Seismic source dynamics, heterogeneity and friction

It has recently been proposed by several authors that stress distribution around active faults may become critical spontaneously. Other authors believe that heterogeneity is a permanent feature of fault planes. We test these ideas on a simple but realistic fault model in the presence of non-linear rate-dependent friction. We find that if friction increases with decreasing slip rate, slip becomes unstable at low slip rates generating supersonic healing phases that lock slip prematurely. Locking of slip in turn produces stress heterogeneity. For a fault model containing a single localized asperity, Das and Okubo found that rupture starts at the asperity and propagates until it either encounters a strong barrier or the stress intensity reduces below a minimum level. Rupture in these models is completely controlled by the physics of the rupture front. Rate dependent friction changes the behavior of the fault in a fundamental way: friction can lock the fault prematurely generating supersonic healing phases. Unlike stopping phases produced by barriers, a healing phase is not a wave phenomenon; it is a direct consequence of the non-linearity of friction. In this case rise time for slip on the fault is no longer controlled by the overall size of the fault as in conventional constant-friction crack models. We find that in our asperity models it is rise time or the healing mechanism that controls the final size of the fault. Studying models involving several isolated asperities we find that stress heterogeneity is preserved provided that friction is strongly rate dependent. Depending on the details of the rupture process, the final state of stress on the fault can be quite complex. This behavior is not universal, it depends on the degree of rate-dependence of friction.