Faulting in the 1986 Chalfant, California, sequence: Local tectonics and earthquake source parameters

The July 1986, moment magnitude (M-w) 6.3 Chalfant, California, earthquake is the largest of a recent series (1978-present) of moderate-sized earthqua kes near the Long Valley volcanic region of east-central California. The se quence 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 per iods 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 st rike-slip faults. Event I involved predominantly left-lateral motion on a N E-striking fault plane initiating at shallow depth (7 km). Event II initiat ed at 10.5 km depth exhibiting right-lateral strike-slip motion on a NW-str iking fault dipping moderately to the southwest. An M-L 5.5 strike-slip eve nt on 31 July (event III) extended the aftershock sequence to the south int o the White Mountains fault zone. P-wave pulse-width stress drops are deter mined for 185 M-L 2.7-4.0 earthquakes that sample the entire sequence. High er 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 mo ving-window b-value of the temporal magnitude distribution shows a general inverse relationship to stress drop with observed changes in both the avera ge stress drop level and the b-value preceding event III. The average after shock stress drop tends to increase as the sequence progresses suggesting t hat the faulted volume is equilibrating to the regional stress. Source para meters have been determined for the principal earthquakes from teleseismic body waves, local strong-motion records, and the extent of aftershock activ ity. The Chalfant sequence appears to be transferring strike-slip motion aw ay 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.