ML13330A162
| ML13330A162 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 12/05/1980 |
| From: | Aki K MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE |
| To: | Reiter L Office of Nuclear Reactor Regulation |
| References | |
| 54-526, NUDOCS 8101070362 | |
| Download: ML13330A162 (2) | |
Text
DEPARTMENT OF EARTH AND PLANETARY SCIENCES MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS 02139 54-526 0
December 5, 1980 Dr. Leon Reiter Seismology Section 44 J Geosciences Branch Division of Engineering Nuclear Regulatory Commission Washington, D.C.
20555
Dear Leon:
This is a letter report to summarize my thoughts on the December 4th NRC-SCECO meeting.
I agree with Stu Smith's statement that DELTA's approach is to provide a framework for extrapolating and organizing data, and this is better than purely empirical approach because one can introduce some physics.
Additional data from the 1979 Imperial Valley earthquake increased the general credibility of their model.
I also agree with Gerry Frazier when he said that changing model parameters and details of physical processes does not affect the site specific prediction for the San Onofre site very much.
DELTA's prediction is probably a good mean of ground motion expected from the earthquake along the particular off-shore fault.
On the other hand, the range of uncertainty in the prediction may depend on model parameters.
I am particularly concerned with the DELTA's consistent use of the 90% of shear velocity as rupture velocity.
My concern was increased by their new analysis of the Imperial Valley earthquake, because the systematic discrepancy between computed and observed horizontal accelerations can be explained by the excessive focusing due to the 90% rupture velocity.
I have pointed out earlier that the 90% rupture velocity cannot explain Eaton's chronograph data obtained near the fault and Filson's and McEvilly's spectral nulls observed at Berkeley.
These observations as well as the station #2 record can be easily explained (Bouchon, 1980) by the rupture velocity of 60% -of shear velocity.
If DELTA used 60% instead of 90% rupture velocity, and calibrated the rest of the parameters against all the data, A o2 0 io01@
Pt
Dr. Reiter P. 2 12/5/80 the resultant prediction for the San Onofre site would not change very much, but the result of the sensitivity study would be different.
In an earlier sensitivity study, DELTA concluded that changing rupture velocity has no significant effect on the prediction. That was because they assumed 90% velocity to start with. If they calibrated the model parameters using the 60% velocity, and considered that the 90% velocity is possible, then for the particular location of San Onofre site relative to the potential fault, the predicted ground motion would be doubled.
Thus, the result of earlier sensitivity study must be modified, if the 60% rupture velocity is used in calibrating the model.
My general feeling is that accepting DELTA's result now is premature. Until accumulated data become sufficient for statistically evaluating the range of uncertainty, we have to consider the extreme case of possible parameter variations.
I would feel comfortable if DELTA's ground motion is multi plied by a factor of 2.
Sincerely yours, Keiiti Aki KA:jar I