Stress drops for foreshocks and aftershocks of the 1979 Petatlán, Mexico, earthquake

Abstract

Stress drops for foreshocks and aftershocks (3.5 ~ ML ~ 4.0) of the 1979 Petatlan, Mexico, earthquake (Ms = 7.6) were examined with the purpose of understanding the mechanical properties of rock and the failure process of a very Young subduction zone. The static stress drop was calculated by estimating the source dimension from the width of the P-wave half-cycle and the seismic moment from a moment-magnitude relation appropriate to the region. Values of stress drop indicate an irregular distribution throughout the aftershock zone and support a two-asperity model in the source area.