Modelling aftershock migration and afterslip of the San Juan Bautista, California, earthquake of October 3, 1972

The San Juan Bautista earthquake of October 3, 1972 (ML = 4.8), located along the San Andreas fault in central California, initiated an aftershock sequence characterized by a subtle, but perceptible, tendency for aftershocks to spread to the northwest and southeast along the fault zone. The apparent dimension of the aftershock zone along strike increased from about 7-10 km within a few days of the earthquake, to about 20 km eight months later. In addition, the mainshock initiated a period of accelerated fault creep, which was observed at 2 creep meters situated astride the trace of the San Andreas fault within about 15 km of the epicenter of the mainshock. The creep rate gradually returned to the preearthquake rate after about 3 yrs. Both the spreading of the aftershocks and the rapid surface creep are interpreted as reflecting a period of rapid creep in the fault zone representing the readjustment of stress and displacement following the failure of a "stuck" patch or asperity during the San Juan Bautista earthquake. Numerical calculations suggest that the behavior of the fault zone is consistent with that of a material characterized by a viscosity of about 3.6??1014 P, although the real rheology is likely to be more complicated. In this model, the mainshock represents the failure of an asperity that slips only during earthquakes. Aftershocks represent the failure of second-order asperities which are dragged along by the creeping fault zone. ?? 1987.