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DavIn S.~FLEISCHAKER ATTORNEY A T TAW SustrSoa 102515rn Snrer.
- WAsna.a.cron. D.C.
(2c2) 833 00N June 9, 1978 Dr. Chester Siess, Chairperson Diablo Canyon Subcommittee
!!uclear Regulatory Commission Advisory Committee on Reactor Sa feguards lia s hi ng to n,
D.C.
20555
Dear Dr. Siess:
I am writing on behalf of the Intervonors in the Diablo Canyon operating license proceeding.
On June 14th and 15th the subcommittee will meet to consider presentations by the iluci ca r itegula tory Commission S taf f ("the S ta ff") and Pa ci fic Gas and Electric Company (." Applicant") regarding the adequacy of*Dieblo Canyon's seismic design.
Our understanding is tha t the Sta f f. believes its analysis is sufficient 1) so permit the ACRS to approve the scismic design criteria p~ ~
adopted for the reevaluation of the design of Diablo Canyon.
and 2) to conmi t to the Staf f's discretion the resolution o f '
"o u t t, t a n d i n g items" relating to the design of the Ca tegory I structures, systems and components.
For the reasons set out belnu, the Intervenors respec t-fully request the ACRS to. rej ec t a ny reques t to approve the seismic design cri teria adopted for reevalua tion or to commi t to the Sta f f's discretion the resolution of the " outstanding items."
I.
Approval of the design criteria adopted for reevaluation is not merited until the Staff and/or Applicant have justified critical assumptions in their analysis.
This is particularly important because the net effect of each of these assumptions is to reduce the seismic design limi ts o f the f acili ty.
flei t h er the Staf f nor the Applicant have provided an adcquate basis for selecting an "adjnstment c o n s ta n t. for effectiveness" equivalent to 0.65 to reduce to.759 the accelera tion used as the zero period limi t for the facilities' design response spectra.
l h e ll. S. Geological Survey Circular 672 concludes _that in. the near field.the peak accelera tion to be expected, nn average, and. corresponding to..a 7.5 8608230454 860001 PDR FOIA HOUCH36-391 PDR
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Dr. Chester Siess, Chairperson June g,1978 Page Two flagni tude ea r thqua ke, is 1.15g.
The Staff and the Applicant apparently accept that conclusion, but proceed to apply the
" adjustment constant" to reduce the accelera tion value to
.75g. !!owhere in the Staf f and ' Applicant presentation is it demonstrated that there exists in the scientific community general. agreement for selection of 0.65 as an " adjustment constant" to reduce peak acceleration values.
In fact,.the Applicant admits that no such agreement exists. 1/
Furthermore, 10 C.F.R.5100, App. A requires that the zero period limit for design response spec tra correspond to peak accelera tions.
In view of this regula tory requirement and the admission that there is no general agreement on selection of "adjus tment cons tants," additional justifica tion is needed for shaving the peak in this case. 2/
Second, neither the Staff nor the Applicant have provided data that justifies the high damping values used in the reanalysis.
Admittedly, the Regula tory Guides permit the use of damping values equivalent t o 71'. - the value used in the reevaluation.
I!ouever, it makes no sense to invoke the Rcgula tory Guides a s jus tification for using high danping va l u c's, (as the Staf f has done here) in lieu of producing scientific data to justify use of these values.
This is particularly the case where, as here, the facility is sited in a zone of high seismic risk. 3_/
Additional data to justify the use of the 70 damping values should be required, and the Staff should be required to analyze the consequences of actual damping values being lower than those assunad.
Third, the tau.effect analysis is based on unrealistic assumptions.
This analysis assumes tha t the structure foundation is rigid, and it is not.
flei ther the Staf f nor the Applicant have demonstrated the applicability of tau to 1/
FSAR, Amendment 50, Volume III, App.
D, a t 11.12.
~2/
Generally, Staf f practice is to use the mean of the peak acceleration as the zero period limit for the design response spectra.
The selection of a lesser value here is an exception to tha t prac tice.
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Use of such hig'ractice.h values represents a departure from l
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Or. Chester Siess, Chairperson June 9,,1978 Page Three rock founda tions.
