ML19343D256
| ML19343D256 | |
| Person / Time | |
|---|---|
| Site: | Monticello, Mcguire, McGuire  |
| Issue date: | 03/09/1981 |
| From: | Thomspon G Advisory Committee on Reactor Safeguards |
| To: | Okrent D Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-1330, NUDOCS 8105040051 | |
| Download: ML19343D256 (1) | |
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,j NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR sAFEGUAR
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d March 9,1981 MEMORANDUM FOR:
D. Okrent, Chairman, ACRS Subcommittee on Sumer South Carolina Plant FROM: Dr. G. Thompson, ACRS Consultant
SUBJECT:
REPORT ON SEISMICITY (RECEIVED BY TELEPHONE MARCH 9, 1981)
The physics of reservoir induced seismicity in the Monticello reservoir is now better understood than anywhere else in the world. The diverse orienta-tion and shortness of fault plane as indicated by focal mechanisms, the smallness of stresses as measured in-situ in boreholes and the decreases in I
earthquake frequency since the reservoir was filled are reassuring.
believe that the family of earthquakes observed so far is most accurately described by the log-quadratic carve of McGuire. This curve yields a return period of 1600 years for Mt = 4.0.
It is conceivable however that water pressure will propagate to greater depths and distances over longer time in these rocks of low permiability and that a new family of earthquakes could Obviously no adequate basis for complete estimates be generated in the future.
of probability is available, but my personal estimate is a return period for magnitude = 4.5 of 1000 years with probability of not being exceeded of 90%.
For magnitude 5.0 with the same return period, I suggest 99% or higher.
U Turning from reservoir induced seismicity to the regional seismic problem our main concern should be about earthquakes of the Charleston type. The 1886 earthquake had an intensity at the site of about seven. Subsurface evidence r.x links this earthquake with the Cooke fault although the association is not certain. The Cooke fault shows progressive movement aggregation of 50 meters in 150 million years.
Thus, earthquakes as large as the 1886 event could have occurred from the Cooke fault at intervals averaging more than a million years.
If other faults and other Jurassic rifts are considered, the re-cuurence interval should be decreased by 100. This kind of reasoning lead to I
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very rough estimates of a return interval of 10,000 years for Charleston-type events within the Piedmont or coastal plan south of Virginia.
I think the Moreover probability of this interval not being exceeded is roughly 90%.
the weight of evidence still links the Charleston seismicity with the Jurassic rift at Charleston.
I think that the ideal of seismicity on a wide spread sub-horizontal detachment thrust can be dismissed.
Finally all evidence to date on the Wateree fault indicates that it has long Pending additional data, the Wateree fault should not be been inactive.
considered any more of a hazzard than other unknown cracks or faults of which I
there must be many.
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