Inversion of GPS data for spatially variable slip-rate on the San Andreas Fault near Parkfield, CA

We analyze GPS data collected from 1991-1998 at 35 sites near the Parkfield segment of the San Andreas Fault. Inverting the resultant site velocities for the distribution of interseismic slip-rate on the San Andreas reveals an area of low slip-rate on the fault extending from between Middle Mountain and Carr Hill to southeast of Gold Hill. This slip-rate pattern is similar to that found by Harris and Segall [1987] using trilateration data collected between 1966 and 1984. We infer a deep slip-rate (33 mm/yr) and depth of the transition between seismogenic and non-seismogenic slip (14 km) that agree better with independent geologic evidence than those found in the 1987 study. In contrast to Harris and Segall [1987], we find no evidence of fault-normal contraction.