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Aftershock occurrence due to viscoelastic stress recovery and an estimate of the tectonic stress field near the San Andreas fault system

EARTHQUAKE RESEARCH INSTITUTE UNIVERSITY OF TOKYO, BUNKYO-KU, TOKYO Japan

Abstract

The possibility of aftershock occurrence due to viscoelastic stress recovery after a main shock is discussed theoretically. The tectonic stress field along the San Andreas fault system is estimated using theoretical results obtained in this study, together with other theoretical and observational results.

The stress gradually rises near the central part of the fault surface if a noticeable primary creep occurs near the fault surface after the main shock and several other tectonic conditions are satisfied. The occurrence of the primary creep is probable since the tectonic stress field near the fault plane of the main shock is strongly disturbed. The static frictional stress drops considerably near the fault plane at the time of occurrence of the main shock so that aftershocks can be caused by the above stress rise.

In seismically active areas along the San Andreas fault system, the effective stress (defined as the static frictional stress minus the sliding frictional stress) and the fracture strength (defined as the static frictional stress minus the initial stress) will be very small near the Earth's surface.