ML20039B144

Forwards Rept on 811109 Small Earthquake Swarm Located Offshore of Oceanside.Events Consistent W/Historic Earthquake Data in Site Region

ML20039B144
Person / Time
Site:	San Onofre
Issue date:	11/18/1981
From:	Baskin K SOUTHERN CALIFORNIA EDISON CO.
To:	Miraglia F Office of Nuclear Reactor Regulation
Shared Package
ML20039B136	List: ML20039B144
References
NUDOCS 8112220336
Download: ML20039B144 (17)

Text

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Director, Office of i;uclear Reactor Regulation Atte, tion: Mr. Frank Miraglia, Branch Chief

Licensing Branch No. 3 U. 5. t;uclear Regulatory Commi ssion

'.'a s hi n gt o n , D. C . 20555 S c 's .e n:

Subject:

Dociet !;os. 50-351 and 50-362 San Onofre fiuclear Generating Station Units 2 and 3 On i;ove .5er 10,1991 we notified the fiRC staff that a s .all l-'

earth cake s.. ara located off shore from 0:eanside had occurred from ';;ve .5er 6 to *;:vem:er 9,1981. The purpose of this letter is to provide the enclosed '

4-report which provides detailed information regarding these events. As indicated in this report it is concluded that these events are consistent with -

, historic earthquake data in the region of the San Onofre site.

please contact If you have any questions or comments concerning this information, me. .

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INTRODUCTICN A sr.all earthquake swarm located of fshor'e from Oceanside was noticed by the seismological Lab, Caltech,,on Monday morning, Neverber 9, 1981.

The largest earthquake detected was LM =3.0; the total nu-3er of located events totaled 20. The sequences started Noverber 6, 1981 a t 2 0: 37 GMT and the last event

'f was recorded at 00: 47 GMT cn Mcverber 9. The eoi- '

central location of the swarm was approximately 12 km SSE from SONGS.

Iraediately upon notification, Southern California Edison initiated a field p ceram to record micrc-earthquakes with portable instrunents.

The first instrument was operational by Menday evening (November 9).

Since the installation of the field seismic recorders, Caltech has not recorded any additional events.

The small size of these earthquakes, v.L <3.0, and the hypo. central _

i distances of 12-13 km precluded recording any data on the strong ground motion instruments located at SONGS.

EARTHOUAKE LOCATIONS AND FOCAL MECHANISMS P- and S- wave arrival times for all 20 events were obtained from Caltech/U.S. Geological Survey. Using the velocity model developed in our previous studies and discussed by Biehler at the recent ASLB -

hearings (in relation to the January, 1975 and June, 1977 earthquakes),

these events were relocated. The resulting epicenter locations are shown on Figure 1.

The associated event data are listed in Table 1.

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FoSal mechanisms for the five largest events were also ccm,ruted.

33 ause,cf the small size of these events and the limited number

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4 of recordings, on1y the mechanisms for the two larger events were rea:enably well ccnstr$ined. Both events indicate predominent.

strike-slip motion. An analysis of the uniqueness of the fault planes for the largest event is shown on Figure 2. The slip-vectors

, indicated with a "+", are simply 'th'e pcles to t'4e fault planes. .

The projecticn shcwn on Figure,2 is a 1cwer hemisphere equal area sterec-net plot. The computer program used in the calculation of this focal mec',anism evalgatec about 200 possible orientaticas of the focal planes.

The best fitting orientation is indicated with a "+". ,

As progressively moie data are violated by the tested orientatien, the printed charac-ter ine.raase frcm "0" t o ',* 3 " . The plotted field of "0" syr.bcls is a gcod indication of the variability or " rattle" in the solution.

3 Similar plots for the compressa;n and tension axis are shown in Figures 3 and 4.

Nigure' 5 shows the traditional lower hemisphere stereonet plot containing

all of the data. The symbol "C" indicates compressions and "D" indicates dilitation.

B This mechanism is very typical of California earthquakes. ~

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The northwest trending f ault plane is considered the correct plane <

based upon geologic conditions, and the sequence would be characterized as representing right-lateral strike-slip motion.

MICRO EARTHOUAKE MONITORING Portable seismic recording instrumentation was installed in the field .

Monday evening, November 9, 1981. In total, 3 stations were installed i

surrounding the preliminary epicentral location-provided by Caltech.

The final. epicentral locations discussed in this report were completed Thursday evening, November 12. At that time, because of the offshore

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I.o':a t i cn , it was concluded that the portable array was still Iqcated in a reasonable configuration. The seismograms frcm the array were reviewed on November 16. No additional events were de-t tected by the array. This is censistent with the report by Ca.ltech that the last event recorded by the regional array occurred at' 0:47 GMT ca November 9. The three portable s'tations were removed from the field en November 17. , 5 .

EISTORICAL SEISMICITY Ficures 6 and,f$7, shew the seismicitv in the SONGS area crer the period 1924-1977 and 1978-1979 respectively. At least six events have c: curred historically in this samp area. Figure S and Table 2 smm-marize these previ:us earthquakes.,

The largest event detected to date is M L=3.5 (1934).

Earthquake clusters in space and time are generally characterized a s either mainshock-aftershock or swar.m sequences. In a swarm the largest event seldom initiates the process but rather occurs in the _

middle of the secuence. i Swarms are not unusual in the recion sur-rcunding' SONGS. -The last swarm of comparable magnitude (Mn < 2.7).

occurred in June, 1977, and was discussed by Biehler during the recent ASLS hearings.

The frequency of occurrence of earthquakes with ML> 3.0 in the regica surrcunding SONGS provides a basis for evaluating the most recent swarm. Seismic recurrence' intervals for the region indicated by the dashed box en Figure 1 have been computed from the historic .

data base. The recurrence interval for this region (8.1x103 km2) for earthquakes of ML >,3.0, is 140 days. Although the general level'

cf_ seismicity in the SONGS region is 3 to 4 tires 1 ewer rhan the sosthern California average, both the~ recurrence intervsis and the presance of historic earthquakes and swar=s with similar magnitudes indicate that the recent swarm is rather nominal. -

The epicentral data are plotted on three figures. The regional data and site specific data points ire initially plotted en a a

digitized ccpy of State of California Geologic Fault Map at a scale of 1:750,000, (Figures 1, 6, 7 6 8). The cap produced by Dr. D. G.

Moore is usei t.c plot the general location of the swar with ri reference to the offshore geologic features and is shewn in s=all scale on Figure 9. Figure 10 is in enlargement of a specific area

. frc: Figure 9 embracing the sware, and presents the epicentral locaticas relative to the-offshore Zone of Defor=ation. Use of the map by

.Dr., Moore to plot the events to exhibit.these relationships is

=ost appropriate 'because of the extensive interpretation by Dr. Moore of the geologic structure and stratigraphy' as interpreted from the nu=erous high resdution seismic profiles obtained in this general -

Iccation. i Although the map by Greene and Kennedy indicates'tata Voids" in the vicinity and suggests the swarm may berder the western edge of their interpretation of the location of the CZD, the accc:panying =aps show that the epicenters of this swarn lie in or near the capped expression' of the Offshore Zone of Defor=ation.

The steep dip and N45W strike, paralleling the OZD, as deter:ined fren the focal plane sol -rion supports this interpretation.

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