ML20213A340

Summarizes ACRS Extreme External Phenomena Subcommittee 861120 Meeting Re Diablo Canyon long-term Seismic Program

ML20213A340
Person / Time
Site:	Diablo Canyon
Issue date:	11/25/1986
From:	Maxwell J MAXWELL, J.C., Advisory Committee on Reactor Safeguards
To:	Seiss C Advisory Committee on Reactor Safeguards
References
ACRS-CT-1871, NUDOCS 8702030227
Download: ML20213A340 (2)

Text

- - -

/

e, dT- /f7/

JOHN C. MAXWELL g /

SEDLOGIET /

5322 WEETERN HILLE DRIVE AUSTAN, TEXA5 *18731 sta 4setest November 25, 1986 ,

To: Dr. C. P. Siess, via Dr. R. Savio From: J. C. Maxwell -

Subject:

November 20, 1986 meetints ACRS~E.E.P. Subcomb mittee, Diablo Canyon Long Term Seismic Program

1. LTSP The Long Term Seismic Program initially seemed too ambitious and broadly focused to be satisfactorily accom-plished in the allowed three years. However, the pre-sentation during the November 20th meeting of the E.E.P.

subcommittee inspires confidence that at least most of the program will be satisfactorily completed. The final report will substantially improve our knowledge and under-standing of the regional tectonics, fault histories, and potential seismic activity within this geologically com-plex region. Of particular importance are the studien near the possible junction of the San Simeon and Hosgri faults which indicate a slip rate on the Hosgri fault of approximately 1/10th that on the San Andreas fault, thus establishing important controls of the rate of strain buildup and release along the Hosgri fault. The extensive restudy of the 19a7 Lompoc earthquake is also of great j

importance, as is the determination that an earthquake exceeding the plant seismic design should originate only on major faults as close as, or closer than the Rinconada fault.

l

2. Attenuation of Seismic Energy a

Calculation of potential seismic damage at the Diable Canyon plant site, and in most of California, must take into account the characteristically high attenuation rate. Large attenuations are to be expected, particularly where Franciscan melange forms the acoustic basement, as in the plant site area and indeed under most parts of the Coast Ranges.

a) Melange characteristics: Where well exposed, this melange resembles a gigantic mud slide. It consists of blocks to large masses of rocks or highly variable G702030227 PDR ACRS 861125 -"

CaGfiel 27 6 _.

. 1

• , r ...

Dr. C. P. Siess November 25, 1986 -

via Dr. R. Savio elastic properties, enclosed in an intensely sheared shaly matrix. Such a mixture effectively disperses and absorbs seismic energy.

b) Distant earthquake: The Diablo Canyon plant site is within a wide belt underlain by melange extending approximately from the Nacimiento fault on the northeas.t, seaward, probably to the vicinity of the Santa Lucia bank (Page, et al, GSA Map and Chart Series, MC-28G,1979).

Energy from distant earthquakes is much attenuated at the plant site, as indicated by the work described at the November 20th meeting.

c) Faults in Melange:.- At the latitude of the plant site the Hosgri and all faults west of the Rinconada fault and east of the Santa Lucia Bank are developed within melange forming the acoustic basement beneath Tertiary sedimentary rocks. The nature and depth of underlying crustal rocks is not known. Reconstructions suggest that melange extends to depths exceeding the limiting depth of earthquakes in this area. The character-istics of melange which cause large attenuation also facili-tate creep and thus inhibit buildup of large elastic strain.

This characteristic of melange was noted by Prescott and Yu (Jour. Geophy. Res. , v. 91, no. B7, p. 7475-7484,1986),

and is also supported by the noteable absence of earth-quakes exceeding magnitude 6 along faults within melange.

d) 1927 Lompoc Earthquake: Determination of the location of the magnitude 7 3 Lompoc earthquake is of great importance for estimating potential seismic intensity at the plant site. Large differences among hypocenters pro-posed by competent seismologists indicate that a unique solution will not be found. The restudy by the LTSP group has established that the earthquake resulted from an up-thrust, probably along a plane dipping 660 NE at 10 km.

depth. The magnitude of the earthquake indicates fault-ing in much more rigid basement rocks than melange would provide. This factor, plus the orientation and dominant-ly upthrust character of the causal fault strongly indicate a location well to the south of the plant site, within the structural domain of the Transverse Ranges where rigid basement rocks may be present at relatively shallow depth.

(by John C. Maxwell

.