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United States Department of the Interior 3ANf f g,
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GEOLOGICAL SURVEY ih rg WASHINGTON, D.C.
20242 5
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JAN 8 - 1973 1
Mr. John S. O' Leary
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Director of Licensing U. - S. Atomic Energy Comunission Washington, D. C.
20545
Dear Mr. O'Imary:
q She U. S. Geological Survey has completed a review of the prograa for additional exploration planned by Pacific Gas and Electric j
Company for the Mendocino Site, Pt. Arena, California.
We under-stand that parts of this program are underway.
The Survey finds 1
gaps in coverage in the program that might influence a decision j
to continue it because acquisition of reliable and necessary off-
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shore information may be beyond present technical capability.
j As you are aware, the U. S. Geological Survey in its " Status of Geologic Review" of the Preliminary Safety Analysis Report (PSAR) on the Mendocino Site-(dated 10/4/72 and transmitted to W.' P. Gammill
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on 1o/27/72), raised considerable doubt over the adequacy of the existing geologic safety analysis for the site.- 'Ihree major -problems centered on the lack of consideration given, in either the PSAR or the AmeM= ants, (1) to the apparent deformation of late Pleistocene marine terraces, (2) to the displacement and possible displacement of Holocene sediments shown in offshore seismic profiles, some of which were not available in the original PSAR or smendments, and (3) to the alterna-tive fault models that would take into account reverse, as well as l
strike-slip, movement on faults vest of the main San Andreas rift valley, including fault zones suggested by the offshore geophysical l
surveys. These problems were discussed at a meeting' of ABC, USGS and PG&E representatives at Menlo Park, California on September 21,.1972.
On October 19, 1972, a program was outlined by representatives and consultants of the Pacific Gas and Electric Co., for further explora-tion of the Mendocino Site, and-discussed informally by ABC, USGS, and 307 87102003520 B71014 PDR FOIA PDR SCHARAB7-462 FoIeB7-%z d%
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s; PG&B personnel. The[codients made at that time by the ABC ans.
USGS vere,'in esaeuce,,tbat.the program as described seemed.
emphasizedbytheprogram',?vastodetectandevaluateanyandall'(q satisfactory 3but that the most-important objective, and the least geologic features that wre relevant to the geologic safety of the
. site, rather than to concentrate on the partial investigations relating only to specific observations included in 'the USGS com-mentary of September 21..
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)i The formally submitted pr4< ram, transmitted by Mr. F. T. Searls, PG&g, with a letter dated October 27,.1972, to the ABC, attention of Mr. R. S. Boyd, ABC, has,been reviewed in the light of the above meetings and informal discussions held during an examination of the site on November.15 It is concluded that the program outlinsd '
could, under very favorable ciretaastances, provide much information of value. The program lacks coverage,ihowever,-of two highly sig-nificant areas of concern related to the problems discussed on.
September 21, and is vague-as to intent on the third.
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Specifically:
(1) Ther,s is no specified provision for detection and dating of strike-slip displacements cn offshore faults. This is conaidered J
a serious omission for1 interpreting the' geologic structure in an area in which strike-slip movement is :to be commonly anticipated.
It should be pointed out that the degree of reliability needed in the solution of these pblems for safety analysis any exceed, in'some cases, the present te ste-of-the-art of offshore geophysical techniques.
(2) There it, incomplete provision for deter =4aina the structure in the had of irregular (" rough") sea bottom along the vest coast, south of Pt. Arena. 'fhis is also considered a serious omission because faults just offshore that are parallel or'subparallel to the regional structtn%1 trend could pass into or close to the site. Because of the physicul characterdetics of this area,-normal marine geophysical techniques are ineffective and it appears that drilling information without either sub-bottom profiles cr geologic control, neither of.
which are presently feasible in this locale, as a. reference model for interpretation would be inconclusive.
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....p (3) There is no clear methodology proposed for considering alternative fault models and the attendant seismic effects, although evidence already presented, for example, of reverse displacement on some splays in the-San Andreas zone and on faults inferred from off-shore geophysical surveys, indicates the need for analysis of such' alternatives.
In view of the above considerations it is our appraisal that, even given the most careful execution of t,he exploration program as out.
lined and the most' favorable return of data for efforts expenied, there would remain certain areas of inadequate coverage and certain resh1 indeterminacies which would preclude final evaluation of the site with the degree of conservative assurance normally required for such applice' ions.
Sincerely yours,
, kt l' Director j
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