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Faulting in the 1986 Chalfant, California, Sequence: Local Tectonics and Earthquake Source Parameters

Seismological Laboratory
Mackay School of Mines
University of Nevada
Reno, Nevada
(K. D. S.)

Bullard Laboratories
Department of Earth Sciences
University of Cambridge, United Kingdom
(K. F. P.)

The July 1986, moment magnitude (M_w) 6.3 Chalfant, California, earthquake is the largest of a recent series (1978–present) of moderate-sized earthquakes near the Long Valley volcanic region of east-central California. The sequence consists primarily of three moderate-sized strike-slip events. High-quality aftershock relocations and short-period focal mechanisms define the temporal and spatial development of the foreshock-mainshock-aftershock periods of these three events. Faulting during the M_w 5.7 (event I; July 20) and the M_w 6.3 (event II; 21 July) events constitute a set of conjugate strike-slip faults. Event I involved predominantly left-lateral motion on a NE-striking fault plane initiating at shallow depth (7 km). Event II initiated at 10.5 km depth exhibiting right-lateral strike-slip motion on a NW-striking fault dipping moderately to the southwest. An M_L 5.5 strike-slip event on 31 July (event III) extended the aftershock sequence to the south into the White Mountains fault zone. P-wave pulse-width stress drops are determined for 185 M_L 2.7–4.0 earthquakes that sample the entire sequence. Higher stress drops are observed near the intersection of event I and II fault planes and at the northern and southern ends of the aftershock zone. The moving-window b-value of the temporal magnitude distribution shows a general inverse relationship to stress drop with observed changes in both the average stress drop level and the b-value preceding event III. The average aftershock stress drop tends to increase as the sequence progresses suggesting that the faulted volume is equilibrating to the regional stress. Source parameters have been determined for the principal earthquakes from teleseismic body waves, local strong-motion records, and the extent of aftershock activity. The Chalfant sequence appears to be transferring strike-slip motion away from the White Mountains front, contributing to the observed increase in the relative normal offset, from south to north, along the White Mountains and the opening of the White Mountains relative to the Sierra Nevada Range front north of Owens Valley.

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