ML20209D089

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Summarizes 840619 & 22 Geological Reconnaissance of Site Vicintiy & 840620 Public Meeting
ML20209D089
Person / Time
Site: 05000000, Diablo Canyon
Issue date: 07/06/1984
From: Brocoum S, Mcmullen R
Office of Nuclear Reactor Regulation
To: Reiter L
Office of Nuclear Reactor Regulation
Shared Package
ML082410749 List: ... further results
References
FOIA-86-197 NUDOCS 8407110084
Download: ML20209D089 (4)
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UNITED STATES 8

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jul 0 61984 MEMORANDUM FOR: Leon Reiter, Acting Chief Geosciences Branch Division of Engineering FROM:

Stephan Brocoum, Leader Geology Section Geosciences Branch, DE

.&#7 Richard B. McMullen, Geologist Geology Section Geosciences Branch, DE

SUBJECT:

GEOLOGICAL INSPECTION AT DIABLO CANYON NUCLEAR SITE On June 19 and June 22, 1984, S. Brocoum and R. McMullen of the Geosciences Branch conducted a geological reconnaissance of the Diablo Canyon site vicinity and the region around the site. They were accompanied by Frank Brady and Forrest Russell (part-time) of Pacific Gas and Electric Company (PG&E) and Douglas Hamilton of Earth Sciences Associates, geological consultants to PG&E.

The afternoon of 19 June was spent examining photographs and logs of the numerous trenches that were excavated across the site in the middle and late 1960's, photographs and maps of the sea cliffs; and regional geology maps.

It is our opinion after reviewing these items that a thorough job was done, and it is highly unlikely that a geological feature significant to the safety of the plant has been overlooked.

On June 20, at the request of D. Eisenhut we attended a public meeting among PG&E and NRC representatives. Geology and seismology issues.were not discussed. We then spent several hours flying around the site and the surrounding region in a helicopter. Several overflights of the site and passes along the sea cliff adjacent to the site were made. The geology exposed appeared to be as depicted on the various maps and logs, except that a large asymmetrical-to-the-southwest fold located at the south end of Diablo Cove probably is more significant now than was given to it at the time it was mapped, in light of the Crouch et al (1984) hypothesis.

It was mapped simply as a drag fold. Rock in the immediate vicinity of the fold was highly deformed. Although the fold is probably very ancient, the mode of deformation and orientation is consistent with large-scale thrust faulting with transport to the west-southwest as postulated by Crouch et al. This feature was examined on the ground the following day and it is described more below. A similar fold, though much smaller, was observed north of Diablo Cove. We next flew along the

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j-1 JUL 0 61984 sea cliff beyond Point Buchon to the Morrow Bay State Park. Several other minor asynnetric folds similar to, though much smaller than the fold at the site were observed. All the asymmetric folds appear to suggest tectonic transport to the southwest, consistent with the hypothesis of Crouch, et al,1984.

The helicopter flew along the shoreline of Estero Bay, past Morro Rock,

'followed the straight coastline west of Canbria which is probably controlled by the-Sam Simeon fault, and crossed the coastline east of Point San Simeon where the San Simeon fault zone comes ashore. The San Simeon fault zone onshore is made up of two major splays, the Arroyas de Lagunas and the San Simeon faults, and numerous minor splays. At San Simeon the zone separates the Miocene Monterey formation to the west from the Jurassic Franciscan formation to the east. The Arroyas de Lagunas fault follows the western front of the Santa Lucia Mountains to i

the north to the vicinity Ragged Point, where it again goes to sea. It j

controls a prominent valley along which drainage channels turn to the i

north. This observation supports right lateral strike s1;p motion along 1

this fault, with the last movement being relatively recent. We then flew over land to the San Luis Obispo Airport for fuel, flying along the line of intrusives beginning with Morrow Rock and extending southeastward for more than 20 miles. After refueling we traveled to the coast south of Avila Beach, flying low over the local topography and j

returned to the site. An asymmetric fold like those described above was observed on the sea cliff south of Avila Beach.

