ML20212M314
| ML20212M314 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Diablo Canyon |
| 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 | |
| ML20150F500 | List:
|
| References | |
| FOIA-86-391 NUDOCS 8608250439 | |
| Download: ML20212M314 (3) | |
Text
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DISTRIBITTION:
4 IO73 Docket File:TR A
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SAB:TR H. R. Denton, Assistant Director. Site Safety, TR THRU:
W. P. Cassaill, Chief, SAB, TR MEETING WITH THE U.S. GEOLOGICAL SURVEY REGARDING THE GEOLOGY AND SEgSMDLOGY OF THE DIABIA CANYON NUCLEAR SITE l
A meeting was held on 31 January,1975 in Denver between the U.S.
l Geological Survey (USGS) and the Nuclear R5gulatory Comunission (MtC) to discuss the geology and seismology input to the Diablo Canyon SER.
A list of attendees is enclossed. The following agenda was agreed on:
- 1. Regional geology, with emphasis' on the definitions of the Hosgri Fault zone (East Boundary fault zone) sad the Santa Lucia Bank Eault zone;
- 2. Seismicity, specifically that associated with the Hosgd and Santa Lucia Bank Fault zones;
- 3. Response spectra for the plant.
Mr. F. McKaown, USGS, discussed his evaluation of the geology which considered that the Hosgri or East Boundary fault zone (EBZ) fault zone and the Santa Lucia Bank fault zone (SLBZ) were capable and could genernte the same order of magnitude earthquake as other ancillary faults of the San Andreas system, the Riconsta, the Sur Nacimiento, etc. The 1927 magnitude 7.3 earthquake is considered by the applicant to have occurred g
on the Santa Lucia Bank Fault Zone. Correlation of structure and
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seismicity is better on the EBZ than the SLBZ and it is at least
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90 miles long whereas the known length of the SLBZ is about 65 miles.
i The SLBZ is better defined on the seafloor than the EBZ but generally n
is not as well defined where it does not offset the seafloor. The two I
zones are similar in that both have offset the seafloor and Holocene deposits, border a Tertiary badin, show predominantly vertical displacement,,
i and are presently involved in transcurrent movement of the San Andreas system.
The USGS criticized our use of fault length versus magnitude to determine the SSE by pointing out that neither the northern nor the southern limit of the EBZ is known. They pointed out that the southern extent of the EBZ cuts across the Pliocene section within the Tertiary basin underlying the physiographic Santa Maria valley. This could i
indicats that the latest activity along the zone is not controlled by the east 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 i
with no geologic evidence of reverse motion. However, there is some Y,g y
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 ise TRjSAB TJtMU
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f.F associated with transcurrent San Andreas cation an.1 not because they both border the same structural basin.
Mr. J. Devine, USGS, briefly sumanarized 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 similar with respect to earthquaka generation capability and assume a magnitude 7.3 as possible of being generated on either structure.
It might be possible to show either that the earthquake did not occur along the SLJZ, or show that the EBZ and SLBs are entirely different structures. On the other hand it may not really matter whether they are similar or not in view of the USGS latest interpretation of the relationship of the EBZ to regional tectonics.
The SLBZ is characterised by a horst on the sea floor (a west facing scarp seaward and an east facing scarp shoreward). The horst is underlain by Frsaeiscan-like rocks (Cretaceous-Jursesic) with Quaternary sediment butting up to the east scarp. There is no evidence of recent movement along these scarps and their presence does not indicate capability. The 1969 earthquake cluster at the south end of the SLBZ indicates source movenent opposite to that which would be expected by the orientation of the scarp.
Some preliminary information was presented regarding the reevaluation of earthquakes in the area. The 1927 earthquake may have occurred 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 earthquakes g
show a definite trend of seismic activity assoicated with the EBZ.
A discussion followad regarding fault length versus earthquake magnitude.
The NRC evaluation was based on the assumptions that (1) the fault is i
90 miles long (2) 1/2 total fault length breaks during a mawimum quake and (3) the Bonilla curve (fault length vs. magnitude) ree-mama =
f' 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 magnitude 7.7 earthquake in 1952 that caused surface rupture over most of the mapped extent of the White Wolf Fault. Regarding these
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two earthquakes, we note that movement on the Manix Fault secondary to a j
larger fault movement depth and the 1952 earthquake is indicated from aftershock studies to have been a complex source with major movement l
l being a thrust on the White Wolf Fault. The 1857 Fort Tejon earthquake l
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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 a
characteristics of the fault may be more controlling than available fault length in determining mayf mine earthquakes.
It was suggested that the ancillary faults vest of the' San Andreas l
are not master breaks but are made up of segments and that earthquakes I
associated with them are multiple events. An example is an 1852 earthquake at san Simeon which had an af tershock of equal magnitude one month following the main event. Mr. Devine stated that although they do not i
agree to t.he letter with cur ev >beton of the response ' spectra, they l
would not take issue with it.
r/saar, USGS sumnarized the UfGS I
point of view as follows:
l (1) the EBZ is similar to other ancil ary faults in the san Andreas systems l
(2) The EBZ and the SLBZ appear to be structurally similar based on 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 EBZ.
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 f,
across the Transverse Range structure. This would involve a reevaluation of the offshore work including that of Wagner, j
and perhaps additional offshore investigations.
I (2) Further evaluation of the 1927 earthquake could be useful in resolving its geologic association.
(3) It was suggested, alternatively, that the largest historic earthquake that has occurred on ancillary faults of the San Andreas system, be asscued possible on the EBZ. This earthquake would be on the order of magnitude 7.
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R. McMullen, Geologist J. C. Stepp, Section Leader Seismology & Ceology Section Seismology & Ceclogy Section l
Site Analysis Branch Site Analysis Branch Division of Technical Review
. Division of Technical Reviev l
Office of Nuclear Reactor Regulation Office of Ifuelear Reactor Regulation i
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