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ON the San Andreas fault near Parkfield, California, rates of seismicity1,2 and geodetic line shortening3,4 have been lower since 1986 than before. Wyss et al.1,3 interpret the rate decreases as precursors to an imminent moderate earthquake (magnitude M = 5.5-6) and estimate the earthquake time to be March 1991 ?? 1 yr. The earthquake was previously predicted to occur in 1988??5 yr based on five of six similar earthquakes which occurred at intervals ???22 yr, the last one in 19665. Here I present a model that attributes the rate changes to the initiation of slip on a subhorizontal detachment fault under the pending rupture area. In this model, the slip starts late in the seismic cycle, and diminishes the loading rate on the nearby San Andreas, thus acting as a buffer.
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Seismic quiescence at Parkfield due to detachment faulting