal depths >20 km). Most of the attenuation relationships available in the literature model the shallow transmission path to some -28-Woodward-Clyde Consultants extent. However, they do not model the deeper path sat- isfactorily. On the basis of analysis of available earth- quake records from both shallow and deep events, Woodward- Clyde Consultants (1978) developed attenuation relationships appropriate for these deep events. A comparison between attenuation relationships developed in that study for events with shallow and deep focal depths for rock sites is shown in Figure 9. The comparison reflects the observation that seismic waves from events occurring in the Benioff zone are transmit ted upward along the zone with relatively less attenuation than the waves from shallow subcrustal zones. Site Conditions In addition to the characteristics of the transmission path, the level of ground motion is also influenced by the char- acteristics of the near-surface materials at the site. For example, surface ground motions are generally larger in soft unconsolidated soil than in hard bedrock. Thus, events with similar magnitudes and transmission path characteristics may generate different levels of ground motion for rock sites as compared to soil sites. For the Bradley Lake area, we have used attenuation relationships for rock sites. 5.3 Ground-Motion Parameters for the Site We evaluated the ground-motion parameters in terms of the estimated maximum ground accelerations and velocities for postulated events associated with various sources. These sources include faults which would be associated with shallow earthquakes, (focal depths <20 km) and the Benioff zone which is as soc ia ted with deep sources (focal depth < 20 km) . For shallow events, associated with the various faults signif- icant to the site, ranges of maximum acceleration and 900 Shallow Events (h<20km) ------Deep Events (h>20km) 800 h = Focal Depth I '\t I I 700 ~r---t----~ ~~-~~ , ' I \+ 1 I 600 ~-' --+-----L~- fJ) \ ! -fJ) -E u c 500 0 ·;:; <l:l .._ ClJ ClJ u u 400 <t: E ::J E X <l:l ~ 300 100 ~---+ OL_ __ L_ __ J_~~~~~~~~ 1 3 10 30 Distance {km) Note: These attenuation relationships were developed specifically for the active subduction zone of Alaska. 100 300 1000 {After Woodward Clyde Consultants, 1978) 1roject: BRADLEY LAKE >reject No. 41229A ATTENUATION RELATIONSHIPS FOR TRANSMISSION PATHS Fig. FOR SHALLOW AND DEEP EVENTS-ROCK SITES 9 WOODWARD -CLYDE CONSULTANTS Woodward-Clyde Consultants -29- velocity values are based on the various published attenua- tion relationships (Schnable and Seed, 1973 and Idriss and Power, 1978). For deep events associated with the Benioff zone, we used the attenuation relationships developed by woodward-Clyde Consultants (1978). For both shallow and deep cases we used relationships for rock sites and appropriate transmission paths. Considerations included in the develop- ment of the estimates for the maximum earthquake on a fault are discussed in Section 2. Table 3 summarizes the ranges of estimated maximum accelerations and velocities for the sources identified as significant to the Bradley Lake project. The OASES report (Woodward-Clyde Consultants, 1978) contains a seismic-exposure evaluation for the Lower Cook Inlet region, which includes the Bradley Lake site. That analysis included the various seismic sources in the region (including Benioff zone as the dominant source) and estimated both recurrence relationships and appropriate attenuation rela- tionships. The analysis considered a period of interest of 40 years and values of parameters for events with a return period of 100 years. This is equivalent to a 33 percent probability of exceedence in the next 40 years. The analysis indicated that for the Lower Cook Inlet region, a maximum acceleration of 0.2g to 0.25g and a maximum velocity of 0.5 em/sec to 9 em/sec have a 33 percent probability of exceedence in the next 40 years. It should be noted that the OASES evaluation (Woodward-Clyde Consultants, 1978) was for a large region, whose overall seismicity is dominated by the Benioff zone. The results of the OASES analysis may not be directly related to the seismic hazards of the Bradley Lake site, which based on the -30-Woodward-Clyde ConsuHants data presented in Table 3 may be domina ted by local faults tllat were not included in the OASES analysis. Thus, ad- ditional site-specific studies and probabilistic evaluations of ground-motion parameters for the project may be needed. 6.0 DISCUSSION The results of this study indicate the earthquake sources that dominate the seismic design parameters for the pro- ject may be those near Bradley Lake; this is because the potential ground motions generated from source faults nearest the site, if active, will overshadow the affects on the design of the more distant and in some cases larger sources. Although sources such as the Eagle River, Bradley River, Bull Moose faults and the many other shorter faults in the site area may be very old (MacKevett and Plafker, 1974 and Magoon and others, 1976); unfortunately, their degree of recent activity has not been studied in detail. Conclusions regarding whether or not recent activity has occurred on these faults are hampered by lack of data. In addition, as discussed in Section 2. 