ML20237F810

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Summary of 750131 Meeting W/Usgs in Denver,Co Re Geology & Seismology Input to Diablo Canyon Ser.Attendees List Encl. W/O Encl
ML20237F810
Person / Time
Site: Diablo Canyon  Pacific Gas & Electric icon.png
Issue date: 02/24/1975
From: Mcmullen R, Stepp J
Office of Nuclear Reactor Regulation
To: Harold Denton
Office of Nuclear Reactor Regulation
Shared Package
ML20236J368 List: ... further results
References
FOIA-87-214 NUDOCS 8708130166
Download: ML20237F810 (3)
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H. R. Denton, Assistant Director, Site Saf .ty, TR THRU: W. P. Cartaill, Chief, SAB, TR MEETING WITH THE U.S. GEOLOGICAL SURVEY REGARDING THE CEOLOGY AND {

SEESMOLOGY OF THE DIABLO CANYON NUCLEAR SITE j l

1 A meeting was held on 31 January,1975 in Denver b6 tween the U.S. l Geological Survey (USGS) and the Nuclear RRgulatory Comission (NRC) to discuss the geology and seismology input to the Diablo Canyon SER. {

j A list of attendees is enclosed. The following agenda was agreed on: l

1. Regional geology, with emphasis on the definitions of the Hosgri l Fault zone (East Boundary fault zone) and the Santa Lucia Bank I Rault zone;
2. Seismicity, specifically that associated with the Hosgri and Santa Lucia Bank Fault zones;
3. Response spectra for the plant.

l Mr. F. McKeown, USGS, discussed his evaluation of the geology which considered that the Hosgri or East Boundary fault zone (EBZ) fault zone l

and the Santa Lucia Bank fault zone (SLBZ) were capable and could generate the same order of magnitude earthquake as other ancillary faults of the San Andreas system. the Riconsta, the Sur Nacimiento, etc. The 1927  !

i magnitude 7.3 earthquake is considered by the applicant to have occurred '

on the Santa Lucia Bank Fault Zone. Correlation of structure and seismicity is better on the EBZ than the SLBZ and it is at least i 90 miles long whereas the known length of the SLBZ is about 65 miles.  !

The SLBZ is better defined on the esafloor than the EDZ but generally ,

is not as well defined where it does not offset the seafloor. The two zones are similar in that both hue offset the seafloor and Holocene l deposits, border a Tertiary basin, show predominantly vertical displacement, e g., E and are presently involved in tranneurrent movement of the San Andreas pC system.

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$e The USGS criticized our une of fault length versus magnitude to z y determine the SSE by pointing out that neither the northern nor the

<> < 1 southern limit of the EBZ is knavn. They pointed out that the southern EE[5 extent of the EBZ cuts across the Pliocene section v1 thin the Tertiary

@ @ basin underlying the physiographic Santa Maria valley. This could in Z indicate that the latest activity along the zone is not controlled by f$5 mau the eest boundary of the basin as maintained by the applicant. The applicant states that the predominant mode of deformation is tensional.

The USGS believes that the current mode of deformation is strike slip with no geologic evidence of reverse motion. However, there is some ) I ^,

seismic evidence for a slight component of reverse movement along with '

strike slip. The EBZ and the SLBZ are similar in the sense that both ime ome a ,- ...TRLSAB _ ,,,,,TR : S AB Tg . i sua ..= s * ..

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4 associated with transcurrent San Andreas motion and not because they both border the same structural basin.

Mr. J. Devine, USGS, briefly summarized the seismological evaluation.

Their conclusion was that although the applicant attributed the 1927 )

earthquake to the SLBZ, the USGS regards the EBZ and the SLBZ as being 6 similar with respect to earthquake generation capability and assume a magnitude 7.3 as possible of being generated on either structure. l It might be possible to show either that the earthquake did not occur l along the SLBZ, er show that the EBZ and SLBE are entirely different i structures. On the other hand it may not really matter whether they are similar or not in view of the USGS intest interpretation of the relationship of the EBZ to regional tectonics.

