Slip Behavior at Depth on the Pajaro Segment of the San Andreas Fault, California

Authors: Wilber, M. M., Bürgmann, R.

We use geodetic measurements in the Pajaro region to invert for a model of the variation of slip in space and time since the Loma Prieta earthquake. The Pajaro geodetic network near San Juan Bautista lies at the transition from the creeping section of the San Andreas fault (SAF) to the south and the locked portion to the north. In 1989 the Loma Prieta earthquake (M = 7) occurred to the northwest of this region. Slow earthquakes have occurred in 1992, 1996, and 1998 south of the Loma Prieta rupture in the Pajaro region. We group geodetic data into two time periods following this event, 1989 - 1992 and 1992 - 1998. These geodetic data include USGS Electronic Distance Measurements (EDM) of the Pajaro network from 1989 - 1996, as well as GPS measurements taken of Pajaro network benchmarks and other southern San Francisco Bay area sites during the 1989 - 1998 time period. Using an elastic half-space model we invert for slip at depth on a 25-km-long Pajaro segment of the SAF. We parameterize this segment of the fault as a rectangular grid of 24 patches, and include simpler geometries for other major faults in the SAF system. Seismic evidence for locking depth of faults is used to help set fault geometry. Geologic slip rate estimates and surface creep measurements are used to constrain the slip model. We then compare the results to the distribution of microseismicity and borehole strain measurements. Our modelling suggests a pattern of high slip at shallow depth migrating southward under the Pajaro network from the first time period to the second after the Loma Prieta event. This seems to correspond well with an apparent migration of slow earthquakes and other seismicity generally southward during this time and would be consistent with a propagation of strain away from the Loma Prieta rupture.