Examining tendencies of in-plane rupture to migrate to material interfaces

We perform a numerical parameter-space study of 2-D in-plane ruptures in a model consisting of two different half-spaces separated by a low-velocity layer and possible simultaneous slip along multiple faults. Ruptures are nucleated by a bilateral expanding stress drop in a limited source region, and may continue to propagate spontaneously (or not) along one or several faults. Most calculations are done for purely elastic media and faults governed by Coulomb friction, but some simulations employ Prakash-Clifton friction and Kelvin-Voigt viscosity. The faults, two of which are material interfaces, are situated equidistant and parallel to each other. Using different nucleation locations, different initial stress, different velocity contrasts, different frictional fault separations, different widths of a low-velocity zone, and different number of faults, we examine the range of conditions for which ruptures migrate to other faults and continue to propagate in a self-sustaining manner. The model produces diverse migration and propagation phenomena represented by several phase diagrams. However, a general result of the study is that ruptures tend to migrate to the material interfaces and become self-sustained wrinkle-like pulses for wide ranges of conditions. The wrinkle-like pulses propagate along each material interface unilaterally in the direction of motion on the more compliant side of the interface (referred to as the 'positive' direction). The existence of a large number of faults produces, like viscosity, distributed deformation that reduces the divergent behaviour of the wrinkle-like pulses. In many cases, ruptures migrate to the interface with the stronger contrast and propagate unilaterally in the positive direction associated with that interface and the overall contrast across the fault zone. In smaller number of cases, ruptures migrate to the interface with the weaker contrast and propagate unilaterally in the opposite positive direction associated with that interface. For various parameter combinations, self-sustained unilateral pulses travel simultaneously, in the two opposite positive directions, along the interfaces on the opposite sides of the low-velocity layer. A low-resolution imaging of these ruptures would lead to an inference on bilateral propagation. The M6, 2004 September, Parkfield California earthquake may provide a natural example of such a case.