ML19250B671
| ML19250B671 | |
| Person / Time | |
|---|---|
| Site: | Skagit |
| Issue date: | 09/21/1979 |
| From: | Menard H INTERIOR, DEPT. OF, GEOLOGICAL SURVEY |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19250B664 | List: |
| References | |
| NUDOCS 7911010292 | |
| Download: ML19250B671 (13) | |
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United States Department cf the Interior
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GEOLOGICAL SURVEY
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b ['T yw , . RESTON, VA. 22092
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,5 " In Reply Refer To: -
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. Mail Stop 905
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. Mr. Haro d Denton Director of the Office of Nuclear Regulations. _ .
U.S. Nuclear Regulatory Commission
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Washington, D.C. 20555
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Dear Mr. Denton:
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Transmitted herewith, in response to the reques t by your staf f, is a supple =ent to our reviev (February 23, 1978) of the geologic and seiscologie data relevant to the Skagit Nuclear Power Proj ect, Units 1 and 2 (NRC Docket Nos. 50-522 and 50-523). . This supplecent was prepared prior to the cenpletion of a review of
. certain proprietary seismic profiles which have-not yet been received by the U.S. Geological Survey. Any i= pact of the review of these profiles will be transmitted at a later date.
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This supplement was prepared by Willia = H. Hays and Stanley R.
,. Brock =an. Assistance was provided by Richard J. 3lakely, Robert H.
Morris and J.ucs F. Devine. .
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Sincerely yours, ~
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b H. Willia = Menard
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Director
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Enclosure
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p p ,, , .. 1280 157
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y.- . i l i k * /_g .\ One Hundred Years of Earth Science in the Public Service
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- O i D Status Review
.- [.. d w N) e .S. I s.i Septo:bc 13, 1979
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Puget Sound Power-& Light C::pany .djn[? . Skagit Nuclear Power Project, Units 1 and 2 fif '
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Project No. 514 Skagit County, Washington
*(: -- NRC Docket Nos. SIN 50-522 and 50-523 7' . ' :.
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Since submitting its last Status Review on the p cposed Skagit
;, Nuclear Power _ Project on February 2_3, 19 7 8, _ the_ U.S. Geological Survey ,
(USGS) has received and reviewed a major sub=ittal f ro= the Puget Sound
Power & L1gh't Company (PSPL) entitled " Report of Geologic Investigations
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in 1978-1979" and dated May 27, 1979. This submittal and a f ew new "r- . questions that have arisen from ongoing stud.ics in the site region ar,e _, .
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discussed in this review.
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c., Geolog7
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The extensive investigations represented by the submittal of
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, May 27, 1979, were sticulated by new ideas regarding the geology of the
. site 'vici'nfty that developed in the coarse of. .the long-ter=.progra=. of _
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the USGS f or mapping the geology of northwestern Washington. The new
- ideas concern a tectonic cixture of meta-igneous acd cetasedi=etary ,
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rocks that is widely exposed south of the plant site in the Table -
, ,- Fbuntain-Raystack Mountain-Bald Mountain region. .(These rocks are hereaf teY 'te'rmed!"CH/Ju rocks", a ter=. coined-from sy=bols for these..,;. . .
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~' ' ' rocks on' the maps of the applicant and of .J. T Whetten.) In the . , , ,,
- Preliminary Safety Analysis Report (PSAR) of early 1978, the applicant
- identified these rocks as part of the Church Mountain thrust plate, -
- - . . dragged or pushed into this region from the east by the overriding
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' Shuksan' thrust, and as being generally correlative to the Chilliwack
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Croep of largely Paleozoic age rwhich co=pos.es that thrust. plate in the, .
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, . Cascade Range f arther east (Misch, 1966, 197.7,.1979). In id.cn,tifyin.g, ,,
'the CH/Ju rocks with the Church Mountain plate and the Chilliwack Group,.
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the applicant was probably influenced by the fieldwork of G. M. Miller
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-(1779, in press); who has sought to extend Misch's capping westward into
-the Cascade foothills. At NRC-USGS nectings with the applicant _ in May,
, 1978, and at. the Atomic Saf ety and Licensing Board (ASLS) hearing in June of' that year, J. T. Whetten of the USGS, who has been: =apping in ~
the San Juan Islands (Whetten and others, 1978) and was extending his work eastward into the mainland, proposed some =arkedly dif f erent , x relationships. He correlated the CH/Ju rocks south of the plant site with the' Mesozoic Decatur Terrane (including the Fi_dalgo Ophiolite cf , Brevn [1977] an d Brown and others (1979}) that he had =apped as a thrust sheet in the eastern part of the islands--a correlation that is
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supported by petrologic and radiometric data. Whetten presented. - > , _ . . .I , .
'- ,7 ' ctidence thnt the:c roche cre not pert cf the Church Mountain thrurt
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. . pl. ate,. structurally underlying the Shuksan. thrdst, but, rather, pcrt of. ,
2' f st; j a higher plate that has been thrust ever the Shuksan plate. These nev
' [,'- ideas, later crpanded in publications (Whetten 'and Za rtman, 1979; t f'-j,",!