2 the seismic record for the area is too short and the potential errors in earth- quake locations too large for an adequate correlation of se1sm1c activity with specific local faults. Data from recently installed seismographic networks in southern Alaska may be of use in increasing the reliability of correlating seismicity with these local faults. It has been suggested that the potential for future seismic activity may be low on the major regional thrust faults such as the Border Ranges, Eagle River, and Contact fault ( Plaf- ker, 1980, personal comlT!unication). This was inferred Woodward-Clyde Consultants -31- primarily from the hypothesis that the active subduction zone has shifted southward away from these older structures. Supporting this interpretation is the paucity of faulting in the young bottom sediments of Lower Cook Inlet reported by Bouma and Hampton (1976) and Bouma (personal communication, 19 80). Also, in the Gulf of Alaska, only two faults, the Patton Bay and the Hanning Bay faults on Montague Island are known to have had unequivocal subsidiary surface displace- ments during the 1964 Prince William Sound earthquake; however, it is very likely that many other off shore faults had similar displacements but were not detected following the earthquake. These two faults are not associated with any of the major ancient thrust faults of southern Alaska, such as the Border Ranges and Contact faults. The active Castle Mountain fault and other documented active faults inland from the continental margin do not support this hypothesis. The evidence collected on the Kenai lineament and the zone of ground cracking in the Kenai lowland in 1964 and evidence collected on the Bradley River fault at the site (Woodward-Clyde Consultants, 1979) introduces suspicions of the level of fault activity in the Kenai Peninsula. Avail- able data is lacking on the level of activity on the Border Ranges, Eagle Mountain, Bradley River, Bull Moose, and the other faults in the area of the Bradley Lake site. We conclude the Army Corps of Engineers should conduct ad- ditional evaluations of the displacement and earthquake potential of these faults. The results of future fault studies may require a re-evaluation of the ground-motion parameters presented in Table 3. The primary basis for the results presented in this report are in large part from Woodward-Clyde Consultants (1978). We Woodward-Clyde ConsuHants -32- believe that the data available in Woodward-Clyde Consultants (1978) apply to this seismicity study for the Bradley Lake Project, considering the purpose and scope of this report. In using the data presented herein, it should be emphasized that the approaches used and the data contained in Woodward- Clyde Consultants {1978) covered large regions and the objectives were influenced by different design criteria than would be used for the Bradley Lake Hydroelectric Project. Therefore, additional studies are required to collect and update the seismicity data in order to obtain results which are most sui ted for the seismic design of the Bradley Lake Project. 7. 0 RECOMMENDATIONS These recommendations for future studies address the problem of determining the level of potential activity on the local faults around Bradley Lake and increasing the knowledge of the seismic conditions which will influence the Bradley Lake Project. Geologic and seismologic studies should be completed. The results of these studies may significantly increase the level of understanding of the seismic hazards at Bradley Lake and may require a re-evaluation of the estimated ground-motion parameters presented in this report. The results of these future studies should be included in a probabilistic evaluation of the ground-motion parameters applicable to the Bradley Lake Project. The recommended geologic and seismological studies are discussed in the following sections. 7.1 Geologic Studies Little or no data is presently available on the degree of activity of the local faults around and within the Bradley Woodward-Clyde Consultants -33- Lake area. A comparison of estimated ground-motion para- meters presented in Table 3 shows the dominating influence these local faults have on the seismic design considerations for the project. This fact stresses the importance of conducting the following recommended detailed geologic investigations into the activity of these local faults. Similar recommendations were provided by Woodward-Clyde Consultants (1979). 1. The Bradley River, Bull Moose, and other faults within the project area should be examined in detail using low-sun-angle aerial photographs and detailed geologic mapping to identify possible evidence of fault activity and locations where Quaternary deposits may be sufficient to trench and log any fault features. One possible location was identified in Woodward-Clyde Consultants (1979) where the Bradley River fault crosses beneath a Quaternary till, near a large landslide. 2. An independent rev 1ew should be made of the subsurface data used by Magoon and others (1976) and if possible any other subsurface data, such as oil company geophysical data, that may provide information on the activity and subsurface characteristics of the Border Ranges fault. This review could help identify the main traces of the fault and should establish the reliability of the data for detecting evidence of past movement on the Border Ranges fault. 3. Investigations of the activity of the Border Ranges fault and the Eagle River fault need to be completed 1n detail. These studies should not only be done 1n the Bradley Lake area but also in other areas where Quater- nary deposits may be located, and used in evaluating the Woodward-Clyde Consultants -34- Quaternary history of the fault. rrhe approach recom- mended here involves: (a) obtaining and analyzing low-sun-angle aerial photographs; (b) conducting hel i- copter reconnaissance; and (c) detailed mapping and trenching of selected critical location. One possible location along the Border Ranges fault where additional detailed geologic investigations may be useful was identified by Tysdal (1976). This location 1s referred to as the Bear Creek Upland (northeast of Tustumena Lake) where several Quaternary glacial deposits reportedly overlie the projected traces of the Border Ranges fault. Other similar areas may be located during investigations along other portions of the Bradley River and Eagle River faults. Particular emphasis should be place on determining as accurately as possible the geometry of the Eagle River fault in the project area; as th fault's subsurface location with respect to the dam site significantly affects the design ground-motion parameters. 