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Published December 1985 | Published
Journal Article Open

Foreshocks and time-dependent earthquake hazard assessment in southern California


The probability that an earthquake in southern California (M ≧ 3.0) will be followed by an earthquake of larger magnitude within 5 days and 10 km (i.e., will be a foreshock) is 6 ± 0.5 per cent (1 S.D.), and is not significantly dependent on the magnitude of the possible foreshock between M = 3 and M = 5. The probability that an earthquake will be followed by an M ≧ 5.0 main shock, however, increases with magnitude of the foreshock from less than 1 per cent at M ≧ 3 to 6.5 ± 2.5 per cent (1 S.D.) at M ≧ 5. The main shock will most likely occur in the first hour after the foreshock, and the probability that a main shock will occur decreases with elapsed time from the occurrence of the possible foreshock by approximately the inverse of time. Thus, the occurrence of an earthquake of M ≧ 3.0 in southern California increases the earthquake hazard within a small space-time window several orders of magnitude above the normal background level.

Additional Information

© 1985, by the Seismological Society of America. Manuscript received 25 April 1985. The author would like to thank J. Pechmann, P. Reasenburg, and A. Lindh for constructive reviews of this manuscript. W. Ellsworth, E. Hauksson, and T. Heaton also gave helpful advice and criticism. This work was conducted while the author was a National Research Council Fellow at the U.S. Geological Survey.

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