ML20234E599

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Personal Impressions on Geology at Bodega
ML20234E599
Person / Time
Site: 05000000, Bodega Bay
Issue date: 02/07/1964
From: Beck C
US ATOMIC ENERGY COMMISSION (AEC)
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ML20234A767 List: ... further results
References
FOIA-85-665 NUDOCS 8709220445
Download: ML20234E599 (4)
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Clif2 erd E. Beck 1/7/64 PER80EhL IlfRESSIAES GE GE M gf AT S g DOCKET NO. 50-205

1. h bedseek fanit in the Bodega shaft escavation is des to seus fesmer earthquake. ]
2. Se fanit ta overlying sediments is probably due to earthquake movement steeg the bedseek fanit, thogh some sliding er slumping ta the ever- i tying sediments may have (a) caused part of the observed faultg er (b) )

obscured or attesed part of the sediment fanit. I i

3. h fault in thd,bedseek is branched, emitiptmaerashed, ete., sad  ;

undoubtedly superienced many diffement movements in the ancient past. j

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4. h fanit in the sediment, because of its general segularity, gemoral lack of br'analiing, absence of para 11a1 breaks, sta., peghably ooeurred in one stagle movement.
5. h re were enanyt many ancient faults observed in the bedsook on Bodega, iiimeloding sans in the shaft essavatten, and testuding at least one in the shaft escevation of a magnitude as lasse as "thePheducek fealt in the adiaft though Sor esos of these other fantts, esse eersospeeding faults Amerved in the everlying ==d4===ts. 1 1
6. b bedseek at the shaft,escavation is en the endar of mesy millimas of i years eldr many faults are also undoubtedly on the order of mittions of  !

years old. I

7. h everlying mediments are deposited in layers, with the oldest en the bedreek and the youngest en the surface. S e lowest lying ==dd - *= are on the order of 300,000 to 800,000 years old, with 400,000 representing

'a minimusa rogh estimate of age. N fault in the made===es was-

./ absorved in the lower oldest layers and entended upward, though not to

/ the surface. substantial thicknesses of deposits of at least 40,000 years old (as detesmined by dating of organic remains Sound there) overisy the highest point at ubich the sad 4==a*'famit was Speed. there is sesong evidenes that the sediment fanit escurred more than 40,000 years age and no evid==e= that indicates a younger age, though some assumptions and interpretations of observa*ia== comid land to a eenus-

, uhat younger age. Celer bandtags of shdiment layers almas tbs.fanit i lead to a definite eenetusica that the fastt is at least aanteries end, ,

/ and me movement of any significasec (1 sager than fractimas of am tash) has securred almas the fanit within this same perine (gehteeber believes not less them "500 to 1,000 years").

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8. he, during a period innen thess have been numerous anajor earthquakes  !

ateng tene gan Andreas fault, of a magnitude semperable to the ene of 1906, perhaps hundreds of this agnitude, as sessitias movement almag

  • k-f.=ita in eka -kart.- envation is in evidense.

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f 9. The wedght (i evidenes end.eptaion Ladteate that the subsidiary earth l ruptuse outside the mais fanit ares d ieb were observed in the i Ft. Reyes area after the 1996 :- I--;-- '- , esaurred along estab11 abed l

' fault lines d ose other movements had previously escurred. This is {

undestedly true of the'Invesness movensat and is probably true of l I

the alt. Wittenburg movement as sett.

10. In the 1996 earthquake the differential ground motion alaes the San Andreas fanit near Pt. Reyes was as er so feet; 16 feet use the motion adjma==t to Bodega Esad. Thus, the magnitude of severity of the earth-geahe was abent b sese at both pleoss.
11. No residual evidense has been found that ground ruptures occurred on Bodega Eeed during the 1906 earth; ' , ed me topographical features skisk would indicate the presemee of established recently active subsidiary fanit 11ame of the kind observed near Pt. Reyes,

.12. There is seasiderable weight of opinian that, La the event of another earthquake of the severity of 1906, the probability of autisidiary faulting is aseos outside but nearby the famic area usuld be )

substantially higher ser the Pt. Beyes area akan for the sedaga area. Ll '

Beusvar, the possibility amists be subsidiary fam1 ting could occur is adjasant asses anydese elseg -the San Andreas fault.

13. sableeker mee admits, as all others believe, that the probability of a significant rupture sa sedega outside the San Andress fault eene, in sane of amether e r" 7 ' aimilar to that of 1906. is "eatremely lew", though the possib(11ty does exist.

