Dynamic Stress Drop Research Articles

Stress drops and radiated energies of aftershocks of the 1994 Northridge, California, earthquake

We study stress levels and radiated energy to infer the rupture characteristics and scaling relationships of aftershocks and other southern California earthquakes. We use empirical Green functions to obtain source time functions for 47 of the larger (M ≥ 4.0) aftershocks of the 1994 Northridge, California earthquake (M6.7). We estimate static and dynamic stress drops from the source time functions and compare them to well‐calibrated estimates of the radiated energy. Our measurements of radiated energy are relatively low compared to the static stress drops, indicating that the static and dynamic stress drops are of similar magnitude. This is confirmed by our direct estimates of the dynamic stress drops. Combining our results for the Northridge aftershocks with data from other southern California earthquakes appears to show an increase in the ratio of radiated energy to moment, with increasing moment. There is no corresponding increase in the static stress drop. This systematic change in earthquake scaling from smaller to larger (M3 to M7) earthquakes suggests differences in rupture properties that may be attributed to differences of dynamic friction or stress levels on the faults.

Slip-weakening distance in dynamic rupture of in-slab normal-faulting earthquakes

We apply the above method to three large in-slab, normal-faulting earthquakes in the Mexican subduction zone. To do this, we calculate the spatial distribution of slip-velocity functions and final slip from kinematic waveform inversion of strong-motion and teleseismic records, and the stress history and final stress change from dynamic rupture calculations. By integrating the slip-velocity functions obtained from the inversion, from the rupture arrival time to the time of peak slip velocity, we obtain slip Dat T pv and then correct it for Dc at T b through dynamic calculations. We also estimate the lowest resolvable limit and probable errors of Dc from the slip-velocity functions, and its upper bound from a theoretical constraint between the dynamic stress drop and Dc. We found that the slip-weakening distance Dc estimated in the frequency band between 0.05 and 0.5 Hz ranges between 40 and 120 cm on the in-slab fault of the 1999 Oaxaca earthquake (Mw = 7.5). The largest Dc is detected in the central fault and in part of the deeper sections, and Dc in the zone around the hypocentre ranges between 50 and 70 cm. The estimated Dc values appear to be less depth-dependent but are rather more dependent on the local maximum slip. This possible slip dependence might be interpreted by the degree of fault roughness, in addition to stress heterogeneities. The fracture energy G in the central section and in the hypocentral zone are roughly estimated to be of the order of 10-15 and 5-8 MJ m −2 , respectively. Both of the estimated Dc and G values are somewhat larger than those on the vertical fault of two recent, shallow strike-slip earthquakes in western Japan.

Permeability evolution during progressive development of deformation bands in porous sandstones

Triaxial deformation experiments were carried out on large (0.1 m) diameter cores of a porous sandstone in order to investigate the evolution of bulk sample permeability as a function of axial strain and effective confining pressure. The log permeability of each sample evolved via three stages: (1) a linear decrease prior to sample failure associated with poroelastic compaction, (2) a transient increase associated with dynamic stress drop, and (3) a systematic quasi‐static decrease associated with progressive formation of new deformation bands with increasing inelastic axial strain. A quantitative model for permeability evolution with increasing inelastic axial strain is used to analyze the permeability data in the postfailure stage. The model explicitly accounts for the observed fault zone geometry, allowing the permeability of individual deformation bands to be estimated from measured bulk parameters. In a test of the model for Clashach sandstone, the parameters vary systematically with confining pressure and define a simple constitutive rule for bulk permeability of the sample as a function of inelastic axial strain and effective confining pressure. The parameters may thus be useful in predicting fault permeability and sealing potential as a function of burial depth and fault displacement.

