Fluid-mediated influence of adjacent thrusting on the seismic cycle at Parkfield

THE San Andreas fault near Parkfield, California has experienced moderate to large earthquakes about every 20 years since 1881, the most recent of which occurred in 1966(1-3). Statistical recurrence and precursory quiescence models(1,4,5) led to a prediction of the next event for 1992 or before, but this event has not yet occurred, Here I present a model(6) that attributes the 'non-occurrence' of the expected earthquake to the influence of adjacent thrusting on the rate of fault weakening from increasing pore pressures within the fault zone, Reductions in fault-normal strain rates(7-9), and the onset of seismic quiescence along the San Andreas fault, both coincide with adjacent blind thrust faulting in the 1982-85 earthquake sequence of New Idria, Coalinga and Kettleman Hills, These events 'turned off' a compaction-induced weakening mechanism and extended the (proposed) seismic cycle at Parkfield, A model simulation of this effect shows a strong correlation in space and time of calculated and observed earthquake hypocentres, and shows that observed seismicity patterns from 1969-95 can be explained by reduced compaction rates brought on by adjacent thrusting, The model may have applications to other linked thrust/strike-slip fault systems, such as are found in southern California and other transpressional regions.