The analysis assumes that seismic waves approach from a horizontal direction and fails to consider the consequence of the scismic uave impact from an upward angle, the angle of wave impact in the near field.
In addi-tion, the analysis fails to consider adequately the torsional effects on the foundation.
There is little discussion of the tau-cffect in the scientific li tera ture.
Thus, the theory has yet to be ex-posed to the criticism of the scientific community.
There is little reason to adopt this approach when better methodo-logies are availabic.
Specifically,.the 3-dimensional soil-structure interaction methodology should be applied to the design of this facility.
Computer programs, such as the CLASSI program, are readily available for such analysis.
Use of such programs would eliminate the need to rely on the questionable tau-effect approach. 4/
II.
Fle turn next to the resolution of outstanding issues relating to the design of Category I structures, systems and RMMWaf components.
The Staff is expected to request that these mattere be committed to its discretion.
~he Intervonor's are opposed to an ACRS " sign-off" on these items and, instead, request the subcom.iittee to maintain jurisdiction over these matters pending submission and evaluation of a more complete SER.
This request is based on the unusually large number o f items-critical to the safe operation and/or shutdown of the facility which remain " outstanding;"
further in its analysis of these items, the Staff has departed from Regulatory Guide procedures and/or Staf f custom and practice.
First, at page 22-1 of the Sa fety Evaluation Report, Supplement No. 7 (."SER, Supp. 7"), the Sta f f summarizes the items which remain "ou ts ta nd i ng. "
The list includes seventeen (17) different categories.
These seventeen (17) categories 4/
S o c,' 11. 0. Trifunac, C011i4Et!TS Oil SEIS!IIC DESIGil LEVELS FOR
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llTAGLO CAftY0tl SITE Ill CALIF 0Rfil A, April, 1978.
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Dr. Chester Siess, Chairperson June 9, 1978 Pag,e Four represent approxima tely forty-six (46) individual outstanding items, as cited elsewhere in the SER, Supp. 7. 5/
Second, of the open items, many are.9ritical to the safe operation and/or shutdown of the facility.
These include:
Containment Interior Structure - discussed at p. 3-28; Intake Structure - discussed at p. 3-34; Turbine Building - discussell at p.
3-39 and 3-40; Reactor Coolant System - discussed at p.
3-48; El ec trical Equipment - discussed at p.
3-75; Emergency Core Cooling System - discussed at p.
6-6.
Third, in its analysis the Staff has departed from Regulatory Guide procedures and/or Staff practice and custom.
g At p.'3-22 and 3-23, the Staff candidly admits that "d7he general, methods of analysis used by the applicant in the seismic reevaluation as outlined above contain three sionifi-cant relaxations relative to the normal, or currently 7cle;;ted.
procedures.
One relaxation is reduction of ground response spectra to account for building size effects.
The-second is use of actual material strengths rather than code specifi'ed minimum material strengths.
The third is allowance for ductili in structures which might be used in tuo specific cases and specifically justified'." /Jmphasis added7 In addition, still outstanding are analyses applying the Regulatory Guide 1.92 method of combining responses to
_5/
Section 3.8, p.
3-45 and 3-46, 22 items; Section 3.9, p.
3 and 3-70, 17 items; Section 3.10, p. 3-75, 6 items; Sectie-6.3,p,.6-6, 1 item.
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Dr. Chester Siess, Cliairperson June 9, 1978 Page Five the reactor coolant system main loops; 6_/
the support capacities for reactor coolant systems branch piping; 7/
the other Category I piping.8_/
Also, the seismic qualificatio'n methods employed in safety-rela ted electrical instrumentation and control equip-ment does not include the a.ging requirements and other general environmental qualification recommendations that are reflec ted i n !!cg u l a to ry Gu i d e 1.100.,9_/
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III..
Apart from the matters raised in preceding paragraphs, we would like to direct your attention to two other matters.
First, there is a case to be made that a 7.5 ilagnitude carth-quake does not represent the maximum earthquake potentic1 of the llosgri-San Gregorio f aul t.
The case is summarized in the m
attached letter from Dr. Eli A. Silver to Drs. iicKeown and Devine. 10/
The' recent work of Drs.
R.
K. 14a r k a n d fi.
G.
