ML20234E673
| ML20234E673 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Bodega Bay |
| Issue date: | 04/28/1964 |
| From: | Wahrhaftig C CALIFORNIA, UNIV. OF, BERKELEY, CA |
| To: | |
| Shared Package | |
| ML20234A767 | List:
|
| References | |
| FOIA-85-665 NUDOCS 8709220473 | |
| Download: ML20234E673 (7) | |
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3 I jd %$$k STATEHENT ON THE 30 DECA HEAD P01fER SITS SHAFT FAULT
[Elle Cupy_gg by Clyde Ifahrhartig '
i Associate Professor of Geology, University of California, Berkeley I 1
On March 18, 1964, I visited the Bodega Reae, tor site in conpany with !
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Carniss H. Curtis and Jack F. Evernden of the Geology Faculty, University of l California, Berkeley; J. Schlocker, Manuel G. Donilla, e and Dwight M. Iemmon of the U. S. Coological Survey, Menlo Park, California; Elmer yarliave, i Consulting Coologist, Sacranento, and the Engineering staff of the Pacific Cas and Electric Company. The purpose of our visit was to exanine the shaft fault described in the report by Schlocker and Bonilla (U. S. Geol. Survey TEI 844), and to resolve, if possible, the questions raised in Curtis' and Evernden's letter of 26 February,1964, to the Pacific Cas and Electric {
Company (and published in the P G & E Progress of April,1964) about the displacement along this shaf t fault. Personnel of the Pacific Cas and ,
Electric Company had dowatered the floor of the shaft, and provided hoses, 1 j
pumps, brooms, and labor for cleaning mud off the rock.
During the visit we observed the following:-
(1) A thin dike, petrologically identical to the dike described by l
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Schlocker and Bonilla, was found east of the shaft fault, approximately 24 ,
feet south of the dike west of the fault (distance measured along the ,
fault)
The westernmost end of the eastern dike segment is about 8 feet east of the trace of the plarie of shearing at which the dike west of the fault ends ,
and in the intensely broken rock in the intervening belt we were unable to locate any segments of the dike.
area. A pond covered nuch of the intervening Assuming that these are offset sognents of the same dike, the i relations betwoon the dike and the fault can be explained in one of two ways:- {
(1) the diko was displaced laterally by the fault a distance of 24 feet ,
or (2) much of the displacement on the dike was on a series of earlier faults ,
..now. inactive, trending about 30'. to the shaft fault, and only part of the 8709220473 851217
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displacement was due to the shaf t fault. Iho minimum displaconent attribu-table to the shaft fault under this latter hypothesis is 16 feet, according.
to measurements made at the tire of our visit, unless the old system of faults was nearly parallel to the present shaft fault.
Either explanation is tenable on the basis of the evidence at hand.
Professors Curtis and Evernden favor the second explanation. I am inclined to agree with Messrs. Schlocker and Bonilh in favoritig the first ---- !
l that tho' offset we see on the dike is the result of displacement along the j shaft fault.
.(2) On the south side of the reactor area, and about five feet above 4 i the top of the reactor shaf t collar and about 30 feet south of the shaft, 1
l we exanined the place where the shaft fault intersected the overlying sedi-monts. In digging at the bank at this phce, Curtis, Everndon, and I q uncovered a sequence of thin black peaty hyere in the sand, that had a total -
vortical'conponent of displacement across the fault of four inches. At the time, we saw no reason to think there had been any horizontal component of l disph conont. Af ter itknch the party, minus Curtis and Everndon, who had to )
l return to Berkeley, returnod to this spot, and we examined the peaty layers l more closely, and photographed them. On the upthrown (east) side of the fault, there were three layers exposed; these three hyers were displaced downward about an inch on the eastern of two fault breaks. West of the western break, however, there were only two peaty layers. Apparently the intermediate layer was missing. This can be explained in two ways -
either the pinch-ou't of the middle layer coincided exactly with the later .
position of the fault, or else the line of pinch-out.of the middle layer was intofsected by the fault and was displaced hterally, so that the layer in vertical section would appear on one side of the fault but not on the other.
