Slip in the 2003 San Simeon earthquake and its effect on the Parkfield segment

The M6.5 San Simeon earthquake, which occurred on 12/22/2003, was a thrust event on the Oceanic fault in the Coast Ranges of central California, ~50 km from the Parkfield segment of the San Andreas fault. It is the largest recorded earthquake in this region. That it occurred just ten months before the highly anticipated, and ~15 year "late", Parkfield earthquake on 9/28/2004, lends itself to speculation that there is a causal relationship. To constrain the fault plane geometry and slip distribution of the San Simeon earthquake, we construct three interferograms from ERS and Envisat data, obtained through WInSAR. Two span both the coseismic and portions of the postseismic period (12/9/2003 - 9/14/2004, 7/9/2003 - 4/14/2004) and one contains purely postseismic deformation (12/31/2003 - 2/4/2004). We also include coseismic and postseismic GPS displacements. All three interferograms and the GPS data are jointly inverted to simultaneously determine both the coseismic and postseismic slip distributions. The distributed slip models are then used to calculate the static stresses imposed on the Parkfield segment. Preliminary results show that the coseismic rupture increased the Coulomb failure stress (CFS) for right-lateral strike-slip on the entire Parkfield segment by more than 0.1 bar, when an effective coefficient of friction (μ^') of 0.4 is used. 0.1 bars is a threshold found by other authors to correlate with aftershock triggering; changes in CFS above this level may be considered significant. The stresses produced by the postseismic slip are much smaller; less than 0.1 bar throughout the Parkfield segment. However the sense of the change in CFS was to encourage right-lateral slip at Parkfield.