ML20203N127

Ack Receipt of Info Re Facility Geological Activities. Program on Track & Objectives Sound.Delineation of Geometry, Physical Characteristics & Potential Seismicity of Hosgri Fault Zone Most Critical Task

ML20203N127
Person / Time
Site:	Diablo Canyon
Issue date:	08/12/1986
From:	Maxwell J Advisory Committee on Reactor Safeguards
To:	Igne E Advisory Committee on Reactor Safeguards
References
ACRS-CT-1855, NUDOCS 8609230153
Download: ML20203N127 (2)

Text

6T-IC55 A -

J RECEIVED 12 Atyust 1986

. '86 AGO 18 All :54 PD2 U.S. NUOLEAR REG. COMM p/f/ff ADV!SORY COMMIT TEE ON E. G. Igne, Senior Staff Engineer REACTOR SAFEGUARDS U.S. Nuclear Regulatory Cbtmission.

Advisor Ommittee on Peactor Safeguards

- H Street Washington, DC 20555

Dear El:

'Ihank you for keeping me up to date on Diablo Canyon N.P.P. geological activities. It seems to me the program is fairly well on track and the objec-tives are sound.

It appears that the delineation of the gemetry, physical characteristics, and potential seismicity of the Ibsgri fault zone is still the most critical task to be accomplished. Because this fault syst s is nowhere exposed, and control by deep drilling is sparse, it is highly unlikely that the total dis-placenent and average rate of slip can be determined with acceptable accuracy within the next fea years. The in:ediate concern is, of course, to arrive at an estimate of the maximum magnitude of earthquake that might occur on this fault zone,adja nt to the plant site. Perhaps the single nost inportant consideration, then, is the capacity of the rocks adjacent to the fault zone to store elastic energy.

North of the Transverse Ranges, all (or nearly all?) earthquakes of magnitude 7 or larger appear to be related directly to the presence of strong crystalline rocks of Sierra Nevada or Salinian bassent (at depths of no ucre than a few km). Faults entirely within Franciscan-type melanges are charac-terized either by creep or by small to medium earthquakes, yet may acccrmodate large regional strains, as in the broad Franciscan outcrop belt extending northward frm San Pablo Bay (Prescott and Yu, Jour. Geot t ys. Res., v. 91, no. B7, p. 7475-7484, June 1986).

In 1979, Ben Page et al. published a geological cross section which extends frm west of the continental slope to Point Buchon, crossing the Santa Lucia Bank and Ibsgri fault zones and continuing to the center of the San Joaquin Valley (G.S.A. !bp and Chart Series, MC-28G,1979) . On the basis of on-shore geology and reflection seismic work off-shore, they conclude that Franciscan or similar melanges form the " basement" beneath the Tertiary sedi-ments, at least as far west as the east Santa Lucia fault zone, and thus form the basemmt displaced by the Ibsgri fault zone. On the basis of seismic characteristics, similar but probably younger nelanges continue to the base of the continental slope beneath the Tertiary and Quaternary sediments. 'Ihe published section indicates that the melanges may extend to depths of 17 km or nore beneath the Ibsgri fault zone, thus exceeding the " brittle-d'.x: tile transition" depth thought to characterize this coastal area. If the Page et al. interpretation is (Drrect, then no large earthquakes are to be expected 8609230153 060012 a c:l m onIGINAL PDR ACRS Cc:G Clod Dy -

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f. 12 August 1986

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along the Ibsgri or other off-shore faults in the area west of the Diablo Canyon plant site.

Quite probably, PG&E &nd consultants are including this proposition among others to be tested by further deep-seismic, magnetic, and gravity surveys.

If not, they may wish to adi it to the list.

Cordially, John C. Mamell JCM:rb e