The Bimaterial Effect on the Earthquake Cycle

Natural faults separate rocks with contrasting (and heterogeneous) material properties, and it is still unclear how these heterogeneities affect earthquake nucleation and rupture. Both laboratory studies and dynamic rupture simulations on a bimaterial fault interface show that material contrast can cause rupture to take a "preferred" direction, defined by propagation in the direction of particle motion of the side of the fault with slower shear wave velocity. Predicted asymmetries are model dependent, but have included partially or strictly unilateral rupture, asymmetric off-fault damage, and asymmetric aftershock distributions. The possibility that rupture direction may be to some extent predictable is of interest to seismic hazard analysis because ground motion intensity and the occurrence of triggered seismicity are both enhanced in the forward direction of rupture.

Much of the understanding of the bimaterial effect is based on single-event dynamic rupture simulations triggered by artificially perturbing some assumed initial stress state. In order to include nucleation effects in bimaterial rupture simulations, we have developed a computational method for studying nucleation and propagation of earthquakes over multiple event cycles, on faults separating heterogeneous materials. As a first step we consider the bimaterial problem on planar, vertical strike slip fault governed by rate-and-state friction with a strong rate-weakening response. The system is loaded at the remote boundaries at a slow plate rate and quasi-dynamic events nucleate spontaneously on the fault. We find that the nucleation site migrates in the preferred direction and rupture propagates with asymmetric properties, the strength of which increases with greater material mismatch. We are currently studying the long term behavior of the earthquake cycle and under what conditions rupture in the non-preferred direction (occasionally observed on natural faults) is possible.