Short slip duration in dynamic rupture in the presence of heterogeneous fault properties

Abstract

Recent studies of strong motion data consistently show that the risetime (duration of slip at particular locations on the fault) is significantly shorter than the overall rupture duration. The physical explanation for this observation and its implications have become central issues in earthquake source studies. Two classes of mechanisms have been proposed to explain short risetimes. One explanation is that velocity-weakening frictional behavior on the fault surface causes the fault to self-heal. This possibility is suggested by rate-dependent friction observed in laboratory experiments and by some two-dimensional dynamic numerical simulations of earthquake rupture. It has recently been demonstrated, however, that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self-heal. An alternative explanation for short risetimes is that spatially heterogeneous fault strength (e.g., barriers) limit the slip duration. In this paper we investigate this second explanation for short risetimes by constructing a three-dimensional dynamic rupture model for the 1984 Morgan Hill, California earthquake (Mw=6.2) using a kinematic model previously obtained from waveform inversion of strong motion data. We assume velocity-independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynamic stress drop and strength excess that reproduces the slip and rupture time of the kinematic model. The slip velocity time functions calculated from this dynamic model are then used in a subsequent inversion to fit the strong motion data. By alternating between dynamic and kinematic modeling, we obtain a dynamic model that provides an acceptable fit to the recorded waveforms. In this dynamic model the risetime is short over most of the fault, which is attributable entirely to the short scale-length slip/stress drop heterogeneity required by the strong motion data. A self-healing mechanism, such as strongly velocity-dependent friction, is not required to explain the short risetimes observed in this earthquake. also been suggested between the seismic moment and the cube of the pulse width for large shallow earthquakes [Furumoto and Nakanishi, 1983] as well as for local deep events [Kikuchi andIshida, 1993]. These results suggest that the duration of slip scales with the fault dimensions; however, the risetime, in the sense that we use it in this paper, is not measured directly in these studies. Near-source observations [A/dCohee and Beroza, 1994;Wald and Heaton, 1994;Haddon, 1995] show that the risetime is significantly shorter than that expected from the overall rupture dimensions. This is somewhat surprising because theoretical models of smooth rupture propagation on equidimensional faults suggest that the risetime should be comparable to the rupture duration [e.g., Madariaga, 1976]. Although this discrepancy had been noted earlier [Bouchon, 1978;Beroza and Spudich, 1988], Heaton [1990 ] drew widespread attention to it by compiling evidence f...

Earthquake Recurrence in Simulated Fault Systems

“…Factors favoring crack-like behavior in the simulations are relatively smooth initial stresses and weak healing (re-strengthening of the fault) following termination of slip, while slip-pulse behavior arises with heterogeneous initial stresses and strong fault healing following rupture termination. This behavior is consistent with fully dynamical rupture simulations (BEROZA and MIKUMO, 1996;ZHENG and RICE, 1998). In our simulations, healing is set by the rate-state frictional properties and by a dynamic stress overshoot parameter that determines the shear stress at the termination of slip relative to the sliding friction.…”

Microscopic and macroscopic physics of earthquakes

“…For example, the stress may increase in the beginning of the slip motion (curve (1) in Figure 4a) because of loading caused by advancing rupture, or of a specific friction law such as the state-rate dependent friction law [Dieterich, 1979a[Dieterich, , 1979b. In fact, seismological inversion studies have shown this increase [Quin, 1990;Miyatake, 1992;Mikumo and Miyatake, 1993;Beroza and Mikumo, 1996;Ide, 1997;Bouchon, 1997]. However, this increase is of short duration and the amount of slip during this stage is small so that little energy is radiated.…”

Scaling Relationships of Source Parameters for Slow Slip Events

“…Two classical models have been proposed to explain the short rise time: the self-healing pulselike rupture behavior (e.g., Heaton, 1990;Beeler and Tullis, 1996) and rupture propagation along a heterogeneous fault (e.g., Boatwright, 1988;Beroza and Mikumo, 1996;Day et al, 1998). The first model assumes that the fault could heal itself shortly after the passage of the rupture front, which allows for the rise time to remain short.…”