- Whetten and others, 1979; Whetten and others, in press) have produced }-yp uncertainty and division in the thicking of the geologic con: unity j,9 regarding the basic structural frs=ework of parts of the Cascade . ,;, _ , foothills. Such uncertainty is of significance to review of the site
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proposal.in that it could limit-1dentification and understanding of, younger', post-thrust deformation of the rocks of the region. _ _ In hope of' resolving quickly _the; question of the basic structural; __
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relationship of the Shuksan thrust plate to' the CH/Ju rocks and also" J--
.nore specific concerns regarding possible high-angle faults of post _ . .
thrust age, the NRC and USGS requested, on June 9,1978, that the.
,, applicant make additional studies, including (1) core boring, to
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determfac the orientation of the fault contact between the Shuksan. and . a Ch/Ju rocks near Little Haystack Mountain; (2) acro =agnetic surveys, to help define tha~ location and the attitude- of the sc=e contact regicnally and to investigate.an anomaly southeast of Butler Rill that had been '. - revealed- on. an earlier, less detailed survey; and . (3) further, studies, .
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including examination of Quaternary deposits, relative to the possible-presence and significance of f an1 ting in -the vicinity of- and parallel to
. . Gilligan and Day Creeks. .
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The applicant responded vigorously to the NRC-USGS requests and
- extended. the scope of its studies slightly beyond;the requests in. order . .
-"to address more fully concerns regarding the hypothesized "B-and - -
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Jault." The applicant was unable to carry _out .co=pletely...the.. reques ted .. .
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- drilling near-Little Haystack Mountain, but drilled 11 core ho.les __there: --
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and elsewhere; carried out an extensive detailed ae'r.ocagnetic survey and 2- ' - analysis thereof; markedly increased the density of data on its regionale .
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geologic map . including data near Gilligan and Day . Creeks; carried outl..__
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on-ground gravity and magnetic surveys across the lower parts of the two creeks; reexamined photographic and radar imagery; and studied Nanaimo-- Chuckanut stratigraphy in the northern San Juan Islands. , ,
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In May and June 1979, the applicant distributed its report, " Report of Geologic Investigations in 1978-1979" (hereaf ter ter=ed "RGI"), on the studies outlined above. The principal topics addressed by the
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applicant in 'this report and o'ther aspects of regional and local geology -
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- " __ tthat hsva sucmed important in._recent considerations of the proposedm r; . _ ,
plant site are discussed briefly below: *
- 1. 1The' basic tectonic fra=cwork of the region--In its 1979 RGI, the
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applicant concludes that the controversial CH/Ju rocks south of the. _ plant. site and other exposures of similar rocks in the region. are
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, indeed parts of the Church Mountain thrust plate and structurally louer than the Shuksan thrust plate--a -conclusion consistent with m c
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- that in the earlier PStJ1 and in conflict s.it.h k'hetten's =edel of fp'w' -
the region. The applicant does =odify itsj earlier position so=cchat (p. 3.1-5) in :::: tin:; thnt the CH'/Ju rc eks probnbly 5.]k.; Z. include sone Mescaoic rockc similar to rocks in the castern San k, f Juan Islands, and the mI includes (p. 3.1-4, 3.1-5) a brief s(M. pff . description of a third structural =odel, that cf Joseph Vance,
~& . which incorporates so=e f eatures of the other two. .$ ', 1 . - " " *
- It see=s clear, regrettably, thc_I_.the issue _ raised by khetten
- regarding the place of the CH/Ju rocks in the basic tectonic _
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.-. framework of the foothill region around the proposed plant site has not been resolved by the considerable new data acquired in thc.past
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- 18 months and that..no..ccascasus on this_ proble= can_be_ expected _ _
. /. - until some future ti=e when cuch = ore of the regica has been capped - _;, . geologically in detail and = ore geophysical data have beca p acquired. The present situation of uncertainty in the geologic-e '" '"
--com= unity is exemp-lified by recent. studies. in the vicinity, of. , m., ,
. -C Little Haystack and Talc Mountains, where the aeromagnetic and
- core-hole data recently supplied by the applicant can apparent 1p be
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acccc=odated both by the applicant's capping and interpretation of the thrust and by L'hetten, Dethier, and Carrol's (1979) very "4 -' ""-diff erent mapping and interpretation.. The. =os t logical and -- . . .
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. responsible present course 1:2 the,_ evaluation.of the Skagit site seems -to be acceptance of the existence of so=e major uncertainty .
'aciottg' geologists regarding the geologic f racevork- of. the .sitenw3
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. region and recogition of this uncertainty in conservative - .-
.. evaluation'of the bearing of specific geologic features on_, site _,,,, _.
-' safety.
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'. " . 2r --T-High-afgIe: north-trending f aults near- the plant site-The,, applicant; _.
. e , u- has*' fairly i conclusively de=ons trated,- by detailed .observatiozigiGif-~- . ,
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- _, mappin,g supported by geophysical surveys, .thatNno significant, _ , . . , 1,
... north ' trending faults follow the valley floors of Gilligan.and. ~
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'7- Iower Day Creeks. The applicant has found little or no evidence _. .
