Imaging the source region of the 2003 San Simeon earthquake within the weak Franciscan subduction complex, central California

Data collected from the 2003 Mw6.5 San Simeon earthquake sequence in central California and a 1986 seismic refraction experiment demonstrate that the weak Franciscan subduction complex suffered brittle failure in a region without significant velocity contrast across a slip plane. Relocated hypocenters suggest a spatial relationship between the seismicity and the Oceanic fault, although blind faulting on a nearby, unknown fault is an equally plausible alternative. The aftershock volume is sandwiched between the Nacimiento and Oceanic faults and is characterized by rocks of low compressional velocity (Vp) abutted to the east and west by rocks of higher Vp. This volume of inferred Franciscan rocks is embedded within the larger Santa Lucia anticline. Pore fluids, whose presence is implied by elevated Vp/Vs values, may locally decrease normal stress and limit the aftershock depth distribution between 3 to 10 km within the hanging wall. The paucity of aftershocks along the mainshock rupture surface may reflect either the absence of a damage zone or an almost complete stress drop within the low Vp or weak rock matrix surrounding the mainshock rupture.