ML19256E329
| ML19256E329 | |
| Person / Time | |
|---|---|
| Site: | Skagit |
| Issue date: | 02/23/1978 |
| From: | INTERIOR, DEPT. OF |
| To: | Case E Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19256E326 | List: |
| References | |
| NUDOCS 7911020106 | |
| Download: ML19256E329 (22) | |
Text
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APPENDIX D U.S. GEOLOGICAL SURVEY REPORTS
- NUQc'
.tc United States Department of the Interici "5f.-r"A t,= D' : :. Y 'I-A.%
GrotocICAL SUR*/EY
/ RESTON, v11tGINIA e.002
'Q~s
- February 11,1978 In Reply Refer To:
" ail St:p 905 .
!!r. Edson G. Case Acting Director of the Office of e,.
i;uclear T'.eactor Fegulation U.S. Nuclear Regulatory Coanission ,
!!ashington, D.Q. 20755
Dear !!r. Case:
Transti cted herewith, in response to the request by your staff, is our review )f the geologic and seismologic data relevant to the Skagit Nuclear Power P oject, Units 1 :nd 2 (r:RC Docket Nos. 50-522 and 50-523).
This review was prepared by William H. Hays and Stanley R. Brcckan.
Assistrice was provided by Robert H. Morris and Jam.2s F. Devine. d e
Sincerely yours, e
.t' ,
Director Enclosure . . . .
^
P00R BRIGINAL 1258 349 D-1 17911020 /Ob
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APPENDIX D (Continued) _
- Puget Sound Power & Light Ccmpany Skagit Nuclear Power Project. Units 1 and 2 Project No. S14 Skagit County,t!ashington e..
- NRC Docket Nos. STN 50-522 and 50-523 Introduction The U.S. Geological Survey (USGS) has reviewed the geologic and seismologic aspects of the Preliminary Safety Analysis Kaport (PSAR) for the Skagit Nuclear Power Project, including amencments 1-20 thereto. .
The Skagit site is in the foothills of the Northern Cascades of Washington,10 km (6 mi) east-northeast of the t:wn of Sedro doolley
The Devils Mountain fault and 35 km (22 mi) southwest of Mount Baker.
zone is 21 km (13 mi) to the south. The site is on a glaciated bedrock bench on the north side of the Skagit River valley, 92 m Foundations of Category I (300 ft) above the floodplain of the river.
structures will rest on steeply dipping beds of the Chuckanut Formation, which consists of interbedded sandstone, siltstone, mudstone, The and subordinate coal of Cretaceous (?) and early Tertiary age. '
sheared but essentially depositional contact be:::een the Chuckanut Formation ar.o the Shuksan schists and phyllites, of Paleozoic age, is wit'hin the site area. Glacial deposits cantle cost bedrock at the site and crop out discontinuously along the Slagit River valley. .
P00R ORG M D-2 1258 350-
AP'PENDIX D (Con.tinued)'
Review of the PSAR involved a survey of =uch published and u infomation on the region and consultatien'with many earth scientists ciated with the USGS, the University of liashington, and Western W -
State College. Representatives of the USGS and the Nuclear Rcgulatory Comt.ission (flRC) made several trips to the site and the surrounding a They met with stat?s of the Washington Division of Geology and Resources and the Geophysics Department of the University of s.. Was E. S. Cheney, representative of an intervenor; and, repeatedly, w '
)
applicant and the applicant's consultants (Bechtel, Inc., and others .
Geoloor The applicant's analysis,of the site is based on published and un lished infomation and original geological and geophysical field investi-gations. In the site area, the applicant carried out an extensive core-d, drilling program, J99ed and tested the Core, i.renched GCross the PTOpOW sites of the reactors and other places where subsurface information was particularly needed, and carried out local seismic-refractica and ma meter surveys.
The applicant mapped the vicinity of the site geologically, investi'!ated recent earthquakes near the site, studied the surface geo of parts of the Devils Mountain fault zone and the Chiwaukum grab obtained and analy:ed magneth, and gravity surveys of much of the Da Mountain faul,t :ene and about 275 km (170 mi) of high-frequency profiling in puget Sound.
During review of the PSAR scme topics were particularly stressed.
These topics are discussed below:
1.
The structure and stability of the i=ediate site area--The applicant studied the surface and near-surface rocks and P00RBRGINAL 2
D-3 1258 351
. . APPENDIX D (Continued). ,
surficial mantle of the site area by. core drilling, trenching, surface observation, and supplementary geophysical techniques and compiled the resulting data en surface and subsurface geologic maps and cross sections. These illustrations and the text of the PSAR seem to adequately portray and describe the stratigraphy and geologic structure of the site. <..
- 2. The structure of the lower Skacit River vallev--Because of the apparent offset of the Chuckanut Formation across the Skagit River valley, close to the plant site, the USGS and the NRC were concerned that a significant left-lateral fault might exist along the valley. The applicant has shown that folds in the Chuckanut Formation have a complex history arld are co=.only irregular, and has ar.alyzed the constraints placed en faulting along the ,5kagit River valley by geologic features that cross it--particularly the ,
Straight Creek fault and the Shuksan thrust. The applicant studied surface exposures of Quaternary sediments within the valley and photoimagery of the valley and of the lowland west of it. No
' substantial evidence for significant east-west faulting 'along the valley has been found.
- 3. Hazards of floods and mudflows alona the Skaait River--The applicant considered the effects of the simultaneous occurrence of two catastrophic events upstream: a huge mudflow frcm Mount Baker, comparable to the Osceola flow from Sunt Rainier (Crandell, 1971), and the failure of two dams that would be in the path of the mudflow. The applicant's calculations that all Category I
- 3 P00ROR21 0-4 1258 352
APPENDIX 0 (Continued) .
.\ .-
structures would be far above the crest of the mud and water appear I
to be valid.
4
- 4. Ash-fall hazard--The only form of eruption that poses a direct I hazard to the plant site is the explosive eruption of ash into the atmosphere. The applicant assumes tnat a very large eruption of this type, comparable to the eruption of 140unt Katmai, Alaska '
i (Griggs,1922), might occur at Gla'cier Peak, 90 km'(56 mi) east- .
southeast of the site, which is the closest volcano with an ex. plosive history. In view of available data on the distribution of ash from 14ount Katr.ai and from Cascade volcanoes and the fa that the site is generally upwind from Glacier Peak, the applicant's conclusion that 15 cm (6 in.) of ash might fall f- the plantc. site in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> appears to be conservative. .,
- 5. The stry'.ucal -elationshios of linear features (lineations) on the around surface--The applicant made a comprehensive survey of lineations in the site region, using aerial photos,and other imagery. Lineations identified within 56 km (35 mi) of the site*
. were discussed with 'censultants knowledgeable of the region, and many lineations were examined in the field. The applicant concluded that most of the lineations are related to bedding or foliation of the rocks, glaciation, activities of man, or previously mapped faults. A few cannot be explained, but they do not appear to represent geologic features that could affect the plant site.
- 6. The Devils f4cuntain fault zone--The Devils Mountain fault :ane bears west-northwest through the Devils Pauntain-Lake Cavanaugh
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P00R ORGINAL
~
e-e 1258 353
. APPENJIX 0 (Continued) -
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Haar region, south of and as close as 21 km (13 mi) from tr.e si:a.
Devils Mountain, this fault zone is about a mile wide and includes three Eastward, these~ three appear faults that produce distinct photolineations.
The fault zone displaces the to converge and form a single structure. .
youngest Tertiary rocks in the Devils Mountain vicinity, which are of
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probab'e Oligocene age. Stratigraphic relations suggest large overall 5
displac.mnt in which.the rocks south of the faults moved relatively downward (PSAR, p. 2.5-10e), but there may have been a substantial or even predominant strike-slip component (Hobbs and Pecora,1941, p. 66-67; Pleistocene Loveseth,1975, p.16-17; J. T. Whetten, oral co=un.,1976).
glacial deposits overlie much of the Tertiary in the Devils Mountain region and cover much of the fault zone.