4. A geologic review, supported by aerial photographic analysis and helicopter reconnaissance should be com- pleted along the Kenai lineament and the zone of ground crack in the Kenai lowlands. The investigation may also require selected geologic mapping and trenching of significant features. These features have trends similar to some of the faults located in the project are and were suspected of being related to active faulting during the 1964 earthquake. Data retrieved from this investigation may provide some insight into the potential activity of the faults near Bradley Lake. Woodward· Clyde Consultants 35- 7.2 s The following recommendations are aimed at increasing the level of confidence in: (1) estimates of maximum earthquakes on given faults; (2) geometry of local faults in the Bradley Lake area; ( 3) activity of local faults in the Bradley Lake area; and (4) available recurrence information. 1. The historical data used in Woodward-Clyde Consultants' OASES report (1978) with regard to location and focal depths should be reexamined. The scope of work for OASES was that of a broad regional assessment and did not include the detailed site specific type of investigations that would be useful for the Bradley Lake area. Improved locations of the smaller and larger historic earthquakes can be obtained using new earthquake location techniques appropriate to the tectonics of the Bradley Lake area. 2. The data set developed by Woodward-Clyde Consultants ( 197 8) for the Lower Cook Inlets reg ion should be aug- mented with recently available and more reliable data on the locations of small magnitude earthquakes. This will allow the determination of the geometry of local faults and their level of activity. 3. The results of recommendations 1 and 2 should be used to assess the geometry 1 extent of activity 1 and recur- rence of significant earthquakes on various features of interest. REFERENCES CITED Woodward-Clyde Consultants Albee, A. , and Smith, J. , 19 6 6, Earthquake characteristics and fault activity in southern California, in Engineering Geology in Southern California: Association-of Engineer- ing Geologists, Los Angeles Section, Special Publication, p. 9-33. Beikman, H., 1979, Preliminary geologic map of Alaska: u. S. Geological Survey, scale 1:2,500,000. Biswas, N. N., 1977, Implications of north Pacific plate tectonics in central Alaska: Focal Mechanisms of earth- quakes: Final Report submitted to the National Science Foundation, Geophysical Institute of the University of Alaska, p. 27. Bonilla, M., and Buchanan, J., 1970, worldwide historic surface faulting: Survey Open-File Report. Interim u. s. report on Geological Bouma, A. H., and Hampton, M.A., 1976, Preliminary report on the surface and shallow subsurface geology of Lower Cook Inlet and Kodiak shelf, Alaska: U. S. Geological Survey Open-File Report 76-695, 9 plates, 36 p. Brogan, G. E., Cluff, L. S., Korringa, M. K., and Slemmons, D. B., 1975, Active faults of Alaska: Tectonophysics, v. 29, no. 1-4. Evans, C. D., Buck, E., Buffler, R., Fisk, G., Forbes, R., and Parker, w., 1972, The Cook Inlet environment, a background study of available knowledge: Department of the Army, Alaska District, Corps of Engineers, in accord- ance with Contract No. DACW 85-72-C-0052. Foster, H. L., and Karlstrorn, T. N. v., 1967, Ground breakage and associated effects in the Cook Inlet area Alaska, resulting from the March 27, 1964 earthquake: u. s. Geological Survey Professional Paper 543-F, 28 p. Idriss, I. M., and Power, M. S., 1978, Peak horizontal accelerations, velocities and displacements on rock and stiff soil sites for moderately strong earthquakes, submitted to Bulletin of the Seismological Society of America for possible publication, April. Kanamori, Hiroo, 1978, Review article Quantification of Earthquakes, Nature, v. 271, p. 411-414. Kelleher, J. A., 1970, Space-time seismicity of the Alaska- Aleutian seismic zone: Journal of Geophysical Research, v. 75, p. 5475-5756. Woodward-Clyde Consultants Lahr, J. c., and Page, R. A., 1977, Earthquake activity and ground shaking in and along the eastern Gulf of Alaska in Environmental Assessment of the Alaskan Continental Shelf: Quarterly Reports of Principal Investigators, October- December 1977, v. 11, u. S. Department of Commerce and u. s. Department of Interior, p. 361-368. Lahr, J. c., Plafker, G., Stephens, c. D., Fogleman, K. A., and Blackford, M. E., 1979, Interim report on the St. Elias earthquake of 28 February 1979: U. S. Geological Survey Open-File Report 79-670, 35 p. MacKevet t, E. M. , and Plafker, G. , 197 4, The Border Ranges fault in south-central Alaska: U. S. Geological Survey Journal of Research, v. 2, no. 3, p. 323-329. Magoon, L. B., Adkison, W. L., and Egbert, R. M., 1976, Map showing geology wildcat wells Tertiary plant fossil locations, K-Ar age dates, and petroleum operations, Cook Inlet area, Alaska: U. S. Geological Survey Map I-10 19, 1:250,000. Malloy, R. J., and Merrill, G. 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