Influence of confining pressure on the mechanical and structural evolution of laboratory deformation bands

We present experimental observations of the influence of confining pressure on the mechanical behavior and structural style of damage in porous quartz rich sandstones. Large (100‐mm diameter) samples of sandstone are deformed in a triaxial deformation apparatus, resulting in deformation expressed as pale interweaving bands of granulated material with finite shear offset and associated microcracking. The deformation fabrics evolve systematically from localized to more pervasive geometries with increasing confining pressure, associated with a systematic reduction in dynamic stress drop. The initial failure envelope is consistent with Mohr‐Coulomb frictional behavior for all tests. These mechanical and structural observations confirm a gradual transition between brittle and semi‐brittle behavior below the threshold for bulk cataclastic flow in a porous granular medium. Our experiments demonstrate that deformation band formation is strongly pressure‐sensitive. The resulting structures are likely to have a strong anisotropic influence on permeability.

Earthquake stress drop and laboratory‐inferred interseismic strength recovery

We determine the scaling relationships between earthquake stress drop and recurrence interval tr that are implied by laboratory‐measured fault strength. We assume that repeating earthquakes can be simulated by stick‐slip sliding using a spring and slider block model. Simulations with static/kinetic strength, time‐dependent strength, and rate‐ and state‐variable‐dependent strength indicate that the relationship between loading velocity and recurrence interval can be adequately described by the power law VL∝trn where n≈−1. Deviations from n=−1 arise from second order effects on strength, with n>−1 corresponding to apparent time‐dependent strengthening and n<−1 corresponding to weakening. Simulations with rate and state‐variable equations show that dynamic shear stress drop Δτd scales with recurrence as dΔτd/dlntr≤σe(b‐a), where σe is the effective normal stress, μ=τ/σe, and (a‐b)=dμss/dlnV is the steady‐state slip rate dependence of strength. In addition, accounting for seismic energy radiation, we suggest that the static shear stress drop Δτs scales as dΔτs/dlntr≤σe(1 +ζ)(b‐a), where ζ is the fractional overshoot. The variation of Δτs with lntr for earthquake stress drops is somewhat larger than implied by room temperature laboratory values of ζ and b‐a. However, the uncertainty associated with the seismic data is large and the discrepancy between the seismic observations and the rate of strengthening predicted by room temperature experiments is less than an order of magnitude.

Rupture directivity and source-process time of the September 20, 1999 Chi-Chi, Taiwan, earthquake estimated from Rayleigh-wave phase velocity

Rayleigh-wave phase-delay differences between the main shock and its aftershocks, regarded as source duration varying with azimuth, at period range of 60 ∼ 120 seconds, were used to rapidly determine the rupture directivity and the source-process time of the 1999 Chi-Chi, Taiwan, earthquake. Results show that the earthquake faulting exhibits an obvious directivity with an optimal rupture azimuth of about 42° in the northeast direction. The earthquake has an average source-process time at 40.6 ± 0.5 sec and a rupture length of 77.0 ± 3.6 km during rupture. In addition, periods at nodes of amplitude spectra of the main shock were adopted to estimate the rupture time, to be about 34.0 ± 0.7 sec; the rise time of 6.7 ± 0.6 sec, and a slightly slow rupture velocity, 2.27 ± 0.15 km/sec, were also obtained. According to Savage’s suggestion, the rupture width of the fault was estimated to be 30.5 ± 4.5 km. Relying on these estimated faults parameters, we calculate the average static stress drop of about 56 bars, lying in between the stress drop of interplate earthquakes and intraplate earthquakes. We also infer the average dynamic stress drop of 52 bars based on Brune’s theory. By the comparison of the two stress-drops, an Orowan’s stress model or a stress model of frictional overshot is probably appropriate to describe the rupture behavior of the earthquake. The radiated seismic energy, then, was estimated at about 1.9 × 1023 ergs, about 16 times larger than the value reported by the USGS.

Stress drop with constant, scale independent seismic efficiency and overshoot

To model dissipated and radiated energy during earthquake stress drop, I calculate dynamic fault slip using a single degree of freedom spring‐slider block and a laboratory‐based static/kinetic fault strength relation with a dynamic stress drop proportional to effective normal stress. The model is scaled to earthquake size assuming a circular rupture; stiffness varies inversely with rupture radius, and rupture duration is proportional to radius. Calculated seismic efficiency, the ratio of radiated to total energy expended during stress drop, is in good agreement with laboratory and field observations. Predicted overshoot, a measure of how much the static stress drop exceeds the dynamic stress drop, is higher than previously published laboratory and seismic observations and fully elasto‐dynamic calculations. Seismic efficiency and overshoot are constant, independent of normal stress and scale. Calculated variation of apparent stress with seismic moment resembles the observational constraints of McGarr [1999].