The length of the Devils Mountain fault zona was investigated by the applicant. Evidence of faulting of the discontinuous 1y exposed bedrock was,-
fcund frcm Deer Creek, on the east, to 1.ne'hillfront south of Mount Vernon, on the west--a distance of about 31 km (19 mi). The applicant found much les shearing and secondary deformation in asscciation with the fault at Deer Creek than farther west. According to J. T. Whetten of the U.S.
Geological Survey (oral cc=un.,1976), the fault extends at least as' far eastward as Boulder Creek valley. The westward extent as inferred from good aeromagnetic data (USGS,1977) is at least as far as a point scutheast of Lope: Isl an'd. In sumary, the length of the fault zone has not been detemined accurately by the applicant. It must be considered to be a minimum of 47 km (29 mi) long (the distance frcm where it has been traced near Bouldar Creek westward to the hillfrent south of Mount Vernen, alcng
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P00R ORBINAl.
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0-6 ,
1258 354
s .
. APPENDIX D (Continued-)
' which the zone is discontinuous 1y exposed). It is almost certainly
' 80 km (50 mi) long (the distance from Boulder Crcek westward to the ,
vicinity of Lopez Island), cnd it very probably extends farther .
t both to the west and southeast. .
The applicant's investigation of the age of the most recent
! movements en the Devils Mountain fault zone has included a search
' of the literature, considerable observation of parts of the zone in the field, the excavation of trenches at several locations along the southern = cst fault in the zone, and core drilling in one area on titis fault. (For comment on the applicant's seismic profiling in puget Sound, see the discussion of profiling near the end of this review.) Neither the published and unpublished s.
literature nor the applicant's originc1 studies revealed any ,
c.onclusive evidence that the Devils Mountain fault zone has displaced the widespread deposits of the last glaciation, which is presumably the Vashon glaciation (about 13,000-19,000 years ago). Radiocarbon dating of organic samples from trenches across the southernmost fault in the zone suggests, not entirely conclusively, that this fault has' not moved in the last 37,000 years. There is no evidence in outcrops or trenches that the ,
fault zone has or has not been active in the interval of 37,000-500,000 years ago. Owing to these uncertainties in age of movement the fcult should be presumed to be capable.
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P00R ORGEL D-7 1258 355
l . _
APPENDIX D.(Continued)
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- 7. The cossibilit/ of a continueus, ncethwest-bearino fault alone the_
_ eastern site of the northern Puoet Sound Lowland--A group called .
Skagetonians Against Nuclear Power (SKANP) was repr'esented by E. S. Cheney who has suggested (1976a, b,1977) that a continuous major fault zone extends from Lake Chaplain, east of the city of s,
Everett, northwestward to Bellingham Bay--a distance of more than 96 km (60 mi)--and probably farther northwestward into the Strait of ,
Georgia. Cheney has mapped a northwest-bearing fault at Lake
- Chaplain. He infers that there are major faults of similar trend east and west of Devils Mountain, that these faults are continuous with the Lake Chaplain fault, and that they extend northwestwa d -
forthwest'-bearing along one or both sides of Lummi Island, r.
contacts and structures are dominent in the Northern C:::: des, e' and it is natural that there are some faults of this trend, such as the fault at Lake Chaplain, near the western front of the range.
Neither Cheney nor the applicant, howeVer, has reported good Such a evidence for a continuous major fault or fault zone.
fault or fault zone was n.t observed on the applicant's BSN profiles. The west-trending steep linear gradient shown on the aeromagnetic map (USGS,1977) appears to preclude extension of any such fault north of latitude 48*20'.
- 8. The local and reoional structural relationshios of the 1872 "earthcuake in central Washinoton, the 1945 earthouake near the northeast coast of central Yancouver Island, and the 1949 and 1955 carthcuakes in southern Pucet Sound--
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P00R ORIGINAL D-8
e g APPENDIX 0 (Continued) .
a; The 1872 earthauake--The applicant's regional analysis of the 1872 earthquake (intensity VIII !E, according ,to .
applicant) suggests that its epicenter is in the Henatchee-Chelan area of central Washington. This location is in '
the general vicinity of the Entiat fault, a major early. "
Tertiary structure that is truncated to the north by a ,,,
Miocene pluton. The area east of the fault has experienced -
u The upwarping in post-Columbia Basalt (post-Miocene) time. .
- applicant's field investigations of the proposed epicentral area and investigations there or nearby by other geologists (for example, F. W. Cater, Jr. , R. L. Gresens, S. C. porter, J. T. Whetten, oral communs., .1975,1976, and R. B. Naitt, Jr.,
and P.. W. Tabor, written commun.,197G) ' ave revealed no r
strong evidence for Quaternary . deformation along these or other structures. On the other hand, Quaternary sedi nents are not ubiquitous in the vicinity, and minor local deformation could remain, undetected.
The applicant proposes that the 1872 epicenter is in .
a different tectonic subprovince than that of the plant site and proposes that a no.rth-bearing tectonic boundary, sep,arating the two subprovinces, passes along the west sides of the Chiwaukum and Methow grabens. This boundary emphasizes what may be scme real differences between Cretaceous and early Tertiary structures of the western and eastern sides of the
- "crthern Cascades. On the other hand, the boundary transects -
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0-9 P00RORCM
\h
l .
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- APPENDIXb(Continued)
I tr.a 3:ructurd a.id mitm r. hic grain of 'he old core of the I
range, and, cost importantly, has no clear relationship to the plate interaction that is generally accepted as having dominated Cenozoic tectonism in western Washington--neither to the sub- '
' duction active until at least pleistocene time, nor to the probably different but as yet uncertainly defined tectonic Thus thara dcas net. appear to be an .
! regime of the present.
I .
adequate basis for establishing tectonic subprovinces.
On the basis of present evidence, it appears that the 1872 earthquake cannot be associated with any known tectonic structure and that it must be considered to have occurred in the same tectonic province as that of the plant site.
- b. The 1945 earthouake--There does not appear to be a relation-ship between the 1946 earthquake and the surficial< geology.
- c. The 1949 and 1965 earthouakes--The largest historic carthquakes' in the puget Sound area were the 1949 event r 'heast of Olympia and the 1965 event south'of Seattle, both of intensity VIII !@t. Their hypocenters were apparently more than 40 km (25 mi) deep. The applicant attempts to associate the 1949 earthquake with a passible northwest-trending fault structure near Olympia and the .1965 earthquake with a strongly defined ~
east-west fault structure near Seattle. The applicant recognizes that these structures are much shallower than the hyp0 centers of the earthcuakes, that the Seattle structure is about 16 km (10 mi) frem the 1965 epicenter, and that
- fault-plane solutions for the earthquakes do not appear to 1258 358 be consistent with the structures.
9 l D-10
- '* APPENDIX D (Continued) ,
The applicant's analysis of proprietary seismic profiles In northern Puget So.nd led to recognition of an east-west
' fault that passes south of the central San Juan Islands and through northern Whidbey Island and probably turns scutheast-ward on the mainland. This fault, which may be termed the
" Southern San Juan Islands _ fault." appears to be subparallel e ,
to and south of the extension of the Devils Mountain fault ,
that is inferred (above) from the aeromagnet.ic data. Largely
- I.
because of the greater historic seismicity south of the Southern San Juan Islands fault than north of it and because the San Juan Islands and adjacent Cascades have apparently been elevated relative to the Puget Sound vicinity during most of Cenozoic time, the applicant proposes that the fault s.
is the boundary between two tectonic subprovinces and suggests -
that the 1949 and 1955 earthquakes, if considered to be random, would be restricted by this boundary. It is generally accepted, as stated above, that ,the interaction between crustal plates and particularly underthrusting of the conti-nental plate has dominated tectonism in this region in Cenozoic time. The Southern San Juan Islands fault is one of many geologic boundaries that may have had only minor .
tectonic significance within the large area centrolled by the underthrusting.
