Inelastic strain distribution and seismic radiation from rupture of a fault kink

Abstract

We extend an elastodynamic finite element method to incorporate off-fault plastic yielding into a dynamic earthquake rupture model. We simulate rupture for models of faults with a kink (a sharp change in fault strike), examining how off-fault plastic yielding affects rupture propagation, seismic radiation, and near-fault strain distribution. We find that high-frequency radiation from a kink can be reduced by strong plastic yielding near the kink. The reduction is significant above several Hz. When rupture propagates around the kink onto a less favorably stressed fault segment, plastic strain tends to localize into bands and lobes. Off-fault plastic yielding also significantly reduces heterogeneity of residual stresses around the kink following a dynamic event. The calculated plastic strain distribution around the kink and the radiated pulse from the kink are nearly grid independent over the range of element size for which computations are feasible. We also find that plastic strain can sometimes localize spontaneously during rupture along a planar fault, in the absence of a discrete stress concentrator like the kink. In that case, a non-dimensional parameter T, characterizing the initial proximity of off-fault material to its yield strength, determines whether plastic strain localizes into discrete bands or is smoothly distributed, with a large value of T promoting localization. However, in the cases of spontaneous localization, the details of the shear banding change with numerical element size, indicating that the final plastic strain distribution is influenced by interactions occurring at the shortest numerically resolvable scales. Off-fault plastic yielding also makes an important contribution to the cohesive zone at the advancing edge of the rupture. Copyright 2008 by the American Geophysical Union.