Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1–8 Hz in daily spectrograms from the continuous seismological records in Guerrero, Mexico. The analyzed data cover a period of 2001–2007 when in 2001–2002 a large slow slip event (SSE) had occurred in the Guerrero‐Oaxaca region, and then a new large SSE occurred in 2006. The tremor burst is dominated by S‐waves. More than 100 strong NVT bursts were recorded in the narrow band of ∼40 × 150 km2 to the south of Iguala City and parallel to the coastline. Depths of NVT hypocenters are mostly scattered in the continental crust between 5 and 40 km depth. Tremor activity is higher during the 2001–2002 and 2006 SSE compared with that for the “quiet” period of 2003–2005. While resistivity pattern in Guerrero does not correlate directly with the NVT distribution, gravity and magnetic anomaly modeling favors a hypothesis that the NVT is apparently related to the dehydration and serpentinization processes.

We estimate the critical slip-weakening distance on earthquake faults by using a new approach, which is independent of the estimate of fracture energy or radiated seismic energy. The approach is to find a physically based relation between the breakdown time of shear stress T b , the time of peak slip-velocity T pv , and the slip-weakening distance D c , from the time histories of shear stress, slip, and slip velocity at each point on the fault, which can be obtained from dynamic rupture calculations using a simple slip-weakening friction law. Numerical calculations are carried out for a dynamic shear crack propagating either spontaneously or at a fixed rupture velocity on a vertical fault located in a 3D half-space and a more realistic horizontally layered structure, with finite-difference schemes. The results show that T pv is well correlated with T b for faults even with a heterogeneous stress-drop distribution, except at locations near strong barriers and the fault edges. We also investigate this relation for different types of slip-weakening behavior. We have applied the method to two recent, strike-slip earthquakes in western Japan, the 2000 Tottori and the 1995 Kobe events. We integrated the slip-velocity functions on the vertical fault obtained from kinematic waveform inversion of strongmotion and teleseismic records from the arrival time of rupture T r to the time of the peak-slip velocity T pv , and we then corrected the slip obtained at T pv for the errors expected from the dynamic calculations. It was found that the slip-weakening distance D c estimated in the frequency window between 0.05 and 0.5 Hz ranges between 40 and 90 cm on the two earthquake faults. However, if we consider the limited frequency resolution of the observed waveforms, probable time errors in the slipvelocity functions obtained from kinematic inversion, and the uncertainty of the slipweakening behavior, the above estimates may be those located between the minimum resolvable limit and the upper bound of their real values. The estimated D c values do not necessarily seem to indicate larger values in the shallower part and smaller values in the deeper part of the fault, but rather a spatially heterogeneous distribution that appears to be dependent on the local maximum slip. This possible dependence might be interpreted by the frictional properties of the fault such as the degree of roughness or the thickness of gouge layers, in addition to stress heterogeneities.

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...

Dynamical rupture process on the fault is investigated in a quasithreedimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress. The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress-fracture criterion, using the finite difference method.If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it. The rise time of the source function and the final displacements are larger around the centre of the fault. In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena. In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage. These regions could be the source of aftershocks at a next stage. The stick-slip faulting and irregular rupture propagation radiate highfrequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults.

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