From the evidence at hand, it.=ust be considered that the 1949 and 1955 earthquakes were randca events and that similar events could occur near the plant site.
6 10
'-" 1258 359
APPENDIX D (Continu~ed) , ,
- 9. The structural relationshios of the 1974-1975 earthauakes in the Skacit River valley--During a period extending from Decenber 15, 1974, to at least May 1975, after which the tempor,ary seismic net was removed, a few shallcw earthquahas =n- numerous minci after-shocks occurred in a small area in the Skagit River valley, about 5 km (3 mi) southeast of the plant site. The largest event had
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a magnitude of 3 and produceif local intensities on valley fill as high as VI MM. The University of Washington established a seismic net in the vicinity,to study aftershocks, and carried out gravity and earthquake-intensity surveys. The applicant sponsored a seismic-velocity sursay, searched the ground and imagery for unrecorded surface faults, supplemented the earlier
~ intensity survey, and made an adcitional gravity survey.<.fleither
,/
prior knowledge of the vicinity ;.or the investications described ; .
above identified a etructure with which the earthquakes can be associated, and the applicant concludes that the carthquakes are j not a factor in plant design.
Recently three high-frequency seismic surveys were carried out nfar the eastern San Juan Islands that are relevant to some aspects of this review. In January 1976, the USGS surveyed various waterways between central Whidbey Island and the south end of Lu::ai Island as part of a larger marine-survey program. In March, the applicant spenscred a supplementary survey, mainly along the scme lines as these of the USGS survey but employing different equipment. The techniques employcd in p%(
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/ l . APPENDIX D (Continued). .
I these surveys produced nanatr ctina af " cu:S as 200 :- (1,000 f!} r.'
sediments (Quaternary but possibly including some pre-Quaternary) but 1
, not penetration of the underlying hard bedrock. The applicant
~
l analyzed the profiles produced by both surveys and found no c1 ear
- indication of faulting of bottom sediments. . Review of the applicant's .
profiles and analysis by the USGS does not alter any of the conclusions prasa,*ad 6 ve. It d::s suggast tha possibility of tectonic defor ation s..
l; of pre-Vashon(?) sediments near Fidalgo Island and northern llhidbey Island, as mentioned above, and the possibility of local minor faulting in sea-floor sediments farther north.
Seismology, Briefly, the Applicant has taken the positions that (1) the largest earthquakes in the site region had maximum epicentral intensities of VIII* and were located either on specific structures or in other tectonic provinces, (2) the-highest intensity experienced at or near the site was VI, and (3) their chosen Safe Shutdown Earthquake (SSE) acceleration
. value of 0.35 g is sufficiently conservative.
During the course of this review the applicant has put ,
forth considerable effort to document the location and intensity of several earthquakes, principally the December 14, 1872 event. The applicant has argued that it occurred between Wenatchee and Lake Chelan and had a =aximum intensity of VIII.
- All intensities are Modified I'.arcalli.
D-13 1259 001
. APPEtiDIX D (Continued)
- The site lies within a region of only moderate seis=le activity and numerous earthquakes nave been fait at thj site, Intensity VI, believed to be the' highect intensity experienced at or near the site, was generated by the 1946 British Columbia, 1965 Seattle, and 1872 Wenatchee earthquakes.
The nearest non-instrumentally located event occurred April 15, 1931, 20 km from the site, had an epicentral intensity of VI, and'may have generated intensity IV motions*. .
rred in the site vicinity. An intensity VI event oc December 15, 1974 which was instrumentally located in Skagit Valley within 6 km of the site and had a magnitude of 2.8 to 3.0. It was not reported felt at the site.
The largest earthquakes within 75 km occurred January 11, 1909' and January 23, 1920, 65 km of the site fren
. the site. The applicant conducted a newspaper search for Their investigation yielded new '
accounts of these events.
data which they feel indicates that the 1909 event may have while been felt in the site vicinity with an intensity of V, the 1920 event may have had a maximum epicentral intensity of only VI (instead of VII as reported in Earthquake History of the United States) and a site vicinity intensity no greater than IV. Since these events do not control the SSE, the -
conclusions regarding maximum intensities have not been critically reviewed for purposes of inclusion in the of ficial record of historic earthquakes.
Other earthquakes having maximum epicentral intensities x
of VII and VIII and ranging in distances frem 75 km to 300 'n O
13 1259 002 D-14
APPENDIX D (Continued) were felt in the vicinity cf the cit up te intensity V Three distant earthquakes - 1949 ueen Charlotte Islands (975 km) , 1958 Southeastern Alaska (1500 km), and 1959 Hebsen Lake (950 km) - were also felt in the site vicinity.
As mentioned earlier in this review, the nearest 15, 1974 instrumentally lo_cated earthquake occurred December in Skagit Valley. The earthquake was shalle , produced ,
locally high intensities (VI) for its mugnitude ( 2. 8-3. 0) , and .
generated a number of af tsrshocks. Special studies were ,
undertaken to locate the af tershocks refine the location of the main shock, to obtain focal mechanism solutions and to investigate the possibility that they were associated with a The previously unrecognized earthquak'e generating structure.
results of the studies were mostly inconclusive. The events were accurately located but no generating structure coul'd be
identified nor could the focal mechanism solutions be
.sufficiently restrained. We ,believe, the effects of this earthquake would be exceeded by the expected effects from the postulated maximum earthquakes described below. ,
' Earthquakes considere'd significant in estimating the maximum earthquake are the 1872 Wenatchee(IX), the 1946 '
British Columbia (magnitude 7. 3) , the 1949 Olympia (VIII) , and In addition, the Devils the 1965 Seattle (VIII) shocks.
Mountain f ault zone and its earthquake-generating potential have been examined.
For the reasons given below, we feel that an event similar to the 1872 event represents the maximum earthquake for the Skagit site.
14.59 003 0-15
APPENDIX D (Continued) An independent study of the 1872 Wenatchee earthquake 2 - data as presented.in the PSAR and its appendices was undertaken by a USGS/NCAA ad hoc working Group on Intensities of Historic Earthquakes. The consensus of the committee was a that the maximtm intensity probably was IX. They also , concluded that the' location of the epicenter has not been accurately determineJ (letter dated April 1,1976) . We J.. recognice that the historic record contains equivocal data; for the Skagit application we recommend the use of ,; however, the maximum intensity of IX as adopted by the ad hoc working group. I Hence, we believe the 1872 Wenatchee event can be characterized as a shallow-focus intensity IX (*) earthquake which (1) has not been associated with a known structure, . (2) has noc been located = ore precisely than within an aren ,, extending from Lake Chelan in central Washington to southern British Columbia, and (3) occurred in a tectonic setting and province that includes the Skagit site. The term " shallow-focus" has been used in the traditional sense that the earthquake hypocenter was probably less than 70 km deep but the evidence for this is weak. That intensity IX was experienced over a broad area argues for at least some depth. The area of maximum intensity was probably quite small which suggests that the hypocenter couldn't have been very deep. No surf ace rupture along a f ault has been identified that could be associated with the earthquake. That _ such a. rupture has not been discovered is not considered 15
-'S 1259 004
APPENDIX D (Continued) i surprising because of the large araa and rugscd cerrcin.sof .
- much .of the epicentral region. There is no reason to believe i
hat a similar earthquake could not occur at some time in the
- future at shallow depths. .
No new information pas been provided that establishes .
.that the Skagit ' site is situated in a tectonic province .
. distinct from that of the locale of the 1872 earthquake.
