ML20043D743
| ML20043D743 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 02/07/1990 |
| From: | Thompson G Advisory Committee on Reactor Safeguards |
| To: | Seiss C Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-CT-1960, NUDOCS 9006110128 | |
| Download: ML20043D743 (2) | |
Text
-.
1.m STANFORD UNIVERSITY g)g g/3y/rf i r, 7
STANFORD, CALIFORNIA 94305 2215 lntra c e cs=
February 7,1990 Dr. Chester P.' Sters 3110 Civil Engineertne, CUg.
University of Illinois y j,, 'ff,J Urbana,IL 61801 p
Dear Chet:
On Monday, January 29 and Tuesday, January 30, I attended the NRC/PG&E Geophysical Data Meeting of the Diablo Canyon Long Term Selsmic Program.
The meeting was held in the PG&E Offices in San Francisco cnd was attended by about 25 people, nearly all of whom weregeologists and geophysicists.
The' format of the meeting was open discussion-about the-interpretation of seismic reflection sections covering about 100 km of the Hosgrl f ault zone. The sections were displayed on the walls in a standard format at a uniform scale. -Each section was exhibited as a migrated (digitally focused) time section, the same section.wlth interpretation, and a depth section-with Interpretation.
This mode-of presentation was extremely useful for critical analysis of points of difference and agreement. Reflection sections. Inherently exhibit best those structural movements that.are in the plane of cross sections (compression or extension),- and there Was only mild disagreement.on : details of the compressional folding 'and thrust faulting.
The sections also show evidence of-strike-slip (in and out of the sections), for example reversed sense of apparent displacement on dif ferent horizons.
- Another limitation of the reflection sections is the depth of coherent Imaging. Because the basement rock is heterogeneous Franciscan formation, faults cannot generally be traced deeper than four or five kilometers with confidence. Thus the debate continued:
Is the Hosgrl faulti s'eep to great ' depths or does it flatten into a subhorizontal
'detachnWnt?"
Those of US who are familiar with all the evidence (earthquakes,
_ current rates of ofIset, regional geology, etc.) seemed to agree that the Hogri Zone near the plant site is taking up mainly strike-slip but is accomodating a smaller component (10-20% ?) of compression.
Jim Crouch vigorously disagreed and two USGS represenatives (McCulloch and -
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2 l.ewis) seemed to. rely heavily on the reflection data -alone and to-4 emphasize dominantly compression.
To.some degree the argument is unnecessary because the design evaluation allows for both strike-slip and compression.
- Moreover, subhorizontal detachment. faults are notably aselsmic worldwide, in both compressional and-extensional environments.
To the exent that they represent a flattening of f aults in the ductile rfon below the brittle upper crust, they present no direct seismic hazard.
Sincerely, 52/
eorge 'A. Thomps n cc ElIgne-
,/t,
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