Regions of Reduced Static Stress Drop near Fault Tips for Large Strike-Slip Earthquakes

We present a case study of slip distributions for the 1992 Landers (M-w 7.3) and the 1999 Hector Mine (M-w 7.1) earthquakes in California's Mojave Desert. Slip distributions, as determined from geophysical inversion of geodetic, strong ground motion, and teleseismic data, are complex, heterogeneous, and often exhibit linearly tapering or concave-upward patterns toward segment tips, distinctly different from the elliptical slip distributions characteristic of uniform stress drop. Mechanical interaction of discontinuous fault segments fully explains the reduced slip near the southern termination of the Camp Rock/Emerson fault segment of the Landers rupture; however, numerical models demonstrate that such interactions are insufficient to explain slip distributions observed at other segment terminations. Numerical models demonstrate that long (5-25 km) zones of reduced stress drop in the vicinity of some rupture segment terminations can explain the slip distributions for these large earthquakes. Zones of reduced stress drop are implemented as regions of increased Coulomb strength. Slip distributions are improved 30%-70% relative to models with uniform stress drop. Regions of reduced stress drop appear to play a relatively greater role near segment tips at which rupture terminates than near segment tips at which rupture jumps to a nearby fault segment. Similar results are obtained implementing discrete stepwise and spatially linear reductions of stress drop. Plausible mechanical explanations for such zones of reduced stress drop include heterogeneous fault strength or friction, spatial or temporal changes in pore pressure, geometric complexity of the fault surface, heterogeneity of normal tractions resolved on the fault surface, inelastic deformation, and dynamic rupture effects.