The J i tectonic boundaries as they are related to the 1872 earthquake 2 are discussed in paragraph 8 (a,\ of the geolog'y section of this report. We do not believe that t.%e evidence that the 1872 earthquake can be associated with a tectonic structure such as the Badger Mountain Anticline is compelling. Although the concentration of earthquake activity in the Wenatchee-Entiat-Chelan area is suggestive of the fresence of an earthquake-generating structure, nc 'ich structure has beef demonstrated on the basis of geology or geophysics. We conclude, then, that a similar event could occur near the the Skagit site and that the site intensity would be greater than that expected to be generated by either the 1949 or the 1965 earthquakes. , The 1946 British Columbia earthquake was i magnitude 7.3 event which, occurred June 23,'1946. The inte: sity reported in
'the United States was VIII; it is not certain what the maximum inte.:sity was. Rogers (1976, personal ccc unication reported in the PSAR, page 2.5-60n) calculated a depth of 30 km. The applicant has argued that it was associated with a northeast-trending structure which has not been recognized 'in 6
.D-17 \259 005
APPENDIX D (Continued) - the surface rocks but which hac been inferred on che basis of tectonic and seismological conciderations. The bulk of these consideratiers .;e presented in the PSAR (pages 2.5-6Dk to The
- 2. 5-61, 10 pag e s.) and will only be summari=ed here.
seismological evidence includes [1; a diffuse band of earthquake epicenters southwest of the site which has a trend
~ '
coinciding in general with the structure $enticned above and , [2] similar focal mechanism solutions have been obtained for the 1946, 1957, and 1972 earthquakes (Rogers, 1976, as The epicenters of these reported in the PSAR, page 2.5 vDn) . , events are coincident with the postulated structure. A fault zone has been postulated on the basis' of the tectonics of the region (ie, Barr , 19 74 ;. Riddihough, f.1977) . USGS geophysicists agree, in general, with the csncept of a fault zone passing beneath Vancouver Island which would correspond to the applicants postulated structure. The sense of motion would be lef t-lateral, whiqh agrees with one of the solutions obtained by Rogers (1975) for the 1972 earthquake. The PSAR goes on to observe that Barr (1974) reports that the igneous basement (oceanic crust) is not involved in the deformation of the overlying sediments. The applicant , concludes t' hat if large earthquakes, such as the 1946 event,
~
are associated wi'..h tectonic boundarie s at depth, structure need not be expected to he visible at the surface. In view of the foregoing discussion, the applicant then . concludes that the 1946 earthquake can be reasonably correlated with a structure trending apprcnimately N60E 17
}}9 Ob D-18
APPENDIX D (Continued) beneath Vancouver Island. The USGS concurs, hence the 1946 . event is not ecnsidered cignitic:nt in establishing the .. . maximum earthquake. The Devils Mountain f ault zone is believed to be at least
~
80 km long, may have had as much as 60 km of strike sl'ip offset, and is considered to be capable. Much, if not all, of this displacement appears to have occurred prior to Quaternary tiae. According to Dobrir (page 2G-1CS, A=end=2nt 19) , oligocene strata do not appear to be disrupted, within the limits of resolution of the geophysical data, along the trace . of the fault where it is transected by the marine seismic reflection profiles (Figs 8-12, Amendment 19) . Other~ evidence indicates that the strata identified as Oligocene by Dobrin may be younger than oligocine in age. Considering that most of the strike slip component of displacement occurred in the Tertiary, strict application of fault length / magnitude relationships based upon California fault modq1s does not seem to b.e appropriate. We believe, however, a magnitude 7.0 to 7-1/4 earthquake, which is consistent with other earthquakes of the region, could occur on the Devils Mountain fault zone. , m 0 0
- 18
} APFENDIX D (Continued) .
- Summarizing, two earthquakcc are postulated es the t
maximum earthquakes of the region i
' 1. an event similar to the 1872 tienatchee (IX) eve.kt very near the Skagit site. "Very near" implies that t be
, attenuation oE ground moti'an for distance no considered. -
- 2. a shallow magnitude 7.0 to 7 1/4 earthquake on the
' Devils Mountain fault, 21 km from the site.
It is the judgement of the USGS reactor site review team that _the proposed use of a bedroca acceleration value of 0.35 g as the Safe , Shutdown Earthquake for use with the Safety Guide 1.60 design spectrum is acceptable for nuclear power plant design. that. On the other hand, the review team is not- confident c.
"a i'e all arpects of the geology and seismology leading -
position have been demonstrated and documented conclusively. m k o m F e 19 h D-20 \
APPENDIX _D (Continued) References Cited . Atwater, Tanya,1970, Implications of Plate Tectonics for the Ceno cic Tectonic Evolution of Western North America, ESA B'ill., 81, . pp 3513-3536. . Barr, Sandra M.,19'4, Structure and Tectonics of the Continental Slope West of Southern Vancouver Island, Canadian Journal of Earth Sciences,11, no. 9, pp 1187-1199. a..
*Cheney, E. S.,1976a, Preliminary report on the geological ha:ards of the Skagit nt clear power site: Unpublished manuscript, 60 p. ,
- 1976b, Preliminary report on the seismic and geologic hazards to the Skagit nuclear power site: Unpublished manuscript, 83 p.
- 1977, Alternative interpretations of the seismic and' geologic hazards to the Skagit nuclear power site: Unpublished manuscript, 117 p. ,-
Coffdan, J. L. and C. A. von Hake,1973, Earthqucke history of the United States, U.S. Dept. of Commerce Pub.1-1. Rev. Ed. (through 1970), 208 p. , Crandell, D. R.,1971, Postglacial lahars from Mount Rainier volcano, Washington: U.S. Geol. Survey Prof. Paper 677. 75 p. Griggs, R. F., 1922, The Valley ~of Ten Thousand Smokes (Alaska): Washington, National Geographic Soc., 340 p. - Hobbs, S. W. .' and Pecora, W. T. ,1941, Nickel-gold deposit near Mount Vernon, Skagit County. Washington: U.S. Geol . Survey Bull. 931-0, 77 p.
- Reports ;repcred by E. S. Cheney for an intervanor group SKANP (Se::getonians Against f:uclear Pcuer), Seattle, Uash.
q 20
- \ lg4' D-21
. APPENDIX D (Continued)
Lovesath, T. P.,1975, The Devils l'ountain fault :one, northitestern Washington: Uachington University M.S. thesis (unpublished), 18 p.
";ddihough, R. P.,1977, A l'.odel for recent Plate Interactioris Off Canada's West Coast, Canadian Journal of Earth Science,.14, pp 384-396. .
Rogers, G. C.,1975, The Vancouver Island Earthquake of July 5,1972, e Canadian Journal of Earth Science,13, pp 92-101. U.S. Gaological Survey,1977, Aeromagnetic map of northern and eastern parts of the Puget Sound area, Washington: U.S. Geol. Survey Open-File Report 77-34.
~
- r.
. e 4
0 21 D-22 % g@ .
