SHORT SLIP DURATION IN DYNAMIC RUPTURE IN THE PRESENCE OF HETEROGENEOUS FAULT PROPERTIES

Recent studies of strong motion data consistently show that the riseti me (duration of slip at particular locations on the fault) is signific antly shorter than the overall rupture duration. The physical explanat ion for this observation and its implications have become central issu es in earthquake source studies. Two classes of mechanisms have been p roposed to explain short risetimes. One explanation is that velocity-w eakening frictional behavior on the fault surface causes the fault to self-heal. This possibility is suggested by rate-dependent friction ob served in laboratory experiments and by some two-dimensional dynamic n umerical simulations of earthquake rupture. It has recently been demon strated, however, that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self-heal. An alternati ve explanation for short risetimes is that spatially heterogeneous fau lt strength (e.g., barriers) limit the slip duration. In this paper we investigate this second explanation for short risetimes by constructi ng a three-dimensional dynamic rupture model for the 1984 Morgan Hill, California earthquake (M(W)=6.2) using a kinematic model previously o btained from waveform inversion of strong motion data. We assume veloc ity-independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynami c stress drop and strength excess that reproduces the slip and rupture time of the kinematic model. The slip velocity time functions calcula ted from this dynamic model are then used in a subsequent inversion to fit the strong motion data. By alternating between dynamic and kinema tic modeling, we obtain a dynamic model that provides an acceptable fi t to the recorded waveforms. In this dynamic model the risetime is sho rt over most of the fault, which is attributable entirely to the short scale-length slip/stress drop heterogeneity required by the strong mo tion data. A self-healing mechanism, such as strongly velocity-depende nt friction, is not required to explain the short risetimes observed i n this earthquake.