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The foreshock activity of the 1971 San Fernando earthquake, California

Ishida, Mizuho and Kanamori, Hiroo (1978) The foreshock activity of the 1971 San Fernando earthquake, California. Bulletin of the Seismological Society of America, 68 (5). pp. 1265-1279. ISSN 0037-1106.

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All of the earthquakes which occurred in the epicentral area of the 1971 San Fernando earthquake during the period from 1960 to 1970 were relocated by using the master-event method. Five events from 1969 to 1970 are located within a small area around the main shock epicenter. This cluster of activity is clearly separated spatially from the activity in the surrounding area, so these five events are considered foreshocks. The wave forms of these foreshocks recorded at Pasadena are, without exception, very complex, yet they are remarkably similar from event to event. The events which occurred in the same area prior to 1969 have less complex wave forms with a greater variation among them. The complexity is most likely the effect of the propagation path. A well located aftershock which occurred in the immediate vicinity of the main shock of the San Fernando earthquake has a wave form similar to that of the foreshocks, which suggests that the foreshocks are also located very close to the main shock. This complexity is probably caused by a structural heterogeneity in the fault zone near the hypocenter. The seismic rays from the foreshocks in the inferred heterogeneous zone are interpreted as multiple-reflected near the source region which yielded the complex wave form. The mechanisms of the five foreshocks are similar to each other but different from either the main shock or the aftershocks, suggesting that the foreshocks originated from a small area of stress concentration where the stress field is locally distorted from the regional field. The number of small events with S-P times between 3.8 to 6 sec recorded at Mt. Wilson each month suggests only a slight increase in activity of small earthquakes near the epicentral area during the 2-month period immediately before the main shock. However, because of our inability to locate these events, the evidence is not definitive. Since the change in the wave forms is definite the present result suggests that detailed analyses of wave forms, spectra, and mechanism can provide a powerful diagnostic method for identifying a foreshock sequence.

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Kanamori, Hiroo0000-0001-8219-9428
Additional Information:Copyright © 1978, by the Seismological Society of America. Manuscript received January 23, 1978. We wish to thank Tai-Lin Hong for providing us with the results of numerical computation of reflected arrivals. Many helpful and stimulating discussions with Don Anderson, Robert Geller, David Hadley, Karen McNally, Seth Stein, and Gordon Stewart are gratefully acknowledged. Carl Johnson and Christine Powell helped us in retrieval of local seismicity data. Takeshi Mikumo and Kazuaki Nakamura made helpful comments on the manuscript. Mizuho Ishida was supported by a fellowship from the Science and Technology Agency in Japan. This research was sponsored by U.S. Geological Survey Contracts 14-08-0001-15893, 14-08-0001-16711, and 14-08-0001-16776.
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Japan Science and Technology AgencyUNSPECIFIED
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Caltech Division of Geological and Planetary Sciences3011
Issue or Number:5
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48930
Deposited By: George Porter
Deposited On:26 Aug 2014 20:53
Last Modified:09 Mar 2020 13:18

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