\1
'e U. S. Geological Survey Recorts
.. n . .'- c. vs u r:dwe Unitea 5tates De'nartment of the Interior
\Rh*:))
GEOLOGICAL SUR\ Ei RESTON, VA. 2252 Septem6er 17, 1979 In Reply Refer To: Mail Stop 905 . l l Mr. Harold Denton Lirec::: cf the Officc of Nuclear Regulations- __.- U.S. Nuclear Regulatory Coc=ission Washington, D.C. 20555
Dear Mr. Denton:
Transmitted herewith, in response to the request by your staff, is a supplemert to our reviev (February 23,19:8) of the geologie and seis=ologic data relevant to the Skagit Nuclear Power Project, Units 1 and 2 (NRC Docket Nos. 50-522 and 50-5'3). - This supplement was prepared prior to the co=pletion 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. This supplement was prepared by William H. Hays and Stanley R. Brock =an. Assistance was provided by Richard J. Blakely, Robert H. Morris and James F. Devine. Sincerely yours,
/
ga h t H. William Menard Director Enclosure 4 1259 011 f .".~ '.'9., i .c. A 2 l} ,' f l One Hundred Years of Earth Science in the Public Senice
, 2 Cr * . , , , ., ,
o<o cic " u-23
^
ht3 sr =. u' - 79092(p$3st
Appendix 0 (Continued) St-tus Reviev Septe=ber 13, 1979 Puget Sound Power & Light Company Skagit Nuclear Power Project, Units 1 and 2 Project No. 514 Skagit County, Washington l NRC Docket Nos. SIN 50-522 and 50-523 Since submitting its last Status Review on the proposed Skagit Nucles' "over_ Project on February 23, 1978, the U.S. Geological Survey _ (USGS) ss received and reviewed a =ajor submittal from the Puget Sound Power & Light Company (PSPL) entitled " Report of Geologic Investigations in 1978-1979" and dated May 27, 1979 This sub=ittal and a f ew new questions that have arisen f nn ongoing studies in the site region are
, discussed in this review.
Geology The extensive investigations represented by the submittal cf May 27, 1979, were stimulated by new ideas regarding the geology of the site vicinity that developed in the coarse of the long-term program of the USGS for mapping the geology of northwestern Washington. The new ideas concern a tectonic mixture of meta-igneous and metasedimetary rocks that is videly exposed south of the plant site in the Table Mountait-Eaystack Mountain-Bald Mountain region. (These rocks are hereaf ter termed "CH/Ju rocks", a term coined f rom symbols for these rocks on the maps of the applicant and of J. T. Whetten.) In the Preliminary Safety Analysis Report OPSAR) 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 Shuksan thrust, and as being generally torrelative to the Chilliwack Group of largely Paleozoic age, which composes that thrust plate in the Cascade Range farther east (Misch, 1966, 1977, 1979). In identifying
, the CH/Ju rocks with the Church Mountain plate and the Chilliwack Group, l
the applicant was probably influenced by the fieldwork of G. M. Miller (1979, in press), who has sought to extend Misch's mapping westward into the Cascade foi thills. At NRC-USGS meetings with the applicant in May
' 1978, and at the Atomic Safety and Licensing Board (ASLB) 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 markedly different relationships. He correlated the CH/Ju rocks scuth of the plant site g
with the Mesozsic Decatur Terrane (including the Fidalgo Ophiolite of i Brown [1977] and Brown and others [1979]) that he had =apped as a thrust I sheet in the e stern part of the islands--a correlation that is 1 -
~**
1259 012
APPENDIX D (Continued) supported by petrologic and radiocetric data. Whetten presented
,.j evidesca that these rocks are not part of the Church Mountain thrust plate, structurally underlying the Shuksan thrust, but, rather, part of a higher plate that has been thrust over the Shuksan plate. These new
~
ideas, later expanded in publications (Whetten and Zartman, 1979; Whetten and others,1979; Whetten and others, in press) have produced uncertainty and division in the thinking of the geologic community , regarding the basic structural frrrework of parts of the Cascade foothills. Such uncertainty is of significance to review of the site proposal in that it could limit identification and understanding of younger, post-thrust deformatica of the rccks of the recion. In hope of resolving quickly the_ question of the basic structural relationship o.* the Shuksan thrust plate to the CH/Ju rocks and also core specific concerns regarding possible high-angle faults of post-thrust age, the NRC and USGS re~iested, on June 9,1978, that the applicant cake additional studies, including (1) core boring, to determine the orientation of the f ault centact between the Shuksan and Ch/Ju rocks near little Eaystack Mountain; (2) aero=agnetic surveys, to help define the location and the attitude of the same contact regionally and to investigate an anomaly southeast of Butler Hill that had been revealed on an earlier, les; detailed .er vey; and (3) further studies, including exa=ination of Ouaternary depecite, relative to the possible presence Gilligan andandDaysignificance Creeks. of faulting in the vicinity of and parallel to 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 B Fault . " The applicant was unable to carry out cocpletely the requested drilling near and elsewhere; Little Haystack Mcuntain, but drilled 11 ci re holes there carried out an extensive detailed aeromagnetic survey and analy;is thereof; =arkedly increased the density of data on its regional Beologic map, including data near Gilligan and Day Creeks; carried out ' on ground gravity and magnetic surveys across the lower parts of the two creeks; reexamined photographic and radar i=agery; and studied Nanaimo-Chuckanut stratigraphy in the northern San Juan Islands. In May and of Geologic June 1979,inthe applicant distributed its report, " Report Investigations 1978-1979" (hereaf ter termed "RGI"), on the studies outlined above. The principal topics addressed by the
, applicant in this report and other aspects of regional and local geology that have seemed i=portant in recent considerations of the proposed l plant site are discussed briefly Lelow:
1 1.
- Jhe basic tectonic fra=evork of the recion--In its 1979 RGI, the i applicant concludes that the controversial CE/Ju rocks south of the plant site and other exposures of similar rocks in the region are indeed parts of the Church Mountain thrust plate and structurally lower than the Shuksan thrust plate--a conclusion consistent with 2
0-25
Appendix 0(Continug_1),
- that in the cerlic- "c
-'d- cenflict with Whetten's redel of
~~ -- the recion. The applicant does modily its earlier position "4 somewhat (p. 3.1-5) in stating that the CH/Ju rocks probably include so=e Mesozoic rocks similar to rocks in the eastern San Juan Islands, and the RGI includes (p. 3.1-4, 3.1-5) a brief description of a third structural model, that of Joseph Vance, -
which incorporates some features of the other two. It seems clear, regrettably, that_ the issue _ raised by Whetten regarding the place of the CE/Ju rocks in the basic tectonic l frantw rh cf the f::thill repien aretnd the prapn==A plant site has not been resolved by the considerable new data acquired in the past 18 =enths and that. no consensus on this. problem can_be _ expected _._ _____ until sc e future ti=e when =uen = ore of the region has been mapped geologically in detail and more geophysical data have been l acquired. The present situation of uncertsinty in the geologic community is exe=plified by recent studier in the vicinity of Little Eaystack and Tale Mountains, where the aeromagnetic and core-hole Jata recently supplied by the applicant can apparently be accommodated both by the applicant's mapping and interpretation of the thrust and by Whetten, Dethier, and Carrol's (1979) very diff erent mapping and interpretation. The most logical and responsible present course in the evaluation of the Skagit site seems to be acceptance of the existence of some major uncertainty among geologists regarding the geologic fra=evork of the site region and recogition of this uncertainty in conservative evaluation'of the bearing of specific geologic features on site safety.
- 2. High-anele north-trendine f aults near the olant site--The applicant has f airly conclusively demonstrated, by detailed observation and mapping supported by geophysical surveys, that no significant nceth-trending faults follow the valley floors of Gilligan and lower Day Creeks. The applicant has f ound little or no evidence for faults within the Shuksan meta = orphic rocks on the slopes above the creeks, but it is prudent to bear in mind that such f aulting might be difficult to detect.
' In two localities in the vicinity of the plant site, on the south side of the Skagit valley, the strong possibility or likelihood of high-angle f aulting younger than any regional thrusting is a present concern east of Gilligan Creek, along the l
contact between the Shuksan and CH/Ju rocks, and in the valley of Loretta Creak, at and near the Chuckanut-Shuksan contact. l The applicant's mappicg (RG1, fig. 3.2-2 and Appendix H, sheet
- 1) of the f ault between the Shuksan and the CH/Ju rocks in sections 12, 13, and 24 or T. 34 N. , R. 5 E. accords closely with Whatten's capping (1979), and all agree that the dip of the fault is steep there. Is section 1 of th.e same township, bedrock exposure is 1259 014
- - _ _ _ _ _ ._ .._. . ... D-26
nuvenu.a v . . . . . . . . . , commonly poor, and the contact can be less accurctely loca:ed.