Bonilla lioister this case.ll/
This work concludes that the methods generally used to E timate earthquake magnitudes freu fault lengths, yield estima tes of the averane size, rather tha:
the maximum size event which could occur oh a fault.12_/
J/
SER, Supp. 7, at'3-69.
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Ibid.
8/
Ibid.
l 9/
Ibid, at 3-72.
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See, Attachment i.
lif See, Attaclimant 2.
12/
He are aware that many scientists are of the opi.nion that argument of whether the Safe Shutdown Earthquake ("SSE")
be Ilagnitude 7, 7.5 or 8, is of secondary importance. (5y At.tachmen t 3, Lct.tcr f rom F. A. fic Keown and J.F. Devine
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.Eli A.
Silver, dated. April 13. 1970).
!!cycr thcl cs s., s i nc. -
case can he made for-an Sr.E :ll.iiin i t ud e 7. 5, i t.i s p,i r t. i c u ' '
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Dr. Chester
. css, Chairperson June g, 1978 Page Six
- ond, the Staff and Applicant selection of an Operat Basis Earthquake (."0BE") corresponding to a peak accele cinn of.20g contravenes the express language of 10 C.r.d. 5100, App. A.
The maximum vibratory ground acceleration of the Operating Casis Earthqual:o shall be at least one-hal f the maximum vibratory ground acceleration of the Sa fe Shutdown Ea r t hqu a l:c. l_3]
Selection of an ODE equivalent to one-half (h) the SSE provides an important margin of safety for the facility.
Elimina ting this requirement may make ser.se in seismically inactive regions.
110 wever, uhcre a plant is located in an active region, as is the case here, it makes sense to adhere to the regulatory requi.rements.
The Staff should provide adequate justification for accepting an OBE equivalent to
.209 and should assess the added safety achieved by designa t ing an OBE equivalent to.409 IV.
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In' conclusion, the Intervenor's submit tha t approval of the scismic design criteria adopted for reevaluation of Diablo Canyon is inappropriate until the Staff has justified the questionable assumptions and/or conducted additional analyses as outlined above.
In addition, the ACRS should not commit to the Staff's discretion the resolution of the "out-standing items."
Ra ther, subuission of. a more complete SER should be required.
Respectfully submitted, J0llN R.
pilILLIpS SI! ION KLEVAliSI'Y 11ARGARET :BLODGETT Center For Law In The public Inte:
10203 Santa Monica Boulevard Los Angeles, California 90067 13/
10 C.F.R. 0100, App. A V(a)(.2).
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Dr. Chester Siess, Chairperson June 9. 19.78 Page Seven DAVID S. FL E IS Cll AKER 1025 Fifteenth Street, 11. W.
Suite 602 Washington, D. C. 20005 Attorneys For Intervenors SCEllIC S!!ORELINES PRESERVATI0t: C0i I' SAll LUIS OBISP0 MOTHERS FOR PEACE ECOLOGY ACTION CLUB SAllDRA A. SILVER J0llll J. FORSTER 4Cffh.Q_.lh*{5C)i'.b.*Uis 0
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5ANTA Citt Z. C U.ll on StA 0,064 TJ*'into$4 of" NA Tt*r.* t. SCifWCf S AHLir.D 3CICr:C.E5 DUILl> LNG February 27, 1978 Frank A. licl'.coun James F. Dcvine U. S. Geol.<;r,1 cal Cur >cy Reston, Virginia 22092
Dear Frank and Jim:
Rece.atly I uns asked by Icuyern at the Center for Lau in the Pabiic Interest to c:: amine the various scismic nafety
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docm:ents par taining to the Diablo Canyon pouer plant.
As usual the PG5E consultants have a strange uay of vieuing the world, but in general your analysis of April 29, 1976 nicely covers many of these odditics and prcsonts the pi'cture in n ;;3 a straight-foruard uay.
I disaa,ree uith your analysis of Ammendm en: 34, NRC question 2.21, horever, and I'll give the reanons in this letter.
At the time your letter uns usitten such disarreement would have been more intuition than documntal ion on my par t, but nou I think the documentation in so ctron ; that perhaps you mi.ght uant to re-consider your recomnendations.
