ML17209A771
| ML17209A771 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 12/30/1980 |
| From: | Devine J INTERIOR, DEPT. OF, GEOLOGICAL SURVEY |
| To: | Rolonda Jackson Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8101060577 | |
| Download: ML17209A771 (4) | |
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I United States Depaztlnent of the Interior GEOLOGICALSURVEY RESTON, VA.22092
%?XX OF THE DDtZC2OR In Reply Refer To:
EGS-Mail Stop 106 december 30, 1980 Qr. Robert E. Jackson Chief, Geosciences Branch Division of Engineering U.S. Nuclear Regulatory Commission.
N.S. P-314 Nashingtcm, D.C. 20555 Var Bob:
%is is in response to your request for an update cm our informatian concerning the occaxnence of earthquakes similar to the Charleston, S.C., event of August 31, 1886.
'Rat earthquake is rated as a Modified Mercalli intensity IX-X and, as such, is the principal controlling event for the determination of engineering design of nuclear power plants and processing facilities in the southeastern United States.
In the past, the seismic engineering input has been'based upon an earthquake of similar intensity occurring in the vicinity of Charleston, S.C., but not occurring randaaly throughout the entire Coastal Plain prRMce e.
Derog the past decade, there has been considerable research by the geologic caamamity directed toward a better understanding of seismicity in the southeast and, in particular, toward identificaticm of the structure that generated the 1886 earthquake.
'Ke spring 1918 regicmal meeting of the Geological Society of America in Chattanoaga, Tennessee, devoted a half-day sympbsium to the discus-sion of recent investigaticms in the Charleston area.
'Ihe USGS has published (1978) Professional Paper 1028, and is planning a second one, which describes the progress of its research at Charleston.
In addition, new ideas about south-eastern U.S.. seismicity are being presented at meetings of geologic and seismolo-gic societies and are being discussed in the various gealogical 5ournals.
As a result, several neer working hypotheses have been presented.
However, although geologic mapping, stratigraphic drilling, seismic reflection profiling, and gravity and magnetic surveys have been underway for several years, no direct carrelaticm between structures and eard~akes has been possible.
We cmly significant structure recognized in the 1886 earthquake aeisoseismal area is a northeast-trending reverse fault called the Cooke Fault.
%his feature has been interpreted prheazily frea seismic reflection surveys as there is no surface expression at all.
To date, however, no evidence has been presented that associates the Cooke Fault directly with the 1886 earthquake.
In fact, the length of the fault, as presently known (1S km), does not appear sufficient for generating an earthquake of iytensity Sl-IX-X, if standard fault length-I'arth+a site relatieeships are usaL Hevever, until further research iilu5h<P ~~
provides mre definitive concepts of scatheastern U.S. seismicity and of its fault length and history of ndvement, the Cooke Fault by virtue of its coinci-dence of location with the Char1cston earthquake shcald remain as a candidate structure to associate with that earthquake.
Ccneequently, it should be con-sidered as having a potcmtial fax generating similar events in the future.
Recently, several other faults have been recognized in the Charleston-Summerville
- areas, but none appear to have the potential of generating the higher intensity earthquakes.
Individual faults recognized in the area appear to have responded to stress fields of different orientation (compressianal or extensional) rather than revexse
- movement, and have inferred slip movements either in a normal or reverse sense depending upca the directian of stress of a given time.
Other faults mapped throughout the eastern Coastal Plain and Piedm~t provinces have had,similar histories but none have been proven to be active at the present time, particularly in context of capability according to NRC criteria.
During the course of recent investigations, various additicmal hypotheses on the probable causative mechanism of the 1886 earthquake have included models of associatica with mafic plutons, stress ccmcentratian along block interface, and projection of the Blike Spur Fracture Zone at depth (basement structure).
Currently, there is much discussica of an interpretation of structure that hypothesizes a large east dipping decollement extending fram Georgia northeast-ward to New England (Harris, L. D., et al, 1979)
(Cook, F. A., et al, 1979).
The decollement has been interpreted primarily from deep seismic reflection surveys de by COCORP (Schilt, F. S., et al, in prep.).
Whether the decolle-ment extends under the Charleston region is controversial (Hamiltcn, R. M.,
et al, in prep.).
Present day seismicity ranges in depth from 3 to 13 km, with two-thirds of the events in the 5-8 km range.
Composite focal mechanism solu-tions indicate sub-horizcntal nodal planes as one of the possible orientations.
However, there is no evidence which suggests that the generaticm of the present low-magnitude seismicity is related to the larger magnitude earthquakes such as the 1886 event.
A structure such is the hypothesized decollement, ho~ever, shmld it exist and still be undergoing sufficient stresses to cause ccatinued
- movement, may be capable of generating a large earthquake.
However, where the hypothesized decollement projects to the surface along the western margin of the valley and ridge province, there has been no recognition of quaternary surface rupture.
Ia additica, this zone is characterized by a relatively law level of seismicity.
The problea regarding identification of specific tectonic structures capable of generating Large earthquakes in the east is far fram resolution; Local structures near Charleston are kncaapletely known at present and the larger strucem6 eleiaent, the decollement, is as yet hypothetical..However, the concentration of seismicity in the Charleston earthquake epicenter bath before and after the August 31, 1886, event and the lack of post Miocene faulting in the Coastal Plain or any evidence far localizing large earthquakes indicate that the likelihood of a Charleston sized event in ather parts of the Coastal
Plain and Piedmont is very lcd.
Consequently, earthquakes similar to the 1886 event should be considered as having the potential to occur in the vicinity of Charleston and seismic engineering parameters should be determined on that basis.
The research on the causative mechanism of the Charleston and other east coast earthquakes must continue if a more definitive resolution of this problem is to be obtained.
Sincirely yours, Enclosure J
.s. F. Devine Assistant Director for
.%agineering Geology
C