ML20235A906
| ML20235A906 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Bodega Bay |
| Issue date: | 04/28/1964 |
| From: | Wahrhaftig C CALIFORNIA, UNIV. OF, BERKELEY, CA |
| To: | |
| Shared Package | |
| ML20234A767 | List:
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| References | |
| FOIA-85-665 NUDOCS 8709230460 | |
| Download: ML20235A906 (8) | |
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l ) b STATDIENT ON THE D0 DECA HEAD POTfER SITE SHAFT FAULT l l by C3yde Wahrhaftig .
Associate Professor of Coology, University of California, Berkeley J . t 4 . On March 18, 1964, I visited the Dodega Reac, tor site in company with Carniss H.. Curtis and Jack F. Evernden of the Goology. Faculty, University of
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California, Berkeley; J. Schlocker, Manuel G. Bonilla, e and Dwight M. Iomnon j of tho U. S. Goohgical Survey, Henlo Park, Ca3ifornia; Elmer Marliave, Consulting Cooit gist, Sacranento, and the Engineering staff of the Pacific ' i
! Gas and Electric Company. The purpose of our visit was to exanine the shaft
[ fault doscribed in the report by Schlocker and Donilla (U. S. Geol. Survey TEI 854), and to resolve, if possible, the questions raised in Curtis' i, ' and Evernden's letter of 26 February,1964, to the Pacific Cas and Electric t Ccmpany (and published in the P G & E Progress of April,1964) about the f k displacement along this shaft fault. Personnel of the Pacific Gas. and . I [' Electric Company had dowatered the floor of the shaft, and provided hoses, {
. pumps, brooms, and labor for cleaning mud off the rock. .
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! During the visit we observed the followings-I <
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(1) A thin dike, petrological 12y identical to the dike described by ch2ocker and Donilla, was found east of the shaft fault, approximately 24
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' foot south of the dike west of h fault (distance measured along the fault),,
The vesternmost end of the eastern dike sogment is about 8 feet east of . E the trace of the plazie of shearing at which the dike vest of the fault ends, ' and in the ' intensely broken rock in ths intervening belt we' were unable to locate any segnants of the dike. A pond covered much of t? o intervening area. D f [ Assuning that these are offset segments of the san s dike, the [. ' relations between the dike and the fault can be explained in one of two vays: 5 . - (1) the dike was displaced laterally by the fault a distance. of. 24 feet,
~ ' or (2) much of the disphcoment on ths dike was on a series of earlier faults, a
snow inactive, trending about 30' to % shaft fault, and only part of the
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c , . .. Q) 0 disp 2acenent was due to the shaft fault. Tho minimum displaconent attribu - ,
.! tahlo to the shaft fault under this lattor hypothesis is 16 feet, according i
g to measurements made at the time of our visit, unless the old system of f faults was nearly para 1101 to the present shaft fault. p Either explanation is tonable on the basis of the evidence at hand. Professors Curtis and Evernden favor the second e:d2anation. I am inclined to agroo with Messrs. Schlocker and Bonilla in favoring the first ---- l ht tho" offset we see on the dike is the result of displacement along the-I shaft fault.
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.(2) On the south side of the reactor area, and about five foot above the top of the reactor shaft collar and about 30 feet south of the shaft, we exanined the place where the shaft fault intersected the over2ying sedi-monts. In digging at the bank at this place, Curtis, Everndon, and I uncovered a sequence of thin black poaty layers in the sand, that had a total L
i vortical cor':ponent of displacement across the fault of four inches. At the-tim, wo saw no reason to think thoro had baen any hori2.ontal componont of j ' displa cement. After itknch the party, minus Curtis'and Everndon, who had to f return to Berkeley, returnod 'to thi$ spot, and we examined h peaty layers more closo2y, and photographed them. 'On the upthrown (east) side of the fault, there were three layers exposod; these three 2ayers were displaced downward about an inch' on tho eastern of two fault breaks. TTest of the
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vesterti break, however, there were on2y two peaty 2ayers. Apparently b intermediate 2ayer was missing. This can be explained in two' wayss-7 either the pinch-ou't of the middle 2ayer coincided exactly.vith the later . r. position of tho fault, or else the line of pinch-out.of the middle 2ayer was intersected by the fault and was disp 2 aced Interally, so that the layer in . vertical section would appear on one side of the fault but not on h 'other.- t We dug back soveral inches into the bank and continued to find, as far as 4 f vo dug, three peat layers on one side of the fault and only two on the other, Sinco the coincidence of the line of pinch-out and the 'lator faulting
} has a low probability, I conclude that tb sefimnts have been disp 2 aced 2ator- ~
l v ally as well as vertica12y, and that the lateral, displacomnt is at 2aast . { four inches. ' y, . . L
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() . l An uppor limit on the possible lateral component of displacornnt on h fault since the sediments were depositod, is provided by the contours mpped by Schlockor and Donilla on the sediment-diorite contact (see the large map of the reactor shaft in TEI 844). These contours define (1) a nose or ridge extending across the fault, and offset ' vertically by it, about 20 feet north of the south
- edge of the shaft;. j and (2) 20--30 feet north of this ridge, the axis of a shallow valley.