We dug back soveral inches into the bank and continued to find, as far as we dug, three peat layers on one side of the fault and only two on the other.
Since the coincidence of the line of pinch-out and the lator faulting
, has a low probability, I conclude that the sedanents have been displaced later- ~
~ ally as well as vertically, and that the lateral displacement is at least' four inches.
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An upper limit on the possible lateral component of displacement q
on the fault since the sediments were deposited, is provided by the contours mapped by Schlocker and Bonilla on the sediment-diorite contact (see the 2arge mp of the reactor shaft in TSI 844). Those 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;.
and (2) 20-30 feet north of this ridge, the axis of a shallow valley. y If a lino is drawn halfway between the segments of the foot contour '
that define the ridgs, this line will lie approximately along the ridge crest.
The lines so drawn on opposito sides of the fault are not displaced laterally a significant amount. The same is true for . Lines defining the valley axis. ,
The total lateral displacement, then, must 1 j
be within the limits of the errors of locating these two axial lines, i which I estimte to be on the order of 2 or 3 feet. Thus the lateral n c !
displaceme, t on the fault since the sedirents wer8..depos.ited is no more than 2 or,,*3..afe e t.
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Thus, the evidence ,that we saw on l! arch 18 bears out, in sty mind, I the concern expressed in Schlocker and Bonilla's report that there is a significant hazard of strike-slip displacement on the fault that goes through the shaft.
However, strike-slip displacement since the sediments were deposited has not been great.
(3)'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. One of these intersects the shaft fault at right a ngles a few feet south of the shaft, and the other trends northwest, passing through the southeast corner of the access j
ramp on the northeast side of the shaft. According to Schlocker and Bonilla, those breaks could be interpreted either as tectonic faults or as landslide fracturts; one, at 3 east, could not be traced into the )
quartz-diorite below, j Their presence, however, indicates that the sediments i l of Bodega 11ead are more broken by small disp 3acements than a cursory exami--} .
, nation would indicate, -
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One other aspect of the controversy, I think, deserves touching upon.
This concerns the evidence of displacements away from the San Andreas Fault during the 1906 earthquake, and the relevance of that evidence to the i l
safety of the reactor site. In Sch2ocker and Boni11a's report reference is mde to four displacements observed by C.12. Gilbert on Inverness Ridge l west of the San Andreas Fault, during the 1906-W California State Earthquake Commission investigation. All references to these ot:currences j inadvertently leave the reader with the impression that these were the only displacements west of the San Andreas Fault Zone proper. 1 Gilbert's statement, however, is as follows (Calif. State Earthquake Comm. Report, Vol.1, part 1, Carnegie Inst. Uash.,1908, p. 7S):- !
"Bodrock cracks occurred at rnny points within the Rif t, usually appearing as branches from the faults. They were seen also at a number of points west of the Rif t, their distribution reaching the ocean in tho vicinity of Point Reyes, ten miles from the fault-trace.
At the more renote points they were quite smil, often baroly l discernible, and no system of arrangemnt 1 ras discovered. They are peculiarly prominent along the sumnit of the ridge constituting the southwestern rim of the Bolinas-Tomles trough. This summit was visited on four lines of road and at each locality conspicuous cracks were found." (Italics mineT-Gilbert went on to discuse the nature of the cracking and displacement at each .of these localities, pointing out that they showed evidence of lateral or vertical displacement of bedrock.
Unfortunately there is no evidence that the persons assigned by the State Earthquake Comtission to exanine the segment of the fault at Bodega 3ay walked over the Head looking for signs of breakage, so we cannot tell t for sure that the Head was free from such breaks.
Farther s outh along the San Andreas Fault, at nany places where there were rouths of travel across the fault, there were displacements some distance from the fault trace. Prof. Oranner of Stanford University reported (Ibid, .
p.1W-108) nunorous faults, sor:e with one focrt of displacement, for three
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miles east of the fault trace on Page Hill Road south of Palo Alto, where also the entire Black Mountain block (which we now believe to be a thrust shoot) was severely shattered. 'Ihe Wright railroad tunnel across the -
Santa Cruz Mountains near Ios Catos was broken at two places 1,000 feet and 1,800 feet west of the main fault break (Ibid., p.111).