The SLBZ is characterized by a horst on the sea floor (a west facing i scarp seaward and an east facing scarp shoreward). The horst is l underlain by Franciscan-like rocks (Cretaceous-Jurassic) with Quaternary j sediment butting up to the east scarp. There is no evidence of recent j r

novement along these scarps and their presence does not indicate capability. The 1969 earthquake cluster at the south end of the SLBZ indicates source movement opposite to that which would be expected by the orientation of the scarp.

Some preliminary information was presented regarding the reevaluation j of earthquakes in the area. The 1927 earthquake may have occurred 4 closer to shore than originally shown and one possible plane of the source solution shows a northwest sense of fault movement. The other plane indicates an east west sense of motion. Recomputed acrthquakes show a definite trend of seismic activity assoicated with the EBZ. l A discussion followed regarding fault length versus earthquake magnitude.

The NRC evaluation was based on the assumptions that (1) the fault is 90 miles long, (2) 1/2 total fault length breaks during a maximum quake and (3) the Bonilla curve (fault length vs. wagnitude) repenseaneddas conservative technique. Dr. Stepp pointed out that some data is available supporting 1/2 fault length rupture from the unpublished work of Dr. Couch at Oregon State where an upper limit of 30 to 40%

of fault length rupture was determined.

The USGS noted that there are occurrences which don't support fault length as being controlling. Examples are: (1) a magnitude 6.4 earthquake in 1947 that caused surface rupture over about a one mile length of the Manix Fault and (2)' a magnituds 7.7 earthquake in 1952 that caused surface rupture l over most of the mapped extent of the White Wolf Fault. Regarding these two earthquakes, we note that movement on the Manix Fault secondary to a larger fault movement depth and the 1952 earthquake is indicated from aftershock atudies to have been a complex source with major movement being a thrust on the White Wolf Fault. The 1857 Fort Tejon earthquake orric s >

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H. R. Denton 1 had 300 kilometers of surface rupture extending from the Transverse Range {

to the area where the San Andreas is characterized more by splays or ]

segments. These observations suggest the need to consider whether physical j characteristics of the fault may be more controlling than available i fault length in determining maximum earthquakes. I l '

It was suggested that the ancillary faults west of the San Andreas are not master breaks but are made up of segments and that earthquakes associated with them are multiple events. An example is an 1852 earthquake j at san Simeon which had an aftershock of equal magnitude one month following the main event. Mr. Devine stated that although they do not agree to the letter with our evaluation of the response spectra, they would not take issue with it. Mr. Houser, USGS summarized the USGS pofut of view as follows: I (1) the EB" is similar to other ancillary faults in the san Andreas systems (2) The EUZ and the SL3; appear to be structurally similar based on l available data. The USGS purposely left room in their report for the applicant to demonstrate the dissimilarity between these faults.

(3) The 1927 magnitude 7.3 should be considered possible of occuring on the EEZ.

l I Regarding additional work the consensus was that:

(1)

(1) Additional studies could define the southern extent of the EBZ and determine whether or not it is playing out to.the south, ];

is truncated by the Transverse Range trend, or is continuous across the Transverse Range structure. This would involve a reevaluation of the offshore work including that of Wagner, and parhaps additional offshore investigations. l (2) Further evaluation of the 1927 earthquake could be useful in I resolving its geologic association. l (3) It was suggested, alternatively, that the largest historic  !

earthquake that has occurred on ancillary faults of the l San Andreas system, be assumed possible on the EBZ. This I earthquuke would be on the order of magnitude 7.

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R. McMullen, Geologist J. C. Stepp, Section Leader Seismology & CeM.,y Section Seismology & Geology Section Site f.nalysis E v..ch Site Analysis Branch Division of Technical Review Division of Technical Review Office of Fuclear Reactor Regulation Office of Nuclear Reactor P.egulation ore can

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