.Sailabic cutcrepc :nd pr:b:ble cutereps suggest that it continues, with a northerly trend, through about the center of the section.
- In the northern quarter of the section, the few outcrops may per:1t an inf erence that the contact turns abruptly east, so as to join the thrust centact between the same rock units in the n rtheast
. quarter of section 6, T. 34 N. , R. 6 E. Even if the Shuksan-CH/Ju contact follows such a course, it see=s likely that the semilinear -
high-angle f ault that for=s the contact for miles south of section I continues on across the section to the north. Rather than a ~~~~--~~ steeply folded seg=ent of a thrust, the fault east'of Gilligan-Creek Cheresf ter tare =d the "Gilligan Fault") is nrobably a younger fa-It that cuts across the thrust.
~
~ '
The extent 6f the Gilligan Faul~t-to the isorth and soufh Ts----~
~
unknown and perhaps almost indeter=inable. If it extends northward beneath and beyond the Skagit valley, it enters a large mountainous mass of rather poorly exposed Shuksan metamorphic rocks; to the south, beyond upper Gilligan Creek, it is within the tectonically mixed CH/Ju rocks. In both the Shuksan and CH/Ju tscranes, the identification and mapping of f aults can be very difficult. South
- of upper Gilligan Creek, the f ault is conceivably related to a discontinuous lineament, apparent on side-looking radar (RGI, p.
3.2-8 to 3 2-11), that extends south to a point near the '.~ast and of Lake Cavanaugh, but there is little or no evidence that this lineament is related to the Gilligan Fault or to any geologic structure. The off s: ts 'n glacial lake deposits about where this lineament crosses tht. Lake Cavanaugh Road (Bechtel, Inc.,1978) are probably nontectonic. A second locality where there is concern regarding high-angle faulting, the valley of Loretta Creek (RGI, Appendix H, sheet 1; Whetten and others, 1979), was called to the attention of USGS reviewers by J. T. Whetten and P. R. Carrol. In the course of mapping geologically the lower part of the valley, Whetten and Ca? rol encountered an exceptionally good exposure of the Chuckanut-Shuksan contact in a small waterfall, at an elevation of about 1,250 f eet. Here the contact consists of a near-vertical shear zone, at least about 6 feet vide, that appears to strike north-northwest. The zone very probably represents a significant high-angle f ault of post-Chuckanut (post-Eocene?) in age. In the absence of strongly supportive data, the acceptance of any other interpretation here would be highly i= prudent. The length of this fault (hereaf ter termed the "Loretta Fault") and the amount and l exact sense of displacement on it are unknown. If it is present north of the Skagit valley, the dif ficulties in capping it there would be similar to those described ia the case of the Gilligan Fault. To the south, the f ault may follow the Chuckanut contact or
, may take another course.
4 .
. 1259 015 D-27
- Apo6ndix_D (Continuea)
- 3. Tn e "Loveseth" Fault-The "Loveseth" Tault, :.:ppci as a fault by T. P. Lovcscth (1975) :=d H. D. Grver (1979) is interpreted by the
- applicant (RGI. Section 3.4.2) and by G. M. Miller (RG1, fig. 3 1-3, sheet 2, and in press), a consultant to beentel, Inc., as the bacal sedi=entary contact of the.Chuckanut Tor =ation, disturbed The
- only by local shearing associated with Tertiary folding.
applicant does not mention the existence of any clear exposures of - the contact, and apparently its dip cannot be accurately measured anywhere. The geologic constraints en its location, though f airly tight in the northeast quarter of section 10 and perhaps in section - - -
- 3. T. 33 N. , R. 5 E.; are co==only loose f arther southr- On the h::10 =f :tidence ef strang sheering and of probable discordance of j
3 beds to the Chuckanut-CH/Ju contact, it seet.s most likely that the j contact mapped in sections 3, 10, and 11 is a fault. The - applicant's interpret'stion cannat, perhaps, be ruled out -here,-but--- it is surely not unique. Farther south, the most likely course of such a f ault lies probably along the southwest side of the large volcanic body in nection 14 and through the Chuckanut beyond, but other courses may be possible. The applicant's mapping, which established the presence of Chuckanut east of the large volcanic i body in section 14, as well as west of it, suggests that the The discordant dips in "Loveseth" Fault is not a major structure. the Chuckanut in the northwest quarter of section 13 suggest that the contact there may be similarly faulted.
- 4. The "B and B Fault"_-The possibility of a "B and B Fault" was discussed at the ASLB bearing in March 1978, largely as a result of USGS concerns regarding a letter received f rom Feter Ward, a stratigrapher who had been carrying on research in the northern San Juan Islands. Ward suggested that the Nanaimo and Chuckanut sedimentary rocks had been deposited in separate sedimentary basins and that their present juxtapcaition across the strait between Barnes and Clark Islands and Lun i Island might be best explained by large-scale strike-slip faulting. The USGS had not fully evaluated Ward's suggestion at the time of the hearing and could
- only speculate regarding the proposed f ault. In informal' consideration of possibly permissible courses for the proposed f ault south of Lummi and aspecially of such courses that were closest to the proposed pI?.nt site, it was thought that such a fault might possibly bend eastward and pass through the Table Mountain vicinity south of Cultus Mountain. It was this highly speculative, " worst-case" fault trace, between the vicinity of Samish Bay on the north and the vicinity of Lake Cavanaugh on the south, that W. H. Hays sketched on a map at the March 1978 hearing
' and that was there so=ehow christened the "B and B Fault."
Since that ASLB hearing, studies by the applicant and others have led to doubt regarding the possibility of large post-Ghuckanut f aulting west of Lucci Island, but seem to have established the presence of faulting near Table Mountain. In the northern San Juan 5. o-28 1259 016 e
_ Appendix D (Continued) Islands, a major f ault between Lum =1 Island and Orcas Island was first suggested by Misch (1966), and considerable evidence points to f aulting there that has aff acted rcchs && y:ung cc the U;;cr Cretaceous Nanaimo Group. The possibility of f aulting younger than the Chuckanut For=aion on Lu==1 Island is, however, more difficult to evaluate. Stratigraphic studies by the applicant (RGI, saction 3.4.1) suggest correlation of the Chuchanut formation on Lu=mi .
- Island with strata on part of Sucia Island and on some other islands vest of Lu=m1 and thus support some of the earlier similar conclusions of Vance (1975,.1977). _ This. c.orrelation see=s to_ lack.
l the support of conclusive evidence for the various deposits being j of che same age but, il valid, it would seen. to pracluda inf arasca of large-scale f aulting vest of Lummi Island since Chuckanut -- (Eocene and older) ti=e. j ___ -- Another developnent since the March 1978 hearing has been increasing realization that the Chuckanut is probably younger than almost all, if not all, of the marine Nanaimo Formaston on Earnes and Clark Islands and elsewhere vest of Lu=mi and thac th e
. Chuckanut probably need not have been deposited in a l asin I distinctly separate from the Nanaimo basin, as Ward r oggested.
Thus, chile large-r:21e pest-Chuckanut f aulting vest of Lummi Island cannot at present be ru1(- out, strong evidence for such faulting appears to be lacking. If there is no such faulting, there is, of course, no necessity of accommodating similar move =ent to the south, along some such structure as the "B and 7 Fault." In the Table Mountain vicinity far to the south, which Eays 9 speculated to be on one possible southerly course of the major fault suggested by Ward, continued geologic mapping has indicated
, the presence of a northwest-trending f ault or f ault zone (Whetten and others, 1979), an interpretation that is not necessarily inconsistent with the applicant's field studies (RGI, p. 3.4.1-4).