E ~ for-faults within the Shuksarr metamorphic rocks. on the slopes. above .3, . g .
...<.. +~" thee creeks; but it is prudents to hear,in mind.'tha.t .s.uch _f.aulting j .,a a n]-"r might be difficult to detect.R .-Y ' O .>- - - . ... .m ,....g .
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'In two localities in-the vicinity of the plant site, on the
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-- -likelihood of high-angle f aulting younger- than- any r.cgional. ,.. uw
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thrusting is a present concern east of Gilligan Creek, along the .
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contac't' between the Shuksan 'and CH/Ju rocks, and in the valley of Loretta Creek, at and near the Chuckanut-Shuksan contact. , The applicant's mapping (RGI, -fig. 3.2-2 and Appendix H, sheet.
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.1-);of the' f ault between the Shuksan; and- the. CH/Ju rocks in . sect,iona.
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- 12, 13, an,d 24 or T. 34 N. , R'. 5 E. accords closely with b'hetten's mapping (1979), and all agree that the dip of the fault is steep
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there.' In section 1 of th,e same township, bedrock exposure is . _
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cot =only poor, and the contac.t can be icss , accurately located. Availabic outcrops and probable outcrops. suggest that it, continues, j'.. J ,- with : northerly trend, threugh abcut the ckmter of the section. .d$' i . , In the northern quarter of the section, the} f ew outcrops =ay pernd t qtif j an inference that the caatact turns abruptly cast, so as to join U the thrust contact betve:n the sa:e rock units in the northeast M.h quarter of section 6, T. 34 N. , 'R. 6 E. Even if the Shuknan-CH/Ju .
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contact follows such a courne, it sce=3'likely t!.ct ti.e sc=illnear
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high-angle f ault that for=s the contact for =iles south of section '24 1 continues on across the section to the north. Rather than a steeply folded seg=cnt of a thrust, the fault cast'of..Gilligat
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, -T , _ . . G. Creek (hereaf ter termed the "Gilligan Fault") is probably a younger
),I. #-"' . . .* . . . *'".* uts acrose the thrust. - - -
The extent of the Gilligin TaulFto theEo? h'an'd Fod'Eh Is -
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.? unknown and perhaps al=ost indeter=inable. If it extends ncrthward
~,i beneath and beyond the Skagit valley, it enters a large mountainous
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= ass of rather poorly exposed Shuksan =cta= orphic rocks; to the 5- " '"so6th, beyond upper Gilligan Creek, It is within the tectonically.
1 ^-' - nixed CH/Ju rocks. In both the Shuksan and CH/Ju terranes, the
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identification and '=spping of f aults can be very dif ficult. South ,
- of upper Gilligan Creek, the fault is conceivably related to a
. discontinucus linc. ament, apparent on_ side-looking radar (RGI, p.
i ~ 3.228 ' t 6 ~3.2-11) , that ex't ends ' south to a point near the' vest end ... , ~ ~' ~
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~ ~ of- Lake Cavanaugh, but there is little or-.no evidence that this. . . . , ,
,- lincament is related to the Gilligan Fault or to any geologic
' TtrEcture'. The of fsets in glacial lake > deposits about where this.
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lincament crosses the Lake Cavanaugh Road (Bechtel, Inc. , 1978) are
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'.,- probably contectonic. , .., , , ,
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~ " E!second locality where there is concern regarding high-angle
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~ faulfittg, the valley of'Loretta Creek (RGI,. Appendix H, . sheet.;.1;; .
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, ~ W erten and others, 1979), was-called to the attention of.USGS , . . .,.
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' ' reviewers by J. T. Wetten and-P. -R.: Carrol. Ii2 the course of',,. . ,,
. ! _. .. . . tapping geologically the lower, part of the valley, Wetten and , ,
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Carrol encountered an exceptionally good exposure of the Chuckanut-
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-Shuksan contact in a s=all waterfall, 'at an elevation of about. -
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r r mI,25 0"f'cet. ' Here the contact consists of a near-vertical.. shear. -...
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zone, at ~1 east about 6 fect wide, that appears to strike north ...; . . northwest. The zone very probably represents a significant high-a'ngle f ault of post-Chuckanut (post-Eocene?) in age. In the .
" 'ib'scnce' of strongly supportive data, the acceptance of.any other s.
' ~ l'n'tcrpretation here would be highly i= prudent. The length of . this_,,.
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fault (hereaf ter termed the "Loretta Fault") .and the a=ount and , , , exact sense of displacement on it are unknown. If it is present north of the Skagit valley, the difficulties _in =apping it there
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would bc s1=ilar to those described in the- case of the Gilligan ~._ Fa ult. ^ To the south, the f ault =ay follow the Chuckanut contact or-
=ay take another course. -.
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- 3. ne "Loveseth" Fault--The "Loveseth" Fau: t, capped as a fault by
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T. P. Loveseth (1975) and H. D..Gower (IE78) 'is interpreted by the applicent (RGI, Sect f,, 3.4.2) and by C. (M. Miller (RGI, f't. 31-M g .A ; 3, sheet .2, and in prean),. a consultant to Bechtel,.Inc., as.the .