Quasi-Static Slip-Rate Shielding by Locked and Creeping Zones as an Explanation for Small Repeating Earthquakes at Parkfield

In a recent study of microearthquakes along the Parkfield segment of the San Andreas fault, Nadeau et al. (1995) have found that much of the seismicity in the region is characterized by quasi-periodic repeating sequences of small earthquakes that are essentially identical in waveform, size and, location. Nadeau and Johnson (1998) interpreted these as repeated slip on a given asperity driven by a steady slip rate of 2.3 cm/yr and concluded that the stress drops needed to be extremely high, of the order of 20 kilobars. We propose another explanation for these small repeating events, namely that an inner asperity is surrounded by a larger creeping zone, which in turn is surrounded by a still larger locked zone. This geometry produces a local slip velocity much less than the overall creep velocity observed on a still larger scale (slip velocity shielding). We have constructed a foam rubber model to illustrate the phenomenon. The time sequences of small events at the asperity, punctuated by large events which rupture the whole block, look very similar to the cumulative moment plots of Nadeau and Johnson. The actual dynamic stress drops are of the same order as for the large events. Thus the results of the model correspond to the observations of Nadeau and Johnson and suggest that the model may be appropriate to explain their observations, without requiring super strong asperities.

Experimental constraints on the mechanical and hydraulic properties of deformation bands in porous sandstones: a review

Abstract Deformation bands form in porous, clay-poor, sandstones in the top few kilometres of the Earth’s surface, involving the sequential growth of a set of discrete fault strands with minimal individual offset, ultimately culminating in the development of a slip surface with a large offset. We review some of our recent experimental results designed to reproduce the early stages of this sequence, obtained at room temperature and low confining pressure ( P < 70 MPa) in a large-capacity (10 cm core diameter) deformation rig. We examine the physical weakening and strengthening mechanisms at work in the experiments, and discuss the implications for fault sealing. We describe laboratory experiments where deformation occurs by the progressive formation of new bands with a finite small offset and a relatively constant fault gouge grain-size distribution, at a relatively constant stress measured at the sample boundaries. The friction coefficient is 0.6, i.e. within the standard range. No large-offset slip surfaces were observed. Cross-fault permeability is transiently increased during dynamic stress drop, associated with the ‘suction pump’ provided by rapid near-fault dilatancy under conditions of constant flow rate. As the deformation band develops quasi-statically, permeability is then reduced further by up to two orders of magnitude as a result of shear-enhanced compaction and porosity loss of the poorly sorted gouge fragments. A simple microstructural model successfully predicts the physical sealing rates in the post-failure stage. Finally, we estimate the chemical sealing rates from mass balance calculations based on direct measurement of the pore fluid chemistry during constant flow experiments at temperatures up to 120°C. When extrapolated to longer timescales, these account quantitatively for the differences between permeability reductions measured in the laboratory and in the field.

The Mw 5.4 Reggio Emilia 1996 earthquake: active compressional tectonics in the Po Plain, Italy