At least I'd like your feelin.:;s on this because I don't cant to appear an disar,rceing with the Survey in mattern of substance here.
In fact, I'd rather not appear at all if I can help it, but I have this deep moral voice tugging, at my conscience.
So much for i ntroduc tion.
fnutt lemtt.h The tuo major arcan of dinar.rcement concern and hi storical o f fset.
I thi nk the data are coupalu.nc, nor a continuou.; nom.;...;an Grec,orio fault ;;one 400 100 ic lexch,
with Miocena or younger of fset i n e >:c e n :. 'o r LOO tua.
Or.ing the very cruda Lon.;th-magni, tude rel ation:..;ctdo indicate a potential magnirnde S carthquake on this Guil P r.onc, and expected recurrence interval Cor such a hypothetical event is in the order of 150 to 300 yearn, ucil i n encess o f avail-able record.
The Cirst in:obl em in f anit 1ength.
Si uce voor April 1"76 f
analyni n uc have obtai ned det ailed anfom:n:ne ti c data alon,c,
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mont. o f thi fa.ul t none,.ani.i S;cyc Gr.iham ha. pu t.
hi.s thesin tor.cthei n'n tlic r er.i onal y,b o lo g,.
Staitin",'in.the north, the-
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Frank A. 14cKeown Page 2
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James F. Devine February 27, 1978 first problem is southuard continuation of the San Gregorio
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fault.
Greene and others interpret the Palo Colorado to be the southern c:-: tension of the San Gregorio.
Two lines of evidence argue against this as the major connec tion.
Steve Graham's mapping indicates the Sur fault shows a taore intense cruch zone, indicative of major faulting, and very dissimilar rock types acronn the fault.
The Palo Colorado in contrast is a narrou zone uith rather similar rocks on either side.
Also, an IIS thesis by Woodson (1973) at the Naval Post Grad.
School, involving detailed bottom gravity studies shoun a pronounced.gravit/ gradient off the coast (nee enclosed figure) which could allow a major fault connection to either
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the Serra 11i11 or sur fcui tc but not to the Palo Colorado.
This doesn't mean the.Palo Colorado does not connect the San Gregorio, only that it is not a major strand of the fault.
Projecting; the sur ser, ment of the fault southward intersects
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a pronounced maguctic anomaly that parallels the coast and i mont probabl-j fault controlled.
IIcCulloch says a faul t rung.
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along this anomaly as seen in hin scismic profiles (see magnetic map enclosed).
The fault can be traced southward in seismic profi!cc to near the ucse side of the large magnetic anomaly off Cape San Martin.
The San Sineon fault runs along the coant south of Cape San Martin and car.not continue on a m
st.raight line through thi n large anomaly.
With a sli.qht westward bcud, however, it would join eith the fault we've been tracing from tha north.o.' Thi s arca aroun.1 Cape San Martin may need more detailed work [g$t there in the probica o f very shallow unter.
Connoc Lion bTiti:cen the liost:ri and San Sim2an faults is nicely discussed in your own analysis.
I think the availabic data are mont consi stent with the e
San Gregorio-l!angri as a _ continuous fault zone, from about Pt. /srr,uello to San Francisco, a 1cngth of 400 hm.
Itow about offset?
I first suggested 80-90 km ca.the San Gregorio baned on offset basement terrancs,. cont relleri by oCCshur'e p,ravity data.
Thi.n interpretation uns based on conniderino, the San Grc5,;or i o to of fset the t',ranitic-Francincan contact at Pt.
Sur northuard to its probable location on the went side of the Farallon ridge, near Ado Mneva, llall (1975) then came one with his interpreted 80-100 km of lior.:;ri of fset.
It uan then that I first nunpected the faults to be continuoun.
Clarence has had an on;,oi.ng battle with Doug flamilton (and nou Seiders). on thin interpreted effnet but everytime I watch the battic I'm more impecuned with l!all'n armmmnt...
- Anyuay, that offnet in not proven but thn cane in strone, enouc.h to
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be corrinome.
Nnu Grnham and nickinnon have a paper in Science discunning a numbrtr of featuren indicatin:- 115 km ef offset mostly nn,the San Gregnein.
