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The foreshock sequence of the 1986 Chalfant, California, earthquake

SEISMOLOGICAL LABORATORY MACKAY SCHOOL OF MINES UNIVERSITY OF NEVADA, RENO, NEVADA

Abstract

The M_L 6.4 Chalfant, California, earthquake of 21 July 1986 was preceded by an extensive foreshock sequence. Foreshock activity is characterized by shallow clustering activity, including 7 events greater than M_L 3, beginning 18 days before the earthquake, an M_L 5.7 foreshock 24 hr before the main shock that ruptured only in the upper 10 km of the crust, and an "off-fault" cluster of activity perpendicular to the slip surface of the M_L 5.7 foreshock associated with the hypocenter of the main shock. The Chalfant sequence occurred within the local short-period network, and the spatial and temporal development of the foreshock sequence can be observed in detail. Seismicity of the July 1986 time period is largely confined to two nearly conjugate planes; one striking N30°E and dipping 60° to the northwest associated with the M_L 5.7 foreshock and the other striking N25°W and dipping 70° to the southwest associated with the main shock. Focal mechanisms for the foreshock period fall into two classes in agreement with these two planes. Shallow clustering of earthquakes in July and the M_L 5.7 principal foreshock occur at the intersection of the two planes at a depth of approximately 7 km. The seismic moments determined from inversion of the teleseismic body waves are 4.2 x 10²⁵ and 2.5 x 10²⁵ dyne-cm for the principal foreshock and the main shock, respectively. Slip areas for these two events can be estimated from the aftershock distribution and result in stress drops of 63 bars for the principal foreshock and 16 bars for the main shock. The main shock occurred within an "off-fault" cluster of earthquakes associated with the principal foreshock. This cluster of activity occurs at a predicted local shear stress high in relation to the slip surface of the 20 July earthquake, and this appears to be the triggering mechanism of the main shock. The shallow rupture depth of the principal foreshock indicates that this event was anomalous with respect to the character of main shocks in the region.

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