We have analysed the seismic sequence that occurred in October 1996 near the town of Reggio Emilia on the southern edge of the Po Plain. The onset of the sequence was marked by a 5.4 moment magnitude main shock, located at 15 km depth. The main-shock focal mechanism is a reverse solution with a strike-slip component and the scalar moment is 1.46 × 1017 N m. We used broad-band digital recordings from a borehole station, located at about 70 km from the epicentre, for a spectral analysis in order to estimate attenuation and source parameters for the main shock. In addition, the empirical Green's function method has been applied to evaluate the source time function in terms of both moment rate and stress rate. We infer an asperity-like rupture process for the main shock, as suggested by the short duration of the stress release with respect to the overall duration of the moment rate function. This analysis also allows us to estimate the average dynamic stress drop of the main shock (600 bar). We analysed the digital recordings of the temporary local seismic network deployed after the main shock and of a permanent local network maintained by the Italian Petroleum Agency (AGIP). During 15 days of field experiments, we recorded more than 800 aftershocks, which delineate a 9 km long, NE-elongated distribution, confined between 12 and 15 km depth, suggesting that the basement is involved in the deformation processes. 102 focal mechanism of aftershocks have been computed from P-wave polarities, showing mainly pure reverse solutions. We calculate the principal stress axes from a selected population of earthquakes providing a constraint on the stress regime of this part of the Po Plain. The focal mechanisms are consistent with a N–S subhorizontal σ1. All the seismological data we have analysed confirm that this region is undergoing active compressional tectonics, as already inferred from recent earthquakes, geomorphological data and other stress indicators. Moreover, the elongation of the Reggio Emilia aftershock sequence is consistent with the regional direction of the thrust fronts cropping out in the area, suggesting that they are still active.

Spatial slip distribution of the September 20, 1999, Chi‐Chi, Taiwan, Earthquake (MW7.6) —Inverted from teleseismic data

The teleseismic waveforms of the MW7.6 September 20, 1999 Chi‐Chi earthquake were examined to obtain the quick information on the fault rupture process. The deconvolution results show the fault ruptured from south to the north, and revealed the west movement of the hanging wall to the footwall on the eastern dipping plane. The spatial slip distribution shows that the earthquake was mainly composed by a large asperity with a dimension of about 45km × 15km. The maximum slip was about 8 m located at about 45 km to the north of the epicenter. The slip distributions obtained in this study have a good agreement with the observed surface breaks. The comparable static and the dynamic stress drops of about 11 Mpa in the large slip region indicate that the melting/fluid pressurization might have taken place in this earthquake. It reduces the dynamic friction and results in the large slip observed.

Dynamic rupture and stress change in a normal faulting earthquake in the subducting Cocos plate

A large nearly vertical, normal faulting earthquake (Mw = 7.1) took place in 1997 in the Cocos plate, just beneath the ruptured fault zone of the great 1985 Michoacan thrust event (Mw = 8.1). Dynamic rupture and resultant stress change during the 1997 earthquake have been investigated on the basis of near-source strong-motion records together with a 3-D dynamic 9model. Dynamically consistent waveform inversion reveals a highly heterogeneous distribution of stress drop, including patch-like asperities and negative stress-drop zones. Zones of high stress drop are mainly confined to the deeper, southeastern section of the vertical fault, where the maximum dynamic stress drop reaches 280 bars (28 MPa). The dynamically generated source time function varies with location on the fault, and yields a short slip duration, which is caused by a short scalelength of stress-drop heterogeneities. The synthetic seismograms calculated from the dynamic model are generally consistent with the strong-motion velocity records in the frequency range lower than 0.5 Hz. The pattern of stress-drop distribution appears, in some sense, to be consistent with that of coseismic changes in shear stress resulting from the 1985 thrust event. This consistency suggests that the stress transfer from the 1985 event to the subducting plate could be one of the possible mechanisms that increased the chance of the occurrence of the 1997 earthquake.

Numerical simulation of dynamic process of the Tangshan earthquake

In this paper, the new LDDA (Lagrangian Discontinuous Deformation Analysis) method is used in modeling the dynamic process of the M S=7.8 Tangshan earthquake on July 28, 1976 and obtain directly the dynamic and quasistatic dislocations, shear stress drops, fracture velocities of the Tangshan earthquake fault. The simulation shows that the slip history at each point of the fault is different. The displacement vectors at the concave side of the fault is greater than that of the convex side of the fault. The "over shoot" of the fault slip is greatest at the middle part of the fault and attenuates to its ends. The rupture velocities of the fault from the epicenter towards south-west and towards north-east are 3.08 m/s and 1.18 m/s, respectively, the average one is 2.13 m/s. The maximum dynamic and quasi-static dislocations are 7.1 m and 6.2 m respectively, the average quasi-static one on the fault is 4.5 m. The maximum dynamic and quasi-static shear stress drops are 8.1 MPa and 5.4 MPa, respectively, the average quasi-static shear stress drop is 3.9 MPa. We found that the rupture velocities and shear stress are related to the initial stress states of the fault.