.C' basal sedinent.ary contact of the Chuckanut For=ation, disturbed ".- A -
only 17 local shearing asso, dated with Tertiary folding. The applicant does not mention t'he. existence of any clear exposures of
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e -[.,(1. 'g the contact, and apparently its dip cannot be accurately ceasured anywhere. The geologic constraints on its location, though fairly .Y- .. tight in the northeast quarter of section 10 and perhaps in section 1.t' - - 3, T. 33 N. , R. S E'.', are co==only loose f arther southv-- On .the
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b'a sis of evidence of strong shearing and of probable discordance of beds. to the Chuckanet ru/.4 centeet, it seecrs ost likely that the
, ? ;, _, ' contact capped in sections 3,10, and 11 is a f ault. ne. ~ .
applicant's- interpr'etation cannot, perhaps , be rule'd out -here r bu6 .
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. . . .it.-is surely not unique. Farther-south, the =os: likely. course.ofc
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such. a f ault lies probably along the. sou,thwest side of the large
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volcanic body in section 14 and through the Chuckanut beyond, but other courses cay' be possible. - 'The applicant's~ espping, which
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. established the presence of Chuckanut cast of the large volcanic
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.. body. in section 14, as wel1 as. vest of -it, suggests that-the - -
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"Loveseth" Fault'is not a =ajor structure. The discordant dips in
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the .Chuckanut in the northwestiquarter of .section 13. suggest.that. . the contact there may be si=11arly f aulted. , . . . . -- -- m- a . . - ~ . .. z . :.r..-. ~.; . .p . .._, m ;
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- 4. m The '.'B and B Fault"-The possibility of a "3 and 3 Fault" was. . ,
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discussed at the ASLB hearing. in March 1978, largely as a result of. USGS concerns regarding a letter received f rom Peter Ward, a -
..._. .stratigrapher who had been carrying on research in the. northern. San . .
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- m. Juan Island.r. Ward suggestedethat the- Nanaimo and Chuckanut - 1- -
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. . . __ sedimentary rocks had been depcsited_in separate sedimentary basins . .
- = ';nnd ;that their prese'nt juxtaposition across the strait - betweenc- - -?
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Barnes and Clark Islands and Lu=..1 Island might be best explained '
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-by:large-scale strike-slip faulting. - The USGS' had not fullym . - >-
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.. .. ...., ..' , .. evalua ted War d's s ugges tion a.t the ti=e of the hearing and could *
.- only speculate regarding the' pr'oposed f ault. In infor=al ,.
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,, consideration of possibly per=issible courses for the proposed',-
f ault' south of Lumni and especially of such courses that were1..-~ "-
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. - - closest to the proposed plantw ite, it was thought that sueh a 2 "
f ault might possibly becd eastward and pass through the Table--
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- Mountain vicinity south of Cultus Mountain. It was this highly speculative, " worst-case" f ault trace, between the vicinity of
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Samish Bay on the north and the vi'cinity of Lake Cavanaugh on the south, that W. H. Hays sketched-on.a. =ap at the March 197S hearing and that was there somehow christened the "B and B Fault." .
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Since that ASLB hearing, st6d'les by the applicant and others have led to doubt regarding the possibility of large post-Chuckanut f aulting west of Lu=mi Island,.: but scem to have established the - presence of faulting near Table Mountain. In the northern San Juan
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Islands, a major fault tetween Lu==1 loland and Orcas Island <was . ,
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- first suggested by Misch (1966), and censiderable evidence pointer f g' . to faultius there t'at has affected rcch as ycung :: the ";per-M4 --Cretaceous Nanaimo Group. The possibility of faulting younger.than N il
- the Chuckanut Formaion en Luc =1 Island is, hawcVer, core dif ficult
..,Q to evaluate. Stratigrr.phic studies by the cpplicant (?.GI, section
' .9, ' . 3.4.1) suggest correlation of the Chuckanut for=ation on Lu =1 .
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. . Island with strata on part of Sucia Island and on se=c other. . .- - .U_ islands west of Lu=si and thus support so=e of the earlier similar n , conclusions of Vance (1915,.19771. _ This_ c.orrelati.pn sec=s to lack
~~. ;' the-:npport of conclusive evidence for the various deposits being
- ,3. - of'the same age but, if valid, it would sce= to preclude inference ~
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~~o f'ic ge-scale faulting west of Lu==1. Island sincc Chuckanut _.
. . . .. _ (Eocene and older) ti=e. -
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g . :. - . . .. .. maAnother development since the March 1978 hearing has been: . . e. . -- T-r"*. - increasing realization that the Chuckanut is probably younger than J.
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.i ,1 .- almost all, if not all, of the marine Nanai=o For=ation on Barnes w.- ~
' '- and Clark Islands and elsewhere west of Lu==1 and that the _- -
2: .,.- Chuckanut probably need not have been deposited in a basin . .
