Constraining fault constitutive behavior with slip heterogeneity

We explore features of rupture dynamics that (1) lead to slip heterogeneity in earthquake ruptures and (2) maintain conditions that permit slip heterogeneity in subsequent events. Our 3-D finite-element simulations of magnitude 7 events on a vertical, planar strike-slip fault show that the conditions that lead to slip heterogeneity remain in place after large events when the initial shear stress, fracture energy, and sliding stress are all spatially heterogeneous. Simulations with spatial heterogeneity in only two of these parameters produce very similar rupture behavior and ground motions but the conditions necessary for slip heterogeneity in subsequent large events are absent. The spatial variations in sliding stress and fracture energy could be a proxy for complex nonplanar fault geometry. A narrow, confined slip pulse appears to be the critical feature required to yield these conditions for slip heterogeneity in subsequent events. An alternative mechanism for generating these confined slip pulses might be fault constitutive models not considered in our study. Fault constitutive models providing significant restrengthening via a different formulation might be able to produce similar behavior with spatial heterogeneity in only one or two of the parameters. Our simulations suggest that confined slip pulses are a very important feature for creating realistic numerical models of rupture propagation and understanding earthquake physics.