ML20151D050
| ML20151D050 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 03/07/1988 |
| From: | Maxwell J MAXWELL, J.C. |
| To: | Seiss C Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-CT-1920, NUDOCS 8804130364 | |
| Download: ML20151D050 (3) | |
Text
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0 JOHN c. MAXWELL eroLosisT 5&a3 5EsTERN HILLS omtyt eumFIN,TEKAs 7s731 sia.4s4 tone 1
March 7, 1988 Dr. Chester P. Siess Advisory Committee on Reactor Safeguards H Street U. S. Nuclear Regulatory Commission Washington, D. C. 20555
Dear Chet:
The studies by P.G.&E. of earthquake history, poten-tial and resulting ground motions for the Diablo Canyon Power Flant site and region set a high standard for simi-lar future studies.
The combination of intensive geologic mapping, reflection and refraction seismic surveys, dating
-of fault movements and analysis of current and past earth-quake data significantly reduce chances of underestimating possible earthquake damage of the DCPP.
The following seem to me to be worth notin6:
Hoscri Fault The Hos6ri fault remains the controllin5 geolo6 c 1
structure for strong ground motion at the plant site.
Ground motion generated by strike slip and by high angle thrusting on elements of the Hosgri fault zone are modeled for the site.
Earlier suggestions that the Hosgri, San Simeon and San Gregorio faults constitute a continuous fracture system capable of yielding magnitude 8 earth-quakes now seem highly improbable.
Offshore seismic map-ping reveals offsets between the three fault zones.
Furthermore, the magnitude 7+ Lompoc event (11-4-1927),
regarded by some as evidence of strike slip displacement of a southern extension of the Hosgri fault, on further detailed study was determined to have resulted from upthrusting on a steep fault west of the Hosgri fault.
Heirarchy of Faults 1
The P.G.&E. study classifies active faults in four
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categories, having annual slip rates which vary by four i
orders of magnitude.
(See attached handout from 2-23-1988 subcommittee meeting in San Francisco).
Over half of the i
5 to 6 cm/ year of shearing motion between the Pacific and North American plates, and a proportional release of seismic energy, occurs along the San Andreas fault system, which is j
r 8804130364 800307 i
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r Dr. Chester P. Siess March 7,1988 characterized by magnitudr. 8+ earthquakes.
The next lower category includes the Boagri, West Huasna and Rinconada strike slip faults.
These faults, characterized by annual movement rates less than one centimeter and by earthquakes to magnitude 7+,
release most of the seismic energy in the coastal area of the plant site.
Paults with smaller slip rates should not produce earthquakes larger than 6+.
The proportionality between fault length, slip rate and charac-teristic maximum earthquake seems to be valid for strike slip faults.
A similar relation may be recognizable among dip slip faults, but the two types cannot be compared on the basis of fault length.
In recent years several damag-ing earthquakes in central and southern California have occurred on faults which do not reach the surface, or are poorly exposed and unrecognized.
Los Osos Fault Exceptionally detailed geologic studies by P.G.&E.
employees and staff consultants have defined the structural framework of the immediate plant site.
New faults have been recognized, ages of latest movement determined, and rates esticated.
Of known capable faults, only the Los Osos st,ucture would seem to pose a threat to the plant.
Upthrusting of the San Luis-Fismo structural block along the Los Osos fault zone north of the plant site has occurred at an estimated annual rate of 1 mm/ year or less.
A recur-rence rate of a thousand years or so could result in a significant earthquake.
If it has not been done, perhaps a thrust type earthquake should be modeled for this fault and its possible continuation offshore.
With best regards, John C. Maxwe I
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