Dynamic Stress Drop of Recent Earthquakes: Variations within Subduction Zones

Stress drop is a fundamental parameter of earthquakes, but it is difficult to obtain reliable stress drop estimates for most earthquakes. Static stress drop estimates require knowledge of the seismic moment and fault area. Dynamic stress drop estimates are based entirely upon the observed source time functions. Based on analytical formulas that I derive for the crack and slip-pulse rupture models, the amplitude and time of the initial peak in source time functions can be inverted for dynamic stress drop. For multiple event earthquakes, this method only gives the dynamic stress drop of the first event. The Michigan STF catalog provides a uniform data base for all large earthquakes that have occurred in the past four years. Dynamic stress drops are calculated for the nearly 200 events in this catalog, and the resultant estimates scatter between 0.1 and 100 MPa. There is some coherent tectonic signal within this scatter. In the Sanriku (Japan) and Mexico subduction zones, underthrusting earthquakes that occur at the up-dip and down-dip edges of the seismogenic zone have correspondingly low and high values of stress drop. A speculative picture of the stress state of subduction zones emerges from these results. A previous study found that the absolute value of shear stress linearly increases down the seismogenic interface to a value of about 50 MPa at the down-dip edge. In this study, the dynamic stress drop of earthquakes at the up-dip edge is about 0.2 MPa, while large earthquakes at the down-dip edge of the seismogenic plate interface have dynamic stress drops of up to 5 MPa. These results imply that: (1) large earthquakes only reduce the shear stress on the plate interface by a small fraction of th absolute level; and thus (2) most of the earthquake energy is partitioned into friction at the plate interface.

Laboratory observations of fault‐normal vibrations during stick slip

We report laboratory observations of interface separation waves during stick slip on a fault in a uniform polymer material. Our observations, made at stress levels expected at midcrustal depths, share many macroscopic properties with ruptures of faults in rocks. We observed a drop in fault‐normal stress shortly before the onset of, and during, stick slip at points along the fault during a rupture. We suggest that P wave energy in front of the propagating rupture tip is responsible for the drop in normal stress. We also interpret that stick slip took place within a traveling slip pulse, and we suggest that the dynamic stress drop within the slipping patch exceeded the overall static stress drop by a factor of at least 5 within a few millimeters of the fault. Our experiments did not resolve whether the fault surfaces actually separate or if fault‐normal stress is just greatly reduced. In either case the net result is that fault slip is permitted to take place with much less factional resistance than that expected from the applied load. Our observations provide laboratory evidence that fault‐normal vibrations may play an important role in sustaining a rupture by facilitating the propagation of a transient instability. Faults may appear weak in part because they are dynamically weakened as they slip during rupture while retaining their strength during the interseismic period.

Dynamic rupture of asperities and stress change during a sequence of large interplate earthquakes in the Mexican subduction zone

Abstract We investigate the spatial and temporal variations of shear stress due to the successive failures over an extensive segment of the Mexican subduction zone during a sequence of large interplate earthquakes that occurred over a period of 13 yr. For this purpose, we develop 3D dynamic rupture models incorporating a shallowly dipping fault located above the subducting plate. The spatial distribution of dynamic stress drop over the fault has been estimated for each of the events, through an inversion procedure using some of the previously derived kinematic fault parameters as observational constraints. The results revealed quite heterogeneous stress changes during these earthquakes coming from medium to high dynamic stress drop due to the rupture of a few patch-like asperities and from stress increase in between and around them. Two weak asperities located southeast of the Michoacan segment were ruptured first by the 1979 Petatlan event. The 1981 Playa Azul event ruptured two asperities in the central zone with a stress drop higher than 80 bars. The largest 1985 Michoacan earthquake resulted from the rupture of two large-size, strong asperities located at both sides of the 1981 fault zone with high stress drop of 80 to 100 bars and from another two asperities at depth. Two days after this largest event, two asperities were broken during the Zihuatanejo aftershock in the southeastern adjacent zone. Many aftershocks of these large events tend to be distributed in the zones of stress increase outside the asperities, while only small numbers of aftershocks have been observed within these asperity zones. It appears that several major asperities that existed in this extensive segment have been ruptured successively so as to fill unbroken gaps on the plate interface. Thus, the stress change left over from the previous earthquake has dominant effects on the next event in this subduction zone.