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" distinctly separate f rom the Nanai=o basin,- as Ward suggested. J . :
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;Thus, while large-scale post-Chuckanut. f aulting west. of. Lu==1.. . .
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' ^IsIrnd cannot at present be ruled out, strong evidence for such .
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, '" f aQlting appears to be lacking. If there is no such f auIting. -' A '-~
ry . .
. . .c ..a..,. theresis ,~;of course,. no -necess,ity of acco==oda ting gi=ilar =ov_enent.. .. .
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, e to-th'e south, along sace such structure as' the "3 and B Fault." -
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- I- 1 LIn the T.ible Mountain', vicinity f ar to the south, which Eays' . ..
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spec 61ated to be on one possible" southerly course of -the '.majore - -w - J- -- --
-Jault(suggested by Ward, continued . geologic .=apping has. Indicate.df. ;.. *- - 1. N ife' presence of a northwest-trending . fault or'. fault. = cue:(Whetten c. .. . ' I. , and others, 1979), an interpretation that is no.t necessarily' ,
fr'- ' - 1 - ' 5- inconsistent with the applicant's -field studies' (RGI, p.;:3.'4 14) ~. . - - .
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Whetten (oral co==. , 1979) believes that this f ault or f ault zoce ---- I s - has near-vertical dip and has considerably d.isrupted the CH/Ju and; .
. Shuksan plate rocks in the Table MountafE-upper Nookachamps Creek.7 '. .
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., , vie ~inity. { The age of this structure '(hereaf ter ter=ed .the. "TableJ '.].J .
--- ~ Mtfustnih-Eault") and its extent to the northwest and .setrtheast are . . -
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unknown. .
.
_S. The age of hich-angle f aulting in the site vicintv--It _see=s_ clear _ _ that a conservative geologic analysis of the site vicinity =ust m .- rvtake=futo account the probable presence of f airly numercus high- - . angle Tertiary f aults, one or two of which =ay pass within a few
-
- --- miles of the plant site. These faults include the Gilligan,. _.
'Loretta, "Loveseth," and Table Mountain Faults ~ discussed above; u ,-
~
probably-some of the f aults mapped by Whetten, Dethier, and Carrol t-C-'(l'979) and the applicant west o'f uiddle and upper Day Creek; and perhaps the Shuksan-CH/Ju contact- at "No Na:e Creek." All of these
. .. .
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.
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.
-
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'l ' ' , f aults should probably be presu=cd to bc younger than latest CliUckanut deposition (Middle Eocene), .thcugh so e cay be -older., _
-. -
'
Sc== pr:bably f or:cd during the enly Tertiary (Socenc?) dcfor=ation. .'
- -b.[d "
7 that folded the Chuckanut, but the near-verticci dips of at 1 cast many of the f ault surf aces and the linearity of the surface traces 'f(,45 ~.
.
of the Gilligan Tault and prohnbly other f aults suggest thst scme i'T. . .,..- of these structures for:.ed af t.er that, deforca:1on. . ... ..
-.
.
- v-3
....
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.
In discussing the possible age of this f aulting, it =ay be
..
'
instructive to consider the nearest faults that have, tcatatively
~
S.i ,
.
at least> been ebasidered cae' abler nc D evi'1's Mo unt'a in- Fau h , . N. .. t . ~. .W - which passes 21 km southwest of the plant site, is considered 5 , capable by the USGS, 1 rgely en the basfe of a lack of evidence'
.
h-JF that it could not be capable and of probable displace =cnts as - ~
' ~ ~
depicted in carine~ profiles located ~ vest ~o f Whidb'ey' Island.~ n e
~
[.Y.'- i.% Straight, Creek Fault, 48 k= cast of the site, has probably =cved a
...
'
' littl'c since =id-Tertiary ti=e and is " tentatively classified as .
. _
%[:
Pc',i capable" (McCleary and,others , 197 3, p . 11) in a study carried-cut
,
. . , _
, ' ' -
c.-{j~ ._ .. 7 g yd consultant to the 'n'ashington Eublic .Fower Supply Syste=.
'31-
- While it see=s legiticate, in a conservative site analysis, to .
consider both of these f aults capable, it should be rc=c: bared, that. _ N/ - ~ '
'
i.f.:;)
"
both are very long,'very large regional structures, which hava
- doubtless coved many times; that displace =ent along the. Straight I,'7
- 1 t.f -L
' ' ^ * "Creck' Fault was very-largely, if not al2:ost entirely, accompli @ed; s,
.
1 O
-
by Miocene time; and that there is no_ evidence that_ =ove=ent alon'g , . M.7 .' the Devil's Mountain Fault was not also largely corpleted by that.
?! - -' ' - ~
. _ ,
"" time.' he capability of two = aster faults. of the region does_ not ~ 3 ,. . .
.F
'
logically establish capability for the cany s= aller f aults. .I t. .
],
seems-likely that most, if not all',. cf .the. high-angle f aults ,of the,. .
~; site vicinity have not . coved in Quaternary time. _ There is ,no . . .
ff.g f' [ sh o6g'cvidence of such cove =ent on any of them. On .the . o.ther .L ., ,, .g .m ~ r ,. - - y hand, Tdisplace=ents, especially' =1nor displacements,..that cight..,, .
