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Aftershocks and Preearthquake Seismicity

Johnson, Carl E. and Hutton, L. K. (1982) Aftershocks and Preearthquake Seismicity. In: The Imperial Valley, California, earthquake of October 15, 1979. Geological Survey professional paper. No.1254. United States Geological Survey , Washington, DC, pp. 59-76.

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Although primary surface faulting was mapped for nearly 30 km, aftershocks extended in a complex pattern more than 100 km along the trend of the Imperial fault. A first-motion focal mechanism for the main shock is consistent with right-lateral motion on a vertical fault striking N. 42° W., in agreement with the strike of the Imperial fault within the limits of resolution. There is evidence that conjugate faulting on a buried complementary northeast-trending structure occurred at the north limit of displacement on the Imperial fault near Brawley, Calif. This faulting was apparently initiated at the time of a magnitude 5.8 aftershock 8 hours after the main shock. A line of epicenters extending along the trend of the San Andreas fault nearly 100 km into the eastern Imperial Valley was noted during the aftershock sequence, in an area recognized as notably aseismic during the preceding 5 years. The main shock was preceded by a 3-month period of significantly reduced seismicity affecting the central Imperial Valley. Although three small events near the incipient epicenter during this interval may be deemed foreshocks, no distinct foreshocks immediately before the main shock were observed.

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Additional Information:© 1982 USGS. We thank the CEDAR-system timers Anne Blanchard, Shirley Fisher, Peter German, Doug Given, Karen Richter, and Vie Lamanuzzi for their great diligence in the face of a seemingly endless task during the timing of initial aftershocks. Parts of the Imperial Valley base map were prepared by Peter T. German. This research was partly supported by U.S. Geological Survey Contract 14-08-0001-16719 and California Division of Mines and Geology Contract 5-0059.
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California Division of Mines and Geology (CDMG)5-0059
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Caltech Division of Geological and Planetary Sciences3459
Series Name:Geological Survey professional paper
Issue or Number:1254
Record Number:CaltechAUTHORS:20141204-133907201
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:52402
Deposited By: Tony Diaz
Deposited On:04 Dec 2014 21:45
Last Modified:03 Oct 2019 07:42

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