Whetten (oral comm. , 1979) believes that this fault or fault zone - has near-vertical dip and has considerably disrupted the CH/Ju and Shuksan plate rocks in the Table >buntain-upper Nookachamps Creek vicinity. The age of this structure (hereaf ter termed the " Table Mountain Fault") and its extent to the northwest and southeast are unknown.
- 5. The are of hich-anele f aultine in the site vicinty--It see=s clear i that a conservative geologic analysis of the site vicinity cust take into account the probable presence of fairly nu=erous high-angle Tertiary f aults, one or two of which may pass within a few miles of the plant site. These faults include the Gilligan, Loretta. "Loveseth," and Table Mountain Faults discussed above; probably some of the f aults =apped by Whetten, Dethier, and Carrol (1979) and the applicant vest of middle and upper Day C cek; and perhaps the Shuksan-CH/Ju contact at 'To Name Creek." All of these 6 ~
1259 017 0-29 e
Appendix D (Continued) f aults should probchly be prc ::cd tc bc 7:en :: th:n 1: tert Chuckanut deposition (. Middle Eocene), though sene =sy be elder. fome probably f orced d ring t.,e eely Tertiary (Eocene?) def or=ation that f olded the Chuchanut, but the ne :-vertical C;.: Of at Ict;t
=any of the f ault surf ace.s and the linearity of the surface traces
; of the Gilligan Fault and probably other f aults suggest that some
, of these structures f ormed af ter that deformation. .
{ I In discussing the possible age of this f aulting, it may be instructive to consider the nearest faults that have, tentatively
- at least, been considered capable. Tne Devil's Mountain Fault, which passes 21 km southwes . of the plant site, is c,nsidered capable by the USGS, largely on tne basis of a lack of evidence that it could not be capable and of probable displacements as -
~~- ' --
depicted in marine profiles located west of Whidbey Isl~an'd.~ %e Straight Creek Fault, 48 km east of the site, has probably coved a little since eld-Tertiary ti=e and is " tentatively classified as capable" (McCleary and others, 1978, p. ii) in a study carried out by a consultant to the Washington Public Pcwer Supply Systen.
' While it seems legitimate, in a conservative site analysis, to consider both of these faults capable, it should be remembered that both are very long, very large regional structures, which have doubtless moved many times; that displacement along the Straight Creak rault was vary largely, if n *. clic:t c:tirely, acce ;lished by Miocene time; and that there is no evidence that movement along the Devil's Mountain Fault was not also largely completed by that The capability of two caster faults of the region does not time.
logically establish capability for the many smaller f aults. It
, seems likely that most, if not all, of the high-angle f aults of the site vicinity have not moved in Quaternary time. There is no strong evidence of such cove =ent on any of them. On the other hand, displacements, especially minor displace =ents, that =ight
! conceivably have occurred on these faults between 500,000 years ago and the end of the last glaciation (about 13,000 years ago) could -
have been covered or obscured by that glaciation; and the applicant (,,,, has presented no strong evidence against such conceivable
' ~
movement. In su= mary, it appears that the hi, angle f aults of the
- site vicinity are very probably but uncertainly incapabic.
- 6. nie 1946 earthouake near the northeast coast of central Vancouver Island--Several papers dealing directly or indirectly with the 1946
. earthquake have been published since submittal of the last USGS Status Review in February 1978. Rogers and Easegava (1978) recoc:pute the epicenter of the earthquake and place it on Vancouver i
Island near its northeast coast, rather then under the Strait of l Georgia. They favor a fault-plane solution calling for a 2 northwest-striking f ault surf ace, possibly in close proximity to
- the similarly striking Beaufort Range Fault, but thef do not rule out a northeast-striking surface. Riddihough (1978) relates the 1946 event to a northeast-trending fault in the subducting plate 7 -
0-30
. - . - - . . . . . . - . - . . - - - _ . - . _ _ . . - - . . _ - . . - _]2.00 N.L _
~
' Appendix' O (Continued) beneath the crust of Tancouver Island. Slawson an' Favage's (1973.?
rerervey of an old triangulation network revealed distortions
- consistent with move =ent on the Beauf ort Range Tault, but
~~~. association of the earthquake with surf ace geology appears to
; remain doubtful. ,
3 . 4 Aeromagnetic Data j T
! The USGS has reviewed the analysis of aeromagnetic data by --
' Exploration Data-Consultants--Incr-(Edcon) and found it to be generally -- - -
aari= factory. Alternative interpretations ar2 possible in some cases, however, t.nd the data do not unambiguously eutablish the correctness of the applicant's mapping and tectonic model. The following are several .
~
general r.onclusions arisibg f rom our Teview of these data and the Edcon ----- -- report: (a) Aeromangetic anomalies help to define the chape and location of highly magnetic rock types, which are pr:bably serpentinites in this area. In particular, ,tromagnetic data were used to deduce the nature of the con;act between Shuksan metamorphic' rocks and the CH/Ju rocks. However, the dips of the surf aces of the serpentinite bodies may be unrelated to the attitudes of the thrust because (1) the so.eco....... __, not be originally associated with thrusting; (2) serpentinite is a highly mobile rock when subjected to stress; and (3) serpentinites possess highly variable magnetic properties. (b) Concern regarding an abrupt change in trend of the aeromagnetic contours southeast of Butler Hill, on small-scale USGS aeromagnetic maps (see RGI, fig. 3.3-2), was expressed in
- the NRC-USGS request to the applicant of June 1978. The
' additional more detailed aeromagnetic survey data provided by the applicant indicate that there is no magnetic basis for a -
f ault with an east-west trici along Skagit Valley south of Butler Hill. (c) Anomalies along the thrust contact in the vicinity of Tale-Mountain are caused by rock bodies (probably serpentinite) which appear to have northeast-dipping northeast f aces. However, these bodies extend to only about 2,000 feet below ground surf ace and could, theref ore, bc confined within a thin thrust sheet. (d) The absence of f ault-related aere=agnetic anomalies near the mouth of Gilligan Creek, south of Butler Hill, and elsewhere does not necessarily preclude the existence of faults in these areas. 8 : 1259 019 o_3i
' * - - - ~ -.
Apoendix D (Continued) (e) Aerocagnetic caps show that ano=alies characteristic of CH/Ju
- chs ::'.:117 ::rmin:tc :: th. threet ce=tect betv--e Shuksan and CH/Ju rocks. This suggests that if CH/Ju rocks underlie Shuksan, as the applicant has suggested, they lie at depths in excess of 10,000 f eet. As CH/Ju #requently crops out in small areas surrounded by Shuksan and away f rom the thrust (e.g., at ,
Butler Hill), this lack of anomalies over Shuksan terrane forces a rather co= plicated geologic model. Sc sm:1 gy i I I detailed (#scussion in the Ceology-and-aeromagnetic _ data saeWe of tt .s r-wiev is presented concerning the existance and age of numerous high-a a faults in the site vicinity. The conclusion offered there is that taese f aults are "very probably, but uncertainly, incapabic " I i On the other hand, there have been nu=erous small earthquakes in the ares containing the site. Their magnitudes range from 1.2 to 3.3 and several have been . felt local.
?. An alignment of epicenters has been noted in the Skagit River Valley but there are doubts about the absolute accuracy of their locations. The stated accuracy cf events located by the University of Washington-operated seismic nervork is 1 to 2 km (Crossen, 1974). However, the seismograph stations in the Skagit Valley vicinity are f ew+r and more videly spaced than tnose further to the south and because the locale is near the northern limit of the network coverage, the accuracy is not likely to be that precise. The re=aining epicenters appear to be essentially random in the site vicinity. None of the small earthquakes have been associated with identified f aults.