.I'm imprenned uith. the
, care tl' icy have unctl' 'to analy,f:e ' t hi n o f fset.
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Frank A. ticl'coun Page 3 James F. Devine February 27, 1978 Still these data are not nearly as tic,ht as the o f fset shorelines covered by well-dated and chemically odd lavas that arc offset by the San Andreas, giving cract piercing points, or the more general but ntill ti;;htly controlled stratigraphic aq;uenents for San Andreas offset.
We might ask, however, whether such data would be seen if the San Andreas unre 907, under water.
I doubt it.
I'm encioning a o f the Grahan-Dickinson paper.
Clearly the lart;c o f fset co py' t proven but I think it is now negligent to disregard the isn po nnibili ty.
What does this mean in terms of crustal plate boundarien?
It goes a long uny to solvi ng a major problem of San Andreas offset.
L'hy are effnetn in central California 300 km but thone in northern California 550 to 600 km (Pt. Arena).?
In part it is because of 115 hm offset on the liongri-San Gregorio feeding into the San Andreas off San Franciso.
The Rinconada fault may add another 35 lua at most.
This leaven about 100 km to either latest Cretaceous movenent, to an incorrect
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estimation of the total value - 550 to 600 1:m, or to as yet undiscovered offsats on other faults in the c.a lini an' b lock.
Thin one 's up for grabs as 7. see it now.
Iloucver, the !!ongri-San Gre;;orio cust be considered as a major fault zone taking up significant plate offset in the late Cenozoic.
SINIOW1 One other interesting observation that only I scem to have made is that the Lnunon report of the 1906 SF carthquake reports measured o f fsets of 15 to 20 1:t (5-6 m) north o f San Francisco and 8 to 10 ft (2.5-3 m) south of the City.
No offsets in excess of 10 ft ocent to the south.
Thi.s can he interpreted in 2 uays.
One in a dy.i ng cut o f slip to the south.
The other is that strain release occurn on both the San Andrean and San Grer,or.io-llon5;ri faults south of SF but only on the San Andreas to the ncrth.
Either is equally acceptable (to me) at present.
Finally, t; hat about the ponnih i.li t.y o f a ma.o.ni.tude S carthquake on the San Grenorin-Itong.ri?
I'm sure you'll a:;rce that if 400 km in accepted as the faul t lent;th, that a ma:>.ni -
tude 8 munt be at least considered.
One question uc now ask in uhy have uc no record of such a larne carthquake on this fault none.
One reason mi.ght be the 1 em;th o f time uc uould have to unit.
Magni tude on strike nl.ip faultn in related to faul t clip and that in a function of nerai.n acconulation.
Let 's take 5 m an a reasonable strain accumulation for a magnitude S, an in obnorved for the 1906 quahc.
Strain aecumo-(
lation depends on the pbynical propertien o f t he faul t plun the ra te o f plette movement acron: it.
Pac i f i c - Amm:i.ca pla t e movement in about 5.6 cm/yr uhich mitnt be'dintributed acronn
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Frank A. 14cKeown Page 4 JamesF.}Devine Feb u ry 27, 1978 at least the entire San Andrean fault nyntem.
Weber and Lajoic--
(1977) report a llolocene slip rate of 1.6 cm/yr on the San Gregorio fault and nurford reports up to 3.3 cm/yr geodetic slip on the central California part of the San Andreas, in the region uhere strain presumably is not accumulating.
Thus we might assume a slip rate on the San Gregorio-Hongri between 1.6 and.1 3 cm/yr.
It would take, there fore, between 217 and 31.2 years to accumulate 500 m on thin fault.
If oc ucre to predict this carthquinhe in 1978 when would the last one have occurred?
Anywhere from 1666 to 1760!
Whe r:e do uc loch for information about the great llosy,ri carthquake of 1666! ?! ?
Clearly our historical record is not long enough to rule out such au carthquake.
Does this nituation have any known counterparts?
Can you get large quakes on slou moving strihc slip faults?
lieu about the 1900 earthquthe (if== 8.4) on the San Schastian fanit just off the coast of Venezuel.a near Caracas?
Slip
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rates there m. cat probably do not exceed 1.5 cm/yr, sugscsting a recurrence interval of hundreds of 3 cars.
