Off-fault Yielding During Dynamic Ruptures: Distribution and Orientations

We use 2D spectral element code to simulate dynamic ruptures on a fault governed by slip- and velocity-weakening friction with off-fault yielding and possible elastic contrast across the fault. The off-fault yielding is implemented with Mohr-Coulomb plasticity, and a continuum brittle damage that accounts for dynamic changes of elastic properties in the yielding zones (Lyakhovsky et al., 1997). The studies attempt to clarify properties of dynamic ruptures and generated yielding zones for different off-fault rheologies, frictional laws, orientations of the maximum regional compressive stress relative to fault Ψ, values of the seismic S ratio and conditions representing different depth sections. In the current simulations, the damage rheology is used with parameters that prevent off-fault instabilities, leading to results that are generally similar to those obtained with plasticity. The location and extent of the yielding zone are found to depend on Ψ, seismic S ratio and the crack vs. pulse mode of rupture, in agreement with previous theoretical and numerical studies. The off-fault yielding zone is wide for conditions representing shallow depth and becomes progressively localized for conditions representing deeper sections. The intensity of plasticity/damage for both rupture modes is enhanced for larger regional stress, but the width can be suppressed due to higher confining pressure and larger rock cohesion. The angle Ψ representing expected microcrack orientations is generally shallow (less than 45 degrees) on the compressional side and steep (greater than 45 degrees) on the extensional side. The Ψ values depend also on the S ratio, conditions representing different depth sections, rupture speed, and existence of velocity contrast across the fault. The latter produces steeper angles of Ψ on the compliant side compared to the stiffer side for situations representing the same type (extension or compression) of quadrant.