.
[$2,' ' ", "" ' ' ' concelvably- have occurred on these. f aults .between 500,000[ years.; agod .
-
- - and the end of the last glaciation. (about_13,000 years ago) .could, - ' . , - -
-
.1 1 _.
".-..V .' ..
.ha,ve~ been covered._or,. obscured by that glaciation;. and the app 11 cant-
.
J. C ,, . _.. _ . , . .
-.-.-
'
.-
%.y-j has presented no ' strong evidence against such conceivable
~ _ ' , - movement. In summary, it appears that the high-angle f aults of the g,. , , .c
$@d ..
,
. ...~ ' site vit:inity are very probabl.y-but uncertainly-ducapable...M *'g,-t g....- .
--
- ..
-
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.
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.
.
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j,*.6 6. he' 19A6' eartheuake near the northeast ceast of central Vancouver- . - .
.
' , Island--Several papers dealing directly or indirectly with the.,1946- ,.
_
'(, , '
,
' ' 'caithquake have been published since submittal of the la.st USGSs
'
'. ' ^
Status Review in Februar*y-1978. Rogers and liasegaua (L973) . _, m,
. recompute the epicenter of the earthquake and place _it on Vancouver. .
'
1sland near its north 'ast coast, rather than under the . Strait of. , Georgia. They favor a fault-plane solution calling for.a
.
'
, _ ._.
,
" . ".
noYthwest-striking f ault surface, possibly in close proximity to. ,_.
, .: ,, '
1the similarly striking Beaufort Range Fa :t, but they do not rule . .
'
"'otit .a,' northeast-striking surf ace. ,. Riddihough (1978) relates the- ,
1946 event to a northeast-trending f ault in the subducting plate .
-
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.
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. s:
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. . bcncath the crust of Varcouver IcInnd. Slawson and Savage's (1913) . . .
. resurvey of an old triangulatior, network .rcicaled distott ens' i
. , '
~
'.'. . consfatent with covement on the Secufort Range Fault, but
' ' .. .. l QM,
- association of the carthpake vith} surf ace geology appears to ,
.,sn j remain couattul.. . . . . . _ .. , m %, y z
.m .n
... . .
. . . . . .. . .
.
.;.. - . .:;. '; - . . .... . . . .. . 9 i '9 9 a 3
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~;' .f.'x' ' R. :-j%y..'.L.'h,.A5 .- >W- '. QU)[ - @ . -
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.
..
.Y'.b.,.\ ...A.ero.na netic Data
-
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. . o .,..<
V.,. . .. ;;;.lT.:z. n..,.
.
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y - . , . , .. . -' - A *,-g. ~ u,.gw ?' 1-
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,
"Ihe USGS has reviewed the analysis of acrecagnetic data by Qi.d '
' Exploration Data-Consultants;-In T-(Edcon) and foual it to be generally-_
- - - -
.$ d - Q,"' ' satis f act'ory. Alternative interpretat' ions are possible'in sc=e caues, ^'-. however, and the data do not una=biruously establish the correctness of V.. .
-
the applicant's capping and tectonic codel. The f ollowing are scvural F ~.Z - "
-
general ~ conclusions ~ arisibg f rom our review of these data and the7.dcon_
-:
. -. . . ; =
.
-l report: '.'.:'%.
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. ., . .a . , p y , d , ; '. . . ,. , . .- . . . . . g , . . c. -
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'
--
(a) . ~ Aeroc. Yetic anomalies help to define the shape and location :,
.'; of high y =agnetic rock ' types, which are probably
~
' ' 2.-
- ,,
In particular, acrocagnetic data
,
.' ; m
~~,
serpentinites in,this area. .
'
were used to deduce the nature of the contact betvect Shuksan , meta =crphic rocks and the CH/Ju rocks. However, the dips of
.,,. , ~,.
- . . . -.
.
..m the surf aces of the serpentinite bodies =ay be unrelated to the attitudes of the thrust because (1) the serpentinite r.ay,.
. .
.
fT not be originally associated .vith thrusting; -(2) serpenti. nit,c
.
'
,; } ' l
'
.m.. ,is a highly obile rock when subjected to stress; and (3)
{,3
.
_. serpentinites possess highly lvariabic bagnetic properties. . _
.
,
-
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.. .. ....r . ... . .. ..
.: .\
-.
. .
-
'
' " ' (b). Concern regarding an a3rupt change in trend of the . -
,II _ , neromagnetic contours southeast of. Butler _ Hill,, on s=al.}, , scal _e USGS'aero=agnetic maps (see RGI fig. 3 3-2), was expressed in ,
,
_ _ " i .- .. 3 [.? .' the NRC-USGS reguest' to the [ applicant of Juna 19 7 3. . .Th e, . . .l . ._
.
~
'~
S.'. '. [..' ',' . .
. ,c.f' -
- e.; .'. .'. additional more detailed aeromagnetic survey data provided by._
.t
. ':.'.'- e .