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14. There is a high degree of probability that, in case of moother 1906 type earthquake, the surfase rupture alens the main San Andreas fault time useld esser at the same pinse as in 1906, i.e., en the eastera '

side of the fault some, though the rupture could possibly occur anywhere within the seas.

15. The westerrreest edge of the Sao Andress fault sene at Bodess seems to )

be unti sotablished, and is at lasst 1,000 feet from the shaft excavatten.

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l Clifford K. Beck 2/7/64 PERSONAL IMPRESSIONS ON GEOLOGY AT BODEGA

1. The bedrock fault in the Bodega shaft excavation is due to some fonner earthquake.

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2. The fault in overlying sediments is probably due to earthquake movement I along the bedrock fault, though some sliding or slumping in the over-  !

lying sediments may have (a) caused part of the observed fault; or (b) j obscured or altered part of the sediment fault. j

3. The fault in the bedrock is branched, multiple-cracked, etc., and undoubtedly experienced many different movements in the ancient past.
4. The fault in 'the sediment, because of its general regularity, general lack of branching, absence of parallel breaks, etc., probably occurred j in one single movement.
5. There were many, many ancient faults observed in the bedrock on Bodega, . including some in the shaf t excavation, and including at least l one in the shaf t excavation of a magnitude as large as "the" bedrock fault I in the shaf t though for none of these other faultsg were corresponding faults observed in the overlying sediments. )
6. The bedrock at the shaf t excavation is on the order of many millions of i years old; many faults ar'e also undoubtedly on the order of millions of );

years old.

7. The overlying sediments are deposited in layers, with the oldest on the j bedrock and the youngest on the surface. The lowest lying sediments are '

on the order of 300,000 to 800,000 years old, with 400,000 representing ,

a minimum rough estbmate of age. The fault in the sediments was l observed in the lower oldest layers and extended upward, though not to the surface. Substantial thicknesses of deposits of at 1 cast 40,000 years old (as determined by dating of organic remains found there) overlay the highest point at which the sediment fault was found. There is strong evidence that the sediment fault occurred more than 40,000 years ago and no evidence that indicates a younger age, though some assumptions and interpretations of observations could lead to a some-what younger age. Color bandings of sediment layers along the fault lead to a definite conclusion that the fault is at least centuries old, and no movement of any significance (larger than fractions of an inch) has occurred along the fault within this same period (Schlocker believes not less than "500 to 1,000 years").

8. Thus, during a period when there have been numerous major earthquakes along the San Andreas fault, of a magnitude comparable to the one of 1906, perhaps hundreds of this magnitude, no resulting movement along the faults in the shaft excavation is in evidence.

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9. The weight of evidence and opinion indicate that the subsidiary earth rupture outside the main fault area which were observed in the Pt. Reyes area af ter the 1906 earthquake, occurred along established fault lines where other movements had previously occurred. This is j undoubtedly true of the Inverness movement and is probably true of  ;

the Mt. Wittenburg movement as well,

10. In the 1906 earthquake the differential ground motion along the San Andreas fault near Pt. Reyes was 20 or so feet; 16 feet was the motion adjacent to Bodega Head. Thus, the magnitude of severity of the earth-quake was about the same at both places, i 1
11. No residual evidence has been found that ground ruptures occurred on Bodega Head during the 1906 earthquake, and no topographical features which would indicate the presence of established recently active l l

subsidiary fault lines of the kind observed near Pt. Reyes.

12. There is considerable weight of opinion that, in the event of another earthquake of the severity of 1906, the probability of subsidiary  ;

faulting in areas outside but nearby the fault area would be j substantially higher for the Pt. Reyes area than for the Bodega area.  ;

However, the possibility exists that subsidiary faulting could occur j in adjacent areas anywhere along the San Andreas fault. j i

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13. Schlocker n9w admits, as all others believe, that the probability of l i

a significant rupture on Bodega outside the San Andreas fault zone, in case of another earthquake similar to that of 1906, is " extremely j

low", though the possibility does exist.

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14. There is a high degree of probability that, in case of another 1906 )

type earthquake, the surface rupture along the main San Andreas fault j l

line would occur at the same place as in 1906, i.e., on the eastern side of the fault zone, though the rupt ure could possibly occur anywhere within the zone.

15. The western-most edge of the San Andreas fault sone at Bodega seems to l be well established, and is at least 1,000 feet from the shaf t excavation. j

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