ANALYSIS OF AFTERSHOCKS AND MODELING OF GPS DISPLACEMENTS FROM THE DECEMBER 22, 2003 SAN SIMEON AND SEPTEMBER 28, 2004 PARKFIELD EARTHQUAKES

On December 22, 2003, a magnitude 6.5 earthquake occurred near San Simeon, California. This earthquake killed two people, injured 40 people, and more than 40 buildings collapsed or were severely damaged in the Paso Robles area. The event was generated on a northwest trending, northeast dipping, blind thrust fault very similar to the 1994 Northridge earthquake. Immediately following the San Simeon earthquake, researchers from the Institute for Crustal Studies (ICS) and the SCEC portable broadband instrument center (PBIC) mobilized instrumentation to record aftershock activity associated with this event. Within the first 24 hours three stations were deployed in the Paso Robles area, one station in Cambria, and one station in Atascadero. In cooperation with ICS, we installed on December 23 two seismographs north of Paso Robles and two more seismographs near San Luis Obispo compliment the permanent station at Cal Poly San Luis Obispo. These instruments then recorded the September 28, 2004 (M=6.0) Parkfield event. In addition, a network of global positioning system (GPS) instruments had been installed in the Parkfield region to measure the earthquake deformation process and recorded both events. The analysis includes the locations of aftershocks and focal mechanisms for the seismicity data, and the modeling the observed GPS displacements using the program DISLOC in order to determine the faulting parameters, independently. The DISLOC program calculates displacements from the analytical solution for a crack in a half-space. The modeling results for the Parkfield fault parameters include: strike=137˚, dip=83˚N, width=15 km, length=30km, and with a maximum displacement of 40cm right-lateral strike-slip.