SAN ANDREAS FAULT SLIP NEAR PARKFIELD: INSIGHTS FROM THE SPATIO-TEMPORAL CHARACTERISTICS OF AFTERSLIP FOLLOWING THE 2004 M6 EARTHQUAKE

A strong postseismic deformation signal was recorded by creepmeters and continuous GPS stations starting in the minutes to hours following the 2004 M6 Parkfield earthquake. Several studies have shown that the observed displacements were likely due to afterslip, and the estimated moment release of this postseismic slip was anomalously large, exceeding the coseismic moment.

We have used a time-dependent inversion of Global Positioning System (GPS) data recorded at 14 continuously-recording and 13 survey-mode GPS receivers during the first 60 days of the postseismic period to infer the spatio-temporal progression of afterslip on a fault surface defined by the spatial distribution of relocated aftershocks. Preliminary results showed that postseismic slip began at shallow depths but spread deeper and surrounded the region of peak coseismic slip estimated from geodetic data. The slip rate diminished rapidly over the first two weeks.

This talk will present recent results from modeling using a model fault geometry that better reflects the resolving power of the data and including additional observations from immediately after the earthquake. These results will be interpreted in the context of the coseismic slip distribution and of interseismic slip at Parkfield where the San Andreas fault transitions from creeping to locked behavior and transient deformation has been observed. We will also explore the relationship between the spatial and temporal evolution of afterslip and aftershocks.