After landing we learned of the magnitude 4.1 earthquake west of Point Sal, which occurred while we were airborne. PG&E supplied the staff with copies of the records from instruments at the site. The earthquake was felt at the plant.

s By automobile we traveled on the Field's Ranch north of Diablo Canyon, past Pt. Buchon and to the State Park. We examined the sea cliff at several locations. The rock, which consisted of the Monterey fomation, the Obispo volcanic intrusive rock, and the Pismo formation, although dipping to the north, was relatively undefomed.

On 21 June we went down and closely examined the large fold southwest of the plant. This fold, the one north of Diablo Cove, and several others observed are oriented in such a way as to indicate tectonic transport to the west-southwest. If this is true it supports the compressional tectonic regime suggested by Crouch et al (1984). Most of these asynnetric folds are relatively small (several feet wide, however, the u

asymmetric-to-the-west-southwest fold southwest of the site is several.

tens of feet wide. This feature was mapped as a drag fold during CP investigations and is in alignment with a fold mapped in the hills east of the plant. The fold axis strikes about N35'W, which is slightly oblique to the Hosgri trend of N40 W.

A possible similar feature was j

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2 JUI. 0 61964 noticed just south of Avila Beach. These features are probably very old as they have no apparent effect on the Pleistocene marine terraces. A refolded fold from an earlier deformation ~ is present in the main fold, indicating multiple deformations.

We traveled by automobile to Hearst Beach at San Simeon where the San Simeon fault zone intersects the coast, separating Monterey formation to the west from Franciscan formation to the east. Also present at this outcrop is a large slab of early Pleistocene rock that Clarence Hall, who had mapped the region for the USGS, correlates with a similar rock in the Point Sal area. He used this correlation as one of the bases for postulating 80 km of right lateral strike slip movement on the Hosgri-San Simeon fault zones. We drove up to Ragged Point where the San Simeon fault zone crossed the shoreline out to sea again. The exposures.of the Arroyas de Lagunas fault, the easternmost splay, in the sea cliff were examined.

It dips about 35' to the northeast and is comprised of relatively young looking gouge. The fault has Franciscan formation on both sides so determining amount of offset is difficult.

The San Simeon fault located several hundred feet to the west is nearly vertical, and the fault zone is wider.

This splay of the San Simeon fault zone separates the Franciscan formation to the east from ophiolite to the west. The San Simeon fault crosses the coast at Ragged Point. The amount of strike slip displacement on the San Simeon fault zone is not known but was estimated, based on several lines of evidence, to be several kilometers.

The vertical offset is at least 5 km based on correlations between strata at the local summit of the Santa Lucia Mountains and the Y1oor of the Santa Maria Basin.

Based on the geological reconaissance and examination of the maps and photos PG&E was informed that there probably was sufficient data available to accomplish an adequate analysis to satisfy element 1 of the licensing conditions. The features on the sea cliff should be m' pped a

and analyzed by an experienced structural geologist.

In line with this, the mapped attitudes of bedding and structural features onshore should be plotted on stereonet projections to determine whether or not the folds are asymmetrical, and, if they are, to show the direction of asymmetry. This information will provide evidence on the direction of tectonic transport in order to help us to better understand the history of movement on the offshore faults.

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,Although this inspection revealed several geological features that need to be reevaluated in light of the thrust fault hypothesis, nothing was observed that would cause us to reexamine our conclusion concerning operation of the plant or to revise the geology elements of the licensing conditions.

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'S ph rocoum, Leader Geology Section Geosciences Branch Division of Engineering

$~M'/>?'77 Richard B. McMullen, Geologist Geology Section Geosciences Branch Division of Engineering cc:

J. Knight L. Reiter G. Knighton S. Brocoum H. Scherling

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R. Rothman R. McMullen j

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