Tho report on the Kern County earthquakes of 1952 (Calif. Div. of Hines Bull.171), the only study of a mdjor California earthquake conparable to the 1906 report, shows (pl. 2) that strike-slip offsets in the bedrock occurred as much as 1.8 miles from the trace of the White Wolf Fault, on the underlying block of this thrust fault.
Prof. G. D. Louderback's historical study of the. California Earthquakes of the 1830's (Bull. Seismological Societycof America, vol. 37, p. 33-74, 1947) gives evidence that the 1836 earthquake on the riayward Fault and the 1838 earthquake on the San Andreas Fault were probab3y of mgnitude comparable -
to the 1900 earthquake on the San Andreas Fault. Th6s earthquakes of this mgnitudo might occur on the average once a ' century or more frequently.
The sedinents of the ' reactor site are older than 40,000 years, according-to radiocarbon dating. Ueathered terrace deposits on the west side of Bodega read nark a sea level mximum at 20 feet above present sea level.; 4
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This sea level maxinun post-dates the sediments in the shaft, because marine deposits,in the shaft sedianents are as high as 100 feet above sea level.
The weathered deposits of the 20-foot maximum probably correspond to the Songamon Interglaciation, and the deposits in the shaft therefore probably, correspond to the proceding interglaciation. He have no reliable date yet on l
the pre-Sangamon interglaciation, but as a reasonable guess we may take -
200,000 years as the age of these sediments. This means that at one time in the last 200,000 years there has been a significant displacement (more than l
one foot) on the shaft fault. There were certainly displacements on the same fault before then, because the granitic rock is much more greatly shattered (and along mny more breaks ) than are the overlying sediments. -
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T Thus the only geological evidence that we have to go on is that there have been several episodes of displacement on this fault, and that the~ 1ast one occurred within the,1ast 200,000 years. This is a totally inadequate sample on which to base an estimate of the probability of recurrence. However, keeping this in mind, the best guess j one can make for' the probability of 'j a repetition in the next 200 years (the life of the ronctor) is 200/200,000 or one in a thousand. The probability is not less than 1/50,000 and not f I
more than 1/50 These figures contrast with the following:-
(1) A probability of close to 1 that if the reactore .were built directly on the trace of the 1906 break, it would be sheared several feet in the next 200 years.
(2). A probability of about 1 in 300 that thousands of people will be' in the University of California Stadium the next time there is a strong earthquake on the Hayward Fault. (The probability that a strong earthquake will take place on the fault within the next 50 years is more q
than 1/2 ---we are overdue now; the proportion of tim that the stadium '
is occupied I estimate to be about 1/150.)
(3) A probability of less than 1 in several million that the ' reactor would be sheared by a fault if it were located in a properly selected place in the mid-continental United States.
In Sumary:-
The shaft fault probably has a disp 3a cement with a total
' net strike-slip component of approximately 24 feet, has moved more than on'ce, and has noved opee in the last 200,000 years with a dip separation of about 1 foot and a strike-slip component of displacement of between 4 inchos and 2 feet. l Reports of the San Francisco 1906 Earthquake and the Kern. County 1952 ,
carthquake show that displacements on minor faults are a comon accompaniment of displacement on the major fault during an earthquake.
There is no good statistical basis for making an estimate of probable
- future brealgage along the shaft fault, but a reasonable guess is that the probability lies between 1/50,000 and 1/50, and may' be about 1/1,000 l
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This is as far as I can go, as a geologist. As a citizen concerned with y own health and safety and with the. welfare of the comunity in which I live, I can speak further. This average likelihood is greater. than the -
risks we individually take each time we take an automobile drive or 'an airplano trip (w&ich I, persona 14, consider hazardous enough). In g mind, it is high enough to justify requiring that the engineers' design-
- a reactor that cannot possibly release significant radioactivity to the .
atmosphere, in the ovent of. a rupture by a ' displacement of as much as 5
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feet through the plant. (A displacement twice that which.we know took p3 ace ).
If-such design specifications cannot be met, then the site should be abandoned._ - .
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Clyde Wahrhaftig.
Derkeley, California-April 28,1964 'l i
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