.. ..
-- ...the applicant. indicate that there is no =agnetic basis. for a ..
. . '
-> f ault with an east-West trend along Skagit Valley south of _ _ , . .
....,., . ~
. , .~-. . ,.- , , ., . , , . . ,.- ,
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Butler Hill __,, . _ , ,
.
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.
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. . . .
'l c{b;..;.- ? .omalies ~the thrust. .. Wcontact in the vicinity.of. Talc f- ,
. - . ,
../ ,U.
t . ? l' aloig;. . .-
.
I .p. . :.'. Mountain are caused by rock bodies (probably serpenticite) :. . . . '
.. :: which appear to have northeast-dipping northeast faces.
Bowever, these bodies extend to only about 2,000 feet below ground surf ace and could, thercfore, be confined within a th'in
. . . . , . ,
thrust sheet.
'
8
-
'(d) The absence of f ault-related aero=agnetic ano=alics near the
.. mouth of Gilligan Creek, south of Butler Hill, and elsewhere
.
_. . .. does not necessarily preclude the existence of f aults. in these . . areas. . _, .
.
> '
. ... l ..
.
1280 :65 S i -
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-
.
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'
.
. j q. . ..;'s E , " (e) Acromagnetic =aps show that ano=clics characteristic of CH/Ju..
.~ reci.. usually ternicar.e at the. thrust , contact between Shuksan .
%.,,.d- . and CH/Ju rocka. nfa a"=.etr that if CE/Ju reeks underlic g.y.g Shukaan, as the applicant has suggested, they lic at depths in
. -. W '
excess of 10,000 feet. As CH/Ju f requently crops out in c=all 3.M -
- M " areas surrounded by Shukaan and away f rc= the thrust (e.g. ,[ a t -
.$5,j /.Ad ~ ~'. Butler Hill), this lack of ano=afies 'over Shuksan terrace ". ~ ~ g 5, .a -['
'
forces a rather co= plicated geologic =edel.' ' " (, ' " - [f' - ;IO& _- . . _. __. _ : _ ;f '_.' ^l ' -e . i~'.2. E { 1; .L-- .
.
-'~C^-*' :~
.
.
- Y
.
*
. Scis=o glll.:(* . .
*
-
b . . ,b.-. . .
,
_
.
. .
.
.
.
.. . ., ..
.. . . ..
.
&_ ,
.. --. .- -
. A de tai _ led d_i_sc.us_s_ ion .in th.e.. C.e.o.l_ogy-and aero=ao n_e. tic. _. data _s ection a--
M,.l y t., a
.
of-this_revicw is. presented concerniog.the existance and age _of numerous
- - - --
.
J. :. . _ high-angle faults in thn site vicinity. The conclusion offered there is N that these f aults are "very probably, but uncertainly, incapable."] ' '- q.;. .
.
' -
.. .
-
. - . 1.-: .;: ': .. : ~; ; o3 :-~ -
-
*p
?,} ';.c.
-
On the other hand, there have been nu=ercus s=all carthquakes in . a.13.f . the . area -containing the site. Their:cagnitudes range f re= 1.2- to 3.3, ._ 'i .F
,
and several have been felt locally. An align =cnt of epicenters has been-g- .noted in the Skagit .31ver Val. ley but there are. doubts about._the. absolute.
@.- - accuracy of' their. locations. The stated accuracy of events located.by.- -
.i f -
.
, the-University. of. Washington-operated reis=1c. network is 1.r. to 2. 'eaa T . n.
~g (Crossen,. 1974). . However, the seis=ograph stations in the Skagit . Valley:
.; . -
-vicinity-are-' f ewer and more videly spaced than those further to the ~ ~ '---
{.[ vl , , south and because the locale is near the northern li=1t of the network .. . .
- -- coveragerthe accuracy is not likely- to- be that precise. -The-re=aining ---
5.L . . .
,, . epicenters appears to be essentially. random in the site . vicinity. ;3one .. .._ _
,
'. ! - -
of; the small earthquakes ' cave been associated with identified f aults.q-3 ~ '.?' . :Itrisrpassible. that these. carthquakes. are associated witht structuresh'y ,Jf/ . W.T -
-
that are eufficiently small' that no surf ace-expression has beenQ-g.'.;;
- 7-3 ~r
.
*
,;;W , rqcognized by- the geologigc investigations., ;-.:.-i.5--
, - _. .
- n. . .
- n. 9 .,g.-^:,s. ce n . -
.
.
.
- - -
,, , ,
-G ..r ~
' . ' :; ,:s.j;::; -- *....- ' ,
. ' . ,.
' - -
. '" , , & y.: _ . '.
. . . . - . . . . . - -
Co'nsequently, it is our judg=ent- that for purposes of nuclear---l---. . . -_:
% reactor design ~it 'should be assu=ed. that' carthquakes as large as'N ' -C'~ ~-7
.7:D '
.- s magnitude 4.0 cduld originate en any"of / he- t identified f aults .ora'a _5.* *./U.S.- - ~
%]. ,*
.