It is possible that these earthquakes are associated with structures that are sufficieatly small that no surface expression has been recognized by the geologic investigations. j Consequently, it is our judgment that for purposes of nuclear- - - reactor design it should be assu=ed that earthquakes as large as magnitude 4.0 could originate on any of the identified faults or an un=apped fault in the region of the plant site. HovcVer, it is our judgment that even if an earthquake of this si=e were to ocent on one of these structures, its consequences at the plant site vould be less than that already postulated to result f rom the two =uch larger earthquakes discussed in our previous reviews (Jan. 30, 1978 and Feb. 23, 1978). Since our last review, other articles relating to the tectonics of the Pacific Northwest, in general, and the Vancouver Island vicfnity, in particular, have been published. They reinf orce the concept that the 1946 earthquake was related to the present-day subduction regine of the region rather than regional north-south co=pression (Rogers and Hasegawa, 1978). Rogers (1979) suggests that the occurrence of earthquakes having a northeast alignment across Vancouver Islard are l related to differential =ove=ects, of the Explorer and Juan de Fuca 1259 020 D-32
Appendix D (Continued) plates. Iwo 'nterpretations of the tectonic settings are presented; his prcierred rc'.:i:s ruggests that the subducted Foothe !sult Tore ("r~ed
#~~ '*
by Hynd=cn, et al,1978) would represent, in effect, a tectonic boundary, north oi which earthquakes would occur becausa of intense local north-south co=pression resulting f rom the Exploret-A= erica pihto interaction. To the south of the Nootka f ault zone, subduction of the . Juan de Fuca plate beneath the America plate would proceed in a nor=al but aseismic fashion. In view of these coneiderations, we believe that the constrain .s on the locus d 2n event si=ilar to the .1946 earthquake- - . would make its significance less than the two postulated controlling certhquahts c. sos aan. au, Asis,c s scs. 2;, Ar.s;. i In s u==ary, there is.4 recognition thac--there bas-been a large amount of new geological and geophysical data provided by the applicant and others since the preparation of our last review. However, there re=ains a major uncertainty as to the completeness, significance, and proper interpretation of both the new and previously discussed data. Consequently, it is still our jugment that for purposes of nuclear reactor design the following two earthquakes should be considered as eortrollin3: .
- 1. an earthquake simila. to the one that occurred Dececher 15, 1872 but having its epicenter suf ficiently close to the site that no attenuation effects be considered, and
- 2. a shallow magnitude 7 0 to 7 1/4 earthquake on the Devils
---. Mountain Fault 21 km. f rom the site. The USGS reactor site review team agrees with the applicant's proposed use of a bedrock acceleration value of 0.35 g as the Safe Shutdown Earthquake for use with the Safety Guid- 1.50 design spectrum for nuclear power plant design. f
?
1 b 1259 021 9-33,
Appendix D (Continued)
. y.c : References
'3 .
Bechtel Incorporated,1978, Subsurface investigation of disturbed . glacial deposits on Lake Cavanaugh Road: Report to Puget Sound ~ Power & Light Co. Brown. E. H. 1977. The Fidalgo Dohiolite, in Brown, E. H., and Ellis, R. C. (eds.), Geological excurrions in the Pacific horthwest: Western Washington Univ., p. 309-320.
- Brown, E. E.g Bradshaw, J. Y. , and Mustoe, G. E. ,1979, Plagiogranite and keratophyre in ophiolite on Fidalgo Island, Washington: Geol.
Soc. of America Bull., Part I, v. 90, p. 493-507. Crosson, R. S.,1974, Co=pilation of earthquake hypocenters in western Washington 1970-1972: Division of Geology ord Earth Resources Infor=ation Circular 53, 25 p. Crosson, R. S. ,1975, Compilation of =.rthquake hopocentars in vastarn Washington-1973: Division of Geology and Earth Resources Information Circular 55,14 p. Crosson, R. S. and Millard, R. C. , 1975, Compilation of earthquake hypocenters in western Washington-1974: Division of Geology and Earth Resources Information Circular 56,14 p. Crosson, R. S. and Koson, L. J.,1978a, Compilation of earthquake hypocenters in western Washington-1975: Division of Geology and Earth Resources Information Circular 64,12 p. Crosson, R. S. and Noson, L. J.,1978b, Compilation of earthauake hypocenters in western Washington-19 76: Division of Geology and
. - Earth Resources Information Circular 65, 13 p.
Hyndman, R. D. , Rogers , G. C. , Bo se, M. N. , Lis ter, C. R. B. , Wade , U. S., Barrett, D. L., Davis, E. E., Lewis, T., Lynch, S., and See ann, D., 1978, Geophysical measure =ents in the region of the Explorer ridge off western Canada: Canadian Journal of Earth Sciences, v. 15, pp. 1508-1525. Miller, G. M., 1969, Western extent of Mid-Cretaceous thrusting in Whatcom and Skagit Counties, Washington: Geol. Soc. of A= erica Abstracts with Programs, part 3, p. 42. Miller, G. M., in press, Western extent of the Shuksan and Church Mountain Plates in Whatcom, Skagit, and Snoho ish Counties, Washington: Northwest Science.
~
1259 022
~ '
0-34
AccendixD(Continued], Misch, Peter, lir66, "ectonic evolution of the Northern Cascaces of Washington State, ,,i_n_ Tectonic history and cineral deposits of the Western Cordillera in British Colu=bia and neighboring parts of the United States: Canadian Inst. of Mining and Metal., 1964 Ann.
- Western Meg. , vaccouver, B.C. , p. 101-148.
Misch, Peter,1977, Bedrock geology of the North Cascades, _i_n Geological
- . excursions in the Pacific _ Northwest:_ Geol._ Soc. of A= erica,1977
! Ann. Etn. Seattle. Washi,eton: oubl. by Dept. of Geol.. Western j Washington State Univ.
I Misch, Peter,-1979 Geologio-cap -of-the _Marblemount-Quadrangh , Washington: Washington Dept. of Nat. Res., Div. of Geol. and Earth Resources, Geol. Map GM-23. ,
~
I Ridd1 hough, R. P.,1978, The Juan de Fuca Plate: A=erican Geophys. I Union, EROS, v.59, no. 9, p. 836-842. Rogers, G. C.,1979, Earthquake fault plane solutions near Vancouver . Island: Canadian Journal of Earth Sciencerr, v. 16, pp. 523-531. l I Rogers , G. C. , and Hasegawa, H. S. ,19 78, A second look at the British Columbia earthquake of June 23, 1946: Seismol. Soc. of America Bull., v. 68, no. 3, p. 653-675. Slawson, W. F. , and Savage , J. C. , 1978, Geodetic deformation associated with the 1946 vancouver Island, Canada, earthquake (abst.): American Geophys. Union, EROS, v. 59, no. 12, p. 1210. Vance,' J. A. ,1975, Bedrock geology of San Juan County, ,i_n, Geology and vater resources of the San Juan Islands: Washington Dept. of Ecology, Water Supply Bull. 46. Vance, J. A.,1977, The stratigraphy and structure of Orcas Island, San Juan Islands,_i_n_ Ceologic excursions in the Pacific Northwest, Geol. Soc. of A= erica Ann. Meg. , Seattle Washingten; publ. by Dept. of Geol., Western Washington State Uiv., p. 170-203. Whetten, J. T. , Jones , D. L. , Cowan, D. S. , and Zart=an, R. E. , 1978, Ages of Mesozoic terranes in the San Juan Islands, Washington, in, Mesozoic Paleogeography of the Western United States, Pacific Coast
. Paleogeography Symposium 2: Pacific Sec., Soc. of Econ. Paleon.
I and Mineral. , p. 117-132.
,' Whetten, J. T. , Zart=an, R. E. , Blakeley, R. J. , and Jones , D. L. , in press, Allochthonous Jurassic ophiolite in northwest Washington:
f Geol. Soc. of A= erica 3ull. 1259 023 12 D-35
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