Perhaps the San Sebastian han a dip-slip component?
Perhaps - but how about the Hosgri?
You raise this point, and Hoshins and Griffiths (1971) shou the Hor.gri a:. a reverse fault in cross section.
WWWW So, chere do uc go from here?
Clearly critical data ucrc not available to you in early 1976 that are availabic nou.
Scismolor;ints may have other reasons for doubtina, the possi-bilit didn'y o f a magnitudu D.
Tom licEri.lly told me the San Gregeric t look like it had 8 's, only 7 's.
My cycn aren't so uc.1
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As an aside, I see an incredible analynis by S. U.
Smi.t h in Appendix DllA, Volume 3 o f the PG&F. report.
Using historic scismicity frem 1930 to 1975 Smi th computes log M - 3.72-0.92 M, where N is no. of carthquakes per year in the region ulth rmde,nitude greater than or equal to that o f if.
If we ask hou of ten a magni tude 7.3 carthquake vill occur {'hc equation telin us once overy 1000 yearn!
Isn't it odd that 3 yearn prior to
.the data base the 1000 year carthquahe ntruck?
Suppone they performed a nimilar analysin on the carthquahc n from San.Tuan nautista to Point Arena on the San Andrea:. from l930 to 197.'i.
Ilow o ften woul d a mag. S.3 he p. edic ted ?
Thenc analysen arc obviounty unclens for frequency of great earthquala - when the time span inn't large enough to include a y,reat varthquake.
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Whi.le the evidence I've outlined here in n:it proo f o f a potent.ial for magni tude 3 carthquaken on i he llom.ri, it ::crounly
. qucntions the liani.n.on which thi.n. pone.i hil i.ty uan elimina ecd
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Frank A. 1tcKeoun Page 5 Jarnes F. Devino rebruary 27, ' 07P-from consideration.
The whole question of naaximuni potential.
carthquake and safe shutdoun earthquake, I think, needs to be re-opened.
With bent rc,gards,
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The San Gregorio-llos;;ri Fault Zone:
An Overview Eli A. Silver Earth Sciences Board tiniversity of California Santa Cruz, CA 95064 The San Greg;nrio-llosgri fault zonc in part of the larger San ^ndrcan fault system in California that forms the maj or locus of shear due to movement between the Pacific and North
/smerican plates.
An enbrmous amuunt of effort has been and
.in presently being devoted to study of the San Andreas fault Itself, and in recent years detailed quantil ative knowledge o f
'('
of fset history, seinmicity, and present-day movement has increased dramatically (see for exampi c Kovach and Nur,1973 ;
ggqrthe Crowell, 1975; Dichinson and Grants, 1968).
The extent of our knowledqc of other faults of the San Andreas system is much less complete, due in part to the lower
, frequency of great earthqunhes and sv. aller of fset on subsidiary faults (and there fore, perhaps, lecnce interent i.n these faultn).
' Another reason may be the location of soSe of the subnidiary faul tn.
The San Gregorin-Itongr! fault.:one is located along the coastline sonth o f San Francisco for a leny,th o f nearly 400 1:m,
and much of it lies just of fshore where it is di Eficult to study.
Major outstandin ; problems of this faul.t zonc include the details oh fault location, continui ty between the San -
Gret;orio and llosgri Giult ner, ment s, ef fner hi nt ory on each ncnment, evidence fnr llolocene movement.n, and noi smicity.
Thenc
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probicms' have importance both for their tectonic implications and their bearing on analysis of scismic hazard for coastal development and power-plant siting.
The papers in this volume were presented as part of a symposium on the San Gregorio-!!ost;ri fault zone at the Cordilleran section meeting of the Geolog,ical Society of Aaurica in Sacramento in April, 1977.
h'ot all of those papers are rep.roduced here but those which 'Collou give a good overview of the p.enent sente of knowledge of this fault zone.
Clarh and Brabb discuss the detailed stratigraphy on either side of thd San Gregorio faul t in its type crea.
Their care ful
(
ohnervations of fundnmental stratigraphic di f ferences, im;dy sir.ni ficant diffe'rences in sedimentation and tectonic history
~
on either side of the fau l t.
