Heterogeneous Distribution of Dynamic Stress Drop and Relative Fault Strength Recovered from the Results of Waveform Inversion: the 1984 Morgan Hill, California, Earthquake

Abstract

The dynamic rupture process of the 1984 Morgan Hill, California, earthquake (M = 6.2) has been investigated on the basis of a three-dimensional (3D) dynamic shear crack model, from previous waveform inversion results. For this purpose, a locking fracture criterion is introduced for rupture propagation, from which a lower bound of the peak shear stress just before rupturing, and hence of the relative fault strength, has been estimated at each fault segment. The distribution of static and dynamic stress drops has also been independently evaluated from that of fault slips by linear and nonlinear inversion procedures, respectively. The results show that large slips located from the hypocenter to about 10 km and 14 to 17 km along the strike at depths between 8 and 12 km result from local dynamic stress drops exceeding 40 and 140 bars, respectively. Negative stress drops down to −15 bars are required to explain very low slips over a shallow fault section. If a non-negative stress-drop condition is imposed, somewhat larger slips are obtained there than those from the waveform inversion. The above results suggest that there could be a zone of velocity-strengthening frictional behavior in the shallow crust, which may have arrested slip motion during rupture propagation. The strength excess is found to be generally small but somewhat larger at a small zone that has delayed rupture propagation. The dynamic rupture initiated from a small nucleus zone with a low stress drop, propagated southeastward, breaking the deeper fault section with high stress drop, and then broke a relatively high strength zone after a short time of arrest, with highest stress drop.