~ .: .ac. un= app.ed. fault. in. the =cgina cf=thc. plant:. site.: Howeve=,; itis'.ouR~cy.~~4'. ,
- .
; --
-
judg=ent that even if an earthquake _ of. this sire were to occur on.one of".'"
'
.
these structures, its consequences at.tha plant site would be less than
'
-
that .riready postulated to result fr=m- the two =uch larger carthquakes --
- discussed ~1n our previous' reviews '(Jan. 30, 1978 and Ieb. 23, 1978). --
'
.
----
--
__ Since our last review, other articles relating to the tectonics of - the Pacific Northwest, in general, and the Vancouver Island vicinity, in, - particular; -have been published. They reinf orce the oncept that the > - 1946 earthquake was related to the present-day subdt regi=e of the region -rather than regional north-south compression t.%ers and
-
- - - -
s. Hasegawa, 1978). Rogers (1979) suggests that the occurrence of _ _
. . .m.earthquahes .having a northeast. align =ent across Vancouver. Island are.. ..
related to differential movements, of '.he Explorer and Juan de Fuca
-
,
..
'..e s
,
'
M...,
._.
.
.
-
- s.I. ...c . '1. 28. 0. YW L.
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.
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- -
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.
.
- ' . . . . , -. .. s.,. .. %. - :. -~ . ..- .-.. .. ...'...
.,: U: .. '~... ,.....YA' 1
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-
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-
.
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- '. .'. ;___-
-
_
.
.
. .
- -
.. "
k, . . % .. v- platen. Two interpretations of the teernnie s, ttings are preaented; his d.M - . t. m .. , preferred setting nuggesta that the subducted Nootka f ault cac (na:cd . . by Hyndean, et al, 1976) would represent, in effect, a tectonic "f%l boundary, north of which earthquakes would occur because o ' intcase local north-south co=pression resulting f rom the Explorcr-A=crica plate 99 Q-
- .Tip
^
.
interaccion. . To the s'outh of the Nootka f ault zone, subduction of the' Juan de Yuca plate bcncath the A=e'rica plate would proceed in a nor=al ., g B- but ascic=1c fashion." In view of these considerations, we belicyc that the constraints' on the . locus .cf .an_ event ai=ilar_ to. the .1946 carthquaka
~
2.@.',.) -
.
4:7. vould make its significance less than the two postulated controlling
-*- earthquakee (USGS Jan. 30, 1979, nd ?ch. 23, 1978).
-
. ..
.9. 7
. -
.- ,3 .. ,. - .
. . . . -
,-'-
'
--- - - In su==ary, there is..a recogaition thac--there has-.beca-a large
- amount of new geological and-geophysical-data provided by the applicant .
, ,1,, and others since the preparation- of our -last review. However, there
; ?, remains a major uncertainty as to the co=pleteness, significance, and
[;;. , proper interpretation of both the new and previously discussed data.
a . Consequently, it is still our jug =ent that f or purposes of nucicar
'
-
reactor design the- folleving two earthquakes should be censidered as
". controlling:
"
, , 4 ,- , ,
. , .. ..*. -
.
' (
..;
,, '
- 1. an earthquake similar to the one that occurred Decc=ber 15,
'
'1872 but having its epicenter suff%iently close to the site- -
j.-
.,
. that n'o attenuation eff ects be cor '.dered, and. * ' ._t , , , ., , T , . ~.
- .
_:
.
.
.. . .. . . ' , .p..- a shallow magnitude 7.0 to 71/4 carthquake on the Devils' . .
_
*
- 2. .
'.. Mountain Fault 21 km. from the site. , ,., ,
,-
.- -
. .- .
. s.
* '
The USGS reactor site review team agrees vith. the applicant's . , . ,
.,' propose,d use of 'a bedrock acceleration value .of .Q.35 g as .the Saf e -'
_. . Shutdown Earthquake f ;r use with the Safety Guide 1.60 design spectrum ,,.
. J. . for nucicar power plant design. . _. ,- '
.
.
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1280 ;67
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f.,.a...*.,^ . r,...* .:. . ... ,. ;. f ..* . e *
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References j c!O, J e . -
.
- i. . . .
.. . ...
. ,. . .
.
.
.
. . .. ..
'* ._...... . -- ... . . . .
- - .
.
.
.
_...
..
*- . .. .
..h.:;;'.;.g l - .
.. <...\,,,..
.
.. ;.
. r...h . :
- ...... ~. .R .. : :
-
..at . . .z.ul
..; : .. ,
..
'. M'd , :.
.
- .. . . , . .
T-;.G.' ... ... Eechtel Incorporated,,19 78, Subsurf ace investig ation of dis turbed..
." ,,'.
glacial depcsits on Lake Cavanaugh Road: Report to Puget Sound 7 ,. :-; .
< =W . :. .-U.. . ...Powe r T.i .l g.at Co. .. ; n ,.--; .. .. . .
. -
-
-
~ - .-
. . .
.u .
,
....... .,.
, . .-~ .
...-
.
.
' : - *. ,..c :: - - -
. l . , . ...$ .' ';
. .
E. .' -
'
'
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