If a line is drawn halfway between the segnants of the foot contour ( S that defhe the ridge, this line will lie approximately along the i ridge crest. 1he lines so drawn on opposite sides of the fault are not ' displaced 2atera11y a significant amount. , j The same is true for linos ' defining the valley axis. l The total lateral displa'coment, then, must i be within h limits of the errors of locating these two axial lines, , 1thich I estirate to be on the order of 2 or 3 feet. Thus % Interal i displacement on h fault since h sediments wer8 deposited is no more than 3 er 3 feet. h Thtis, the evidence ,that we saw on liarch' 18 bears out, in ny rdad, ' the concern expressed in Schlocker and Boni11a's report ht there is a significant hazard of strike-slip displacement on h fault ht goes through ths shaft. However, strike-slip / displacement since h sediments were deposited has not been great.
- F (3 Y While wo were at the reactor site, but after Curtis and Evernden -
had left, I was shown two other steeply dipping faults displacing the
- sediments by several inches to a foot. ,
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One of these intersects the
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shaft fault at right angles a few feet south of the shaft, and the other i j y trends northwest, passing through the southeast corner of h access ranp on the northeast side of the shaft.
! According to Schlocker and '
j I Bonilla, hse breaks could be interpreted either as tectonic faults or as landslide fractures; ! quartz-diorite below. one, at least, could not be traced into h . \ { { Their presence, however, indicates ht..the s'ediments , l ,~nation of Eodega Ibad ' are more broken by erall displacements hn.a' cursor vould indicate. - l
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... O ..J one other aspect of the controversy,- I' think, deserves touching upon. .
This concerns the ' evidence of displacements away from the San Andreas Fault- , f
.during the 1906: earthquake, and h relevance of ht evidence to the. -
p safety of the reactor site. In Sch2ocker and Doni11a's report reference is - rade to four displacements observed by G.' R. Gilbert on Invorness Ridge j west of the San Andreas Fault, during the -1906-W California State Earthquake Comission investigation. All references to these ' occurrences 4 i inadvertently leave % reader with the impression ht these were the j on2y disp 3acements west of the San Andreas Fault Zone proper.' { <
, Gilbert's statement, however, is as follows (Calif. State Earthquake w
Com. Report, Yol.1, part 1, Carnegie Inst. Wash.,1908, p. 75) - nDedrock cracks occurred at mny points within.the Rift, usua12y
' appoaring as branches from the faults. They were seen also at a h number of points west of the Rift, their distribution reaching the '
ocean in tho vicinity of Point Reyes, ten miles from the fault- - trace. At the more remote points ther vero quite smn, often baro 3y
.L discernible, and no system of arrangenant was discovered. They are '
)* peculiarly prominent along the sumit of the ' ridge constituting +
h southwestern rim of the 2olinas-Tomalas trough. This sumit was ', visited on four lines of road and at each locality conspicuous cracks were found." (Italics mine t Gilbert went on to discuss the nature of the cracking and disp 2acomont [ at each.of these localities, poin&g out that they showed evidence of lateral or vertical displacement of bedrock.
- Unfortunately there is no evidence that the persons assigne_d by the f' State Earthquake Comission to exanine the segment of the fault at Bodega e
7, 3ay walked over the Head looking for signs of breakage, so we cannot tell, j for sure ht the Head was free from such breaks. - b - i ' Farther south along the San Andreas Fault, at many places where there were rouh of travel across the fault, there were displaceman'ts some distance
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from the fault trace. Prof. Dranner of Stanford University reported (Ibid, p.1W-108) numerous faults, soms- with one foot 'of disp 3acement, for three
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' miles east of the fault trace on Page Mill Road south of Palo Alto, whoro also W entiro D2ack Mountain block (which we now believe to be a thrust shoot) was soverely shattered. 'Jhe Tiright railroad tunnel across the t Santa Cruz Mountains near los Catos was brokon at two p2 aces 1,000 feet j
and 1,800 feet vest of the main fault break (Ibid., p.111). The report on the Kern County earthquakes of .TS52 (Calif. Div. of I Hines Dull.171), the only study of a m:fjor California earthquake conparable < j to' the 1906 report, shows (pl. 2) ht strike-slip offsets in the bedrock !