Shear zone stress release heterogeneity associated with two mining-induced events of M 1.7 and 2.2

We analyse space–time variations in stress release estimates for microseismic events ( M<0) and two larger seismic events of M 1.7 and 2.2 associated with a schist shear zone at Lockerby mine, Sudbury (Ontario). These events occurred following sequential sill pillar extractions between October 1992 and January 1994 and were recorded with a local underground array. Also, we calculate shear and normal stress variations along the schist using 3-D numerical models of the excavation. The distribution of post-event microseismicity suggests that the two larger events were related to the failure of local adjacent asperities. They, however, exhibit different source mechanisms as a result of local variations in the orientation of both the schist and principal stresses. Similar heterogeneous patterns along the schist zone are found in static stress drop, apparent stress, and dynamic stress drop distributions. Numerical modelling results show that the stress release patterns are not only influenced by the shear stress, but are also strongly dependent on the resisting stress level. We conclude that high stress release estimates are not necessarily indicative of higher slip potential, and that differences between the conditions at failure of the two larger events are likely due to variations in the normal relative to shear stress levels.

The state of stress on some faults of the San Andreas System as inferred from near‐field strong motion data

We present a simple method to calculate the stress produced on an earthquake fault during rupture. This method allows the complete evaluation of the stress spatio‐temporal history over the fault. We apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the San Andreas fault system over the last 20 years: the Imperial Valley earthquake of 1979, the Morgan Hill earthquake of 1984, the Loma Prieta earthquake of 1989, and the Northridge earthquake of 1994. We use for this study the tomographic models of the fault rupture obtained from the inversion of the near‐field seismic data recorded during these earthquakes. The results obtained show that the static and the dynamic stress drops vary greatly over the fault. The peak values obtained for the four earthquakes studied range from about 20 to 100 MPa. These high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault. The regions of the fault which break with a high stress drop are also the regions where slip is large. This suggests that most of the slip produced in a large earthquake takes place over the “strong” areas of the fault. Low slip regions tend to break with low stress drops. After the earthquake, the shear stress is increased over a significant portion of the fault, which corresponds to low slip regions. Aftershock activity tends to be concentrated in these areas of stress increase. The apparent strength of the fault before the earthquake (that is the local shear stress increase which is required for rupture) is also extremely heterogeneous. The rupture velocity seems to be inversely correlated with this apparent fault strength, the rupture accelerating over the “weak” areas of the fault and slowing down over the high strength areas.

Complexity of seismicity due to highly rate‐dependent friction

We study a simple antiplane fault of finite length embedded in a homogeneous, isotropic, elastic solid in order to understand the origin of seismic source heterogeneity in the presence of nonlinear rate‐dependent friction. All the mechanical properties of the medium and friction are assumed to be homogeneous. Starting from a heterogeneous initial stress distribution, we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for more than 20,000 events, in some cases. The style of seismicity produced by this model is determined by a control parameter which measures the degree of rate dependence of friction. For classical friction models with rate‐independent friction, no complexity appears and seismicity is perfectly periodic. For weakly rate‐dependent friction, seismicity becomes slightly nonperiodic but most events are still characteristic earthquakes. When friction is highly rate‐dependent, seismicity is completely irregular and ruptures of all sizes occur inside the fault. Highly rate‐dependent friction destabilizes the healing process, producing premature healing of slip and partial stress drop. Premature healing causes rupture to take the form of narrow, propagating slip episodes, the so‐called Heaton's [1990] pulses. Partial stress drop produces large variations in the state of stress which, in turn, produce earthquakes of different sizes. We make the conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity.

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

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.