Complex faulting in the 2003 Mw6.5 San Simeon, California, earthquake sequence: observations from seismicity, seismic velocity structure, and InSAR imagery

We present new observations and interpretations of the highly complex aftershock sequence and postseismic deformation following the 2003 Mw6.5 San Simeon thrust-faulting earthquake. The mainshock and aftershocks were relocated using a 3D seismic velocity model obtained by inverting combined arrival time data from the CISN and PG&E seismic networks and refined using double-difference. The new mainshock depth is 9.7 km; most of the aftershocks are above 11 km. The major features of the new 3D velocity model are a low Vp anomaly directly above the mainshock high-slip zone, perhaps due to weaker Franciscan rocks, and a high Vp/Vs anomaly ~2 km above the mainshock hypocenter suggesting high-pressure fluids. The diffuse and complex nature of the aftershocks makes it difficult to visualize the 3D seismicity structure when viewing the earthquakes as individual points. Instead we represent the aftershock patterns as 3D contour surfaces of constant earthquake density. This reveals the full ~30 km length of the NW-trending NE-dipping mainshock fault plane, of which only ~10 km was previously observed. The complex fault structures also include a ~10- km-long SW-dipping back-thrust fault parallel to the mainshock thrust fault in the hypocentral zone, several smaller SW-dipping structures in the SE, and perhaps additional NE-dipping or subvertical structures SE of the mainshock plane. The earthquake focal mechanisms are predominately oblique thrust, with a notable change from deeper thrust to shallower strike-slip events. A month-long InSAR interferogram (31 Dec. 2003 4 Feb. 2004) shows postseismic uplift in the hanging wall block between the mainshock and back-thrust faults as distinct interference fringes remarkably consistent with the aftershock patterns. The mainshock fault defined by the seismicity projects up-dip to the SW edge of the InSAR fringes, while the back-thrust and the smaller SW-dipping structures project up-dip to the NE edge of the InSAR fringes. The western limit of the InSAR fringes, and the up-dip projection of the seismicity, falls west of the mapped trace of the Oceanic Fault in the NW and central parts of the aftershock zone, consistent instead with the break in topography of the range front of the Santa Lucia Range. In the SE aftershock zone, the InSAR fringes and projected mainshock plane are consistent with the mapped Oceanic Fault trace as it traverses into the range. The area of postseismic uplift between the mainshock and back-thrust faults generally coincides with the highest topography, implying that earthquakes similar to San Simeon have occurred repeatedly to generate the modern topography.