. occurred as much as 1.8 miles from the traco of h ifhite Tfolf Fault, !
j on the underlying block of this thrust fault. - 1 J s i Prof. G. D. I4uderback's historical study of the. California Earthquakes n ! of h IE30's (Dull. Seismological Societycof America, vol. 37, p. 33-74, i t 1947) gives evidence ht the 1836 earthquake on the Hayward Fault and the I 1838 earthquako on the San Androas Fault were probably of magaitudo corparable to the 19Q6 earthquake on h. San Andreas Fault. This earthquakes of this magnitute. might occur on. h average caco a century or more frequently. * { i L Ihe sedironts of the reactor site are older than 40,000 years, according to radiocarbon dating. 1Teathered terraco deposits on the west sido of Dodoga Pead mark a sea level eny4em at 20 feet abovo present sea level. p This sea level **v4"m post-dates the sodirents in the shaft, because rarine deposits,in the shaft sedimonts are as high as 100 feet above sea level. !
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The weathered deposits of the 20-foot r274 "'" probably correspond to ' the SanCamon Interglaciation, and the deposits in the shaft therefore probably .
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correspond to the proceding interglaciation. ITe have no reliab2a date yet on the pre-Sanganon interg2aciation, but as a reasonable guess we may take { 200,000 years as the age of those sedimonts. This means that at one time i ( in the last 200,000 ( years there has been a significant disp 2aceront (more than' one foot) on the shaft fault. There were certain2y displacements on the sann i j ; fault before then, because the granitio rock is much more greatly shattered l l (and along many more breaks) than are the overlying sediments.- e '
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Thus the only geological evidence that we have to go on is ht thoro l j
.have been several episodes of displacement on this fault, and that the last one
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I occurred within the last 200,000 years. This is a totally inadequate sa=ple 5 on which to base an estimate of the probability of recurrence. However, keeping this _n mind, the best gfess one can make for the probability of [ a repetition in the next 200 years (the life of the roactor) is 200/200,000 or one in a thousand. [
" The probability is not less than 1/50,'00Q and not l more than 1/50 j These figures contrast with the followings-I. {
f (1) A probability of close to 1 that if the reactoro . vere built i i j directly on the t ace of h 1906 break, {t would be sheared several feet k([ .in the_ next 200 years.
' (2). A probability of about 1 in 300, that thousands of people vill !
i be in the University of California Stadium the next time there is a 1 strong earthquako on the Hayward Fault. (The probability that a strong earthquake vill take place on the fault within the next 50 years is more l than 1/2 '
--we are overdue now; the proportion of timo that the stadium '
it, occupied I estimate to be about 1/150.)
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{ (3) n probability of less, than 1 in several million that the reactor l i vould be sheared by a fault if it were located in a properly selected place in the mid-continental Unitdd Staters. '
; g Sur=ary:- The shaft fault i pobably has a displacement with a total not strike-slip co=ponent of approximately 24 feet, has moved more than !
L '" once, and has moved opee in the last , 200,000 years with a dip separation j of about 1 foot and a strike-slip component of displacement of between !
" 4 inchos and 2 feet. I I
i , {o Reports of the San Francisco 1906 Earthquake and the Earn County 1952 .. i { earthquake show that displacements on minor faults are a comon ment accompan t of displacement on the pajor fault during an earthquake. j
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There is no good statistical basis for mking an esticate of probabl e ' t
. future breakage along the shaft fault, '. probability lies between but a reasonable guess is that the i
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. 2/50,000 and 1/50, and my'be about 1/1000 , .
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e This is as far as I can go, as a geologist. As a citizen concerned L l with y own health and safety and with the' welfare of.the comunity in which. I live, I can spotk further. This average likelihood is greater than the , risks wo individually take each tim we tako an automobile drive or an'
. airplano trip (viitich I, personally, considor hazardous enough). In
- g mino, it is high enough to justify requiring that the engineers d' esign .
a reactor ,that cannot possibly release significant radioactivity to the atmosphere, in the event of a rupture by a displacement of as much as 5 feet through the plant. (A disp 2acement twice that which.ve know took place). - If such design specifications cannot be met, then the site should be abandoned. ' L S' Clyde Wahrhaftig l 3erkeley, California ,
. April 28,1964 - ,
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0FFICE __. CP).IRMAN ' l t
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l TO: ,> I 1 l [ For Info =ation M For Appropriate Handling For preparation of reply for Chaiman's I
- signature (Refer to Manual Chapter 0240) l For discussion at Co::=1ssioner's Information Meeting i' t
i For distribution to other Co::missioners i i
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l I i i t t REMARKS,: $.
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j Cowcrd C. Orc a, Jr. ; f For the Chai= n . i IV i
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