DYNAMIC STRESS CHANGES DURING EARTHQUAKE RUPTURE

We assess two competing dynamic interpretations that have been propose d for the short slip durations characteristic of kinematic earthquake models derived by inversion of earthquake waveform and geodetic data. The first interpretation would require a fault constitutive relationsh ip in which rapid dynamic restrengthening of the fault surface occurs after passage of the rupture front, a hypothesized mechanical behavior that has been referred to as ''self-healing.'' The second interpretat ion would require sufficient spatial heterogeneity of stress drop to p ermit rapid equilibration of elastic stresses with the residual dynami c friction level, a condition we refer to as ''geometrical constraint. '' These interpretations imply contrasting predictions for the time de pendence of the fault-plane shear stresses. We compare these predictio ns with dynamic shear stress changes for the 1992 Landers (M 7.3), 199 4 Northridge (M 6.7), and 1995 Kobe (M 6.9) earthquakes. Stress change s are computed from kinematic slip models of these earthquakes, using a finite-difference method. For each event, static stress drop is high ly variable spatially, with high stress-drop patches embedded in a bac kground of low, and largely negative, stress drop. The time histories of stress change show predominantly monotonic stress change after pass age of the rupture front, settling to a residual level, without signif icant evidence for dynamic restrengthening. The stress change at the r upture front is usually gradual rather than abrupt, probably reflectin g the limited resolution inherent in the underlying kinematic inversio ns. On the basis of this analysis, as well as recent similar results o btained independently for the Kobe and Morgan Hill earthquakes, we con clude that, at the present time, the self-healing hypothesis is unnece ssary to explain earthquake kinematics.