Graham and Dickinson use this Wtew and other regional data to infer up to 115 km of right lateral of fset on t!ic faul t since Miocene time.
This figure is larr;cr than an earlier su.ggesti on o f 80 t o 90 laa (siver, 1974) based
~
~
on offset basement terrancs using o ffshore geophysical control.
An estimate of 80 cc 100 km of post !!iocene right lateral o f fset on the 11oscri faul t (Itall,1975) tien ra ther nic el y with the above estimates for the San Gregorio segment, but the Ilosgri estimate has been questioned (llamilton nnd t-fillin:; ham, 1977),
llall (this volume) briefly addrennes these questions and proposes il pu l.1 -a par t origin for the Sant a Maria basin
(
onshore.
The question of continuity of. the San Grer,orin-linsgri fault: zone. (ocuses on fpor prqblem brea: :
Point Nor, Cape ::an
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Martin,. San.4imeon, and south o f Point Sa i. (Fig. 1).
The Point
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Sur region is discussed in detail by Graham and Dickinson.
Their interpretation that the San Gret;crio probably connects uith the Sur fault is supported by detailed gravity studies (tloodson, 1973) and argues against a previous suggestion that the innin San Gregorio fault trace turnn inland south o f Monterey to join the Palo Colorado fault (Greene and others,
'n 1973).
11all (1975) first sugr.csted that the San Simeon fault i n part of the !!osr,ri fault zone (Fig. 2).
The detail.ed connection between the liongri and San Simeon faults has not been establinhad
(
and some maps shou an en-echelon of fset betueen these faults (Hall, 1975; licculloch and othern, 1977).
The San Simeon i
(Hovgri) ncgment trends of fshore to the north toward Cape San
~
HWR4401 Martin (Fig. 2).
Recently flown acromagnetic data (USGS-Calif.
3 Division of liines and Geology unpublished data) reveal a high amplitude ano::al: trendint; northuent across Cape San Martin 1
and neem to require the liongri-San Simeon fault either to bend around the anomaly (Fig. 2) or to step 4 km inland to a fault bounding the cant side o f the anomaly.
If the fault bends o
around this anomn.ly it could j oin a major offshore fault north of Cape San thrtin (McCulloch and othern, 1977) that trends k-toward the Sur fault (Fig. 2).
McCulloch and others (1977)
(their Fig. 2) shou a northwest treneling fault uent of Point
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Sur (Fig. 1) uhich they extend noutheastward to the coast, I
cutting acronn and neparatinr, the liongri and Sur fa u l t s.
Thi n Q.;"
intei i retation wo'il.l imply a ele finite 1.ach o f continuity bet ween y
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the San Gregorio and liongri r,3tts i,. this area.
Iloweve r,
a their. north:est trending fault must cross a high amplitude magnetic anomaly that lies parallel to the coast (ano:aaly
~
bounded by -1.5 nT contour in Fi;. 2) and thin anomaly shows t
no evidence of a crosscuttiin; structure.
The anomaly also trendo parallel to the Sur and Hosgri faults and may be caused by serpentine intrusions alont; the fault.
Structural relations in this nearshore area are obscured by nurface sitaiping (McCulloch and others - tiieir Fig. 2), and I conclude that the bull; o f evidence at prcscot favors or at least al. lows continuity betuenn t he sur and !:ougri faults.
{
The soothern entensi en of the liongri is also in dispute.
McCulloch and othern (1977) map the fault south o f.Pt. Argutllo, but IInmilton and WI)lingham (1977), usint; much the same data, ppgg map it no farther south than o ffshore Point Sal.
Either version rai.nes grometrical problems of cadin;; a fault with approximately 100 hn of lar e Cenozoic lateral o ffset.
Various
~ '
. solutions to thin probl.em have been proposed in oral com.uai-cations, incl.uding a bend o'f the fault into the Transverse rangen ubere the notion trould be t al:en up in cotapression (D.
McCulloch, oral ce mnun., 1977 ; llemii.l. ton and Wil.lingham, 1977) or an of fset of the fault by enne-trending fa ul ts in the Santa 11arbara region (J. Crnuch, oral common., 1976).
