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Published April 1982 | Published
Journal Article Open

P-wave complexity and fault asperities: The Borrego Mountain, California, earthquake of 1968


Results from a synthetic seismogram analysis of the short-period P waves from the Borrego Mountain earthquake of 9 April 1968 (M_L = 6.4) are used to model the strong-motion recording at El Centro. A short-period-long-period deconvolution analysis of the teleseismic P waves suggested that a two-source model would fit the data much better than the single-source model presented by Burdick and Mellman (1976). Forward and inverse modeling of the data demonstrated that two sources, each of less than 2-sec duration, the second occurring 2.2 sec after the first and both being at about 8-km depth, best fit the short-period waveforms. From this model, long-period synthetics were generated which were found to be quite compatible with the data. This source model was also used to synthesize the strong-motion SH displacement, velocity, and acceleration records from El Centro, California. The close match of synthetics and data is used to argue that short-period waveforms contain much information about asperities which play a crucial role in the near-source strong motions from an earthquake. The Borrego Mountain event probably began with the failure of a fault asperity. The evidence for this is the several-hundred-bars stress drops of the two short-period sources and the probable location of these sources in a place where there was almost no aftershock activity or postseismic creep on the fault.

Additional Information

© 1982, by the Seismological Society of America. Manuscript received 13 November 1980. Special thanks must be made to several people for their assistance with this study. Gladys Engen ran much of the analysis on the strong-motion data and was responsible for finding the fits to the acceleration record. George Mellman kindly provided his inversion program and made up for the lack of written comments m the program with oral instructions about its inner workings. Dave Boore and Allen Olsen reviewed the manuscript, and their comments prompted some unexpected improvements m the analysis. Joe Galvan and Laszlo Lenches helped prepare the figures. This research was supported by the National Science Foundation under Grant PFR-7921769 and also by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Office of Scientific Research under Contract F49620-81-C-0008.

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