Bimaterial effects in an earthquake cycle model using rate-and-state friction

We have developed a computational framework to study earthquake cycles in 2-D plane strain and apply it to faults separating dissimilar material. We consider a planar, strike-slip fault governed by rate-and-state friction where quasi-dynamic events nucleate spontaneously due to remote, tectonic loading. We investigate the influence of material contrast over the course of many hundreds of years. For the parameters we consider, we find that the presence of bimaterial properties influences the earthquake nucleation site, such that rupture in the preferred direction (that is, in the direction of particle motion of the side of the fault with lower shear wave velocity) is favorable. For large values of the critical slip distance Dc, events propagating in the preferred rupture direction occur for a wide range of material contrasts. For smaller values of Dc, small events emerge, even in the monomaterial case. With material mismatch present, some of these small events propagate in the nonpreferred direction, made possible by a favorable stress distribution left on the fault from previous ruptures. These results may shed light on our understanding of rupture directivity on large strike-slip faults (like the San Andreas Fault in California) which occasionally host events rupturing in the nonpreferred direction.