Dynamic modeling of the 2004 Mw 6.0 Parkfield, California, earthquake

We present two spontaneous rupture models of the 2004 M-w 6.0 Parkfield earthquake constrained by near-source ground motions. We start with a stress drop distribution calculated from a kinematic slip distribution. Using a linear slip-weakening friction law, we utilize trial and error to obtain both the stress conditions and frictional parameters on the fault that produce synthetics consistent with records. The material contrast across the San Andreas Fault is incorporated using different one-dimensional velocity structures on each side of the fault. An approximately constant S parameter of 0.3 and a uniform slip-weakening distance of 0.15 m are used in the dynamic models. In our preferred dynamic model, consistent with the ground motion and GPS, the slip is bounded by seismicity streaks at 5 and 10 km depths, confirming a locked zone at depth. The stress drop is approximately 10 MPa in the hypocentral region and about 2 MPa elsewhere. The material contrast across the fault causes significant normal stress variations (similar to 1 MPa), leading to a larger strength drop to the southeast than to the northwest. The main rupture front propagates at nearly a constant subshear rupture velocity similar to 3 km/s in both directions. The total radiated energy determined from the preferred dynamic model is 1.1 x 10(13) J, seismic moment is 1.0 x 10(18) Nm, and fracture energy is 3.0 x 10(13) J. The limited number of aftershocks in the slipped area suggests the important role of stress on the distribution of seismicity in the locked zone.