Sati s fac tory Cic1d documentation, however, han not been reported and this remains an outatandini; neruct ural pi nblem.
C-llolocene movemenen are wel.1 document ed for the San Gre>;orio faul.t - (Weber and 1. G oi c, 1977; copperumith and' Gel:ggn, thin'
~
+*
- + =
i s
(.
5-(.
(
volume), and 'ntudies of scismicity contirm tile prencn t -day activity on both the liont,ri and San Gregorio segments (Cau thro p,
1975 and this volume).
This infonnation is crit.ical to any planned develornent along the central Cal.ifornia coast becaur.c the San Gregorio-1!asgri is very nearly a coastline fault over most of i ts longth.
The San Gregorio-llongri appears to be the largest of the subsidiary fault:c uitbin the Sat /.ndreas syst cm, both in length and cTfset.
Other faul.ts, nuch as the !!ay. ard-Cal.averas and Rinconada ha.e inscer documented offset but also play an impo):t ant role in the tectonic development of the Cal.ifornia
(
coast ranges and are dennrving Of intensive study.
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Re ferences Ci. < em.
Croucil, J. C. (Ed.),1975, San Andreas fault in nouthern Cali forni a :
California Division of Mines and Ccology Special Report 118, 272 p.
Dickincon, W. R., and Grant::, A. (Eds.), 1968, Proceedings of the conference on geologic probican of San Andreas fault sys t em :
Stanford Univ. Pubs. Geol. Sci., v. 11, 374 p.
Gautiuop, Willian, 1975, Seismicity of tbn central California const21 reg!.on:
U.S. Geol. Survey Open-file Report 75-13',
(
87 p.
Greene,11. G., Lne, W. 11. K., McCulloch, D.
S., and Brabb, E.
E., 1973, Faults and carthquahen in the Monterey Bay rdWibir4 region, Cali fornia:
U.S. Cect. Survey Misc. Field Study M. F. - 518, 14 p.
llall, C.
A., Jr., 1.975, San Simeon-llosgri fault syntcm, constal Cali fornia : economic and environmental impli.cationc :
Science, v. 19 0,
- p. 1291-1294.
Ilamilton, D. H., and Will i n,r, ham, C. 11., 1.977, llongri fault
- .one; ntructure, ainuunt of di splacement, c.nd rel.ationship to s truc turen o f the ucstern Tranverse ran;;en :
Ccol.
Soc. America Abs. uith programs, v. 9, no. 4, p. 429.
1 ovach,11.
L., and Nnr, Amon (1:ds. ), 1973, Proceedin.:;n o f the
(
conference on tect.onic problemn of t-hc San Andrean' faIult nyntem:
Stan ford liniv. Puhn. Ocol. Sci.,
v.
ll, 494 p.
Mc'ulloch, D. S., C la r ke, S. 'll.,.ir., P f 614, 11.
E.,
!;en t e, M. U.,.
C
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e
-~
E'
e
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and Utter, P. M., 1977, A summary report on the regional geology, petroleum potential, and environmental r,cology in the area of proposed Icase sale 53-A, central and northern California outer continental shelf, part A, 39 p.
Silver, E. A.,1974, Structural interpretation from free-air gravity on the Cal.ifornia continental margin, 35 to 40 N:
Geol. Soc. America Abs. uith pregrams, v.
6, no. 3, p. 253.
Weber, G. E., and Laj oie, K. R.,' 1977, Ltte Pleistocene and llolocene tectonics of the San Gregorio fault zone between Moss Picach and Point Ano Mucvo, San Mateo Ccunty, Cal.1-fornia:
Geol. Sec. America Abs. t:ith programs, v. 9,
(
no. 4, p. 524 Woodson, W. B., III,1973, A bottora gravity sur!.cy of the
~
continental shelf betueen Point Lobos and Point Sur, Califognia:
Thesis, Havn1 Postgraduate School,112 p.
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Figure Captions Figure 1..
Map of central California coant showin;; geographic locations and faul ts cited in text and location of Fir;urc 2.
Figure 2.
Detailed aeromagnetic nap of central California coast betteen Point Sur and San Siircon.
Flight lines had 1 mile spacing, floun nr;rmal to the coast.
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