Abstract

The ML ≃ 6.5 earthquake that occurred near Coalinga, California, on 2 May 1983 induced changes in near-surface fault slip along the San Andreas fault. Coseismic steps were observed by creepmeters along a 200-km section of the San Andreas. Some of the larger aftershocks induced additional steps, both right-lateral and left-lateral, and in general the sequence disrupted observed creep at several sites from preseismic long-term patterns.

During the 6 months preceding May 1983, five of the six creepmeters on the San Andreas fault nearest to Coalinga measured creep rates that were faster than the long-term averages. Seasonal rapid creep has been observed in previous winters, but never as pronounced as during early 1983. A single station near Parkfield, California, recorded an abrupt increase in creep rate beginning 16 hr before the main shock.

Static dislocation models can approximately explain the magnitudes and distribution of the larger coseismic steps on 2 May. The smaller, more distant steps appear to be the abrupt release of accumulated slip, triggered by the coseismic strain changes, but independent of the strain change amplitudes.

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