Coseismic and postseismic slip from the 2003 San Simeon earthquake and their effects on backthrust slip and the 2004 Parkfield earthquake

Abstract

The 2003 M6.5 San Simeon earthquake occurred less than a year before and 50 km west of the long‐awaited Parkfield earthquake of 2004. Aftershocks of the San Simeon event occurred not only on the presumptive rupture plane but also delineated subconjugate structures to the east (backthrusts). InSAR data show a complex pattern of deformation during the postseismic time interval and suggest that shallow postseismic slip may have occurred on the backthrust structures. In this paper we present a model inversion for slip during and after the earthquake from four interferograms and GPS data. We find that the data are well‐fit when coseismic slip is allowed on two planes, the main rupture plane and a backthrust structure to the south. The backthrust structure accommodates slip amounting to about one third of the coseismic moment release. Shallow postseismic slip also occurred on the main rupture plane, updip from the coseismic slip, and on the southern backthrust. Postseismic slip on the more northern backthrust structure is not well‐constrained and the data is fit nearly as well without including it. The areas of postseismic slip on the backthrusts correspond well with areas that experienced unclamping and little slip is found in regions of clamping. No value for the coefficient of friction below 1.0 results in a Coulomb Failure Stress field that matches the aseismic slip distributions better than the normal stress alone. We also investigate whether static stress changes from the San Simeon earthquake acted to encourage right‐lateral strike slip on the Parkfield segment of the San Andreas fault. We find that the San Simeon earthquake's rupture planes were oriented in such a way that the maximum increase in Coulomb failure stress on the San Andreas fault occurred on the Parkfield segment. The 2004 Parkfield earthquake differed from its expected behavior by nucleating in the southern portion of the Parkfield segment; an area that experienced increased shear stress of 0.1–0.14 bars from coseismic slip in the San Simeon earthquake. Postseismic slip from San Simeon further loaded the southern portion of the Parkfield segment preferentially over the northern portion, though the size of the stress changes was small (0.03–0.04 bars).