Numerical and theoretical analyses of in-plane dynamic rupture on a frictional interface and off-fault yielding patterns at different scales

We perform numerical simulations of in-plane ruptures with spontaneous Mohr-Coulomb yielding in the bulk and analyse properties of the ruptures and yielding zones at different scales. Using a polar coordinate system, we show that the overall shapes and patterns of the simulated yielding zones can be well explained by combining the slip-induced Coulomb stress change and the background stress. Although there is no apparent mechanism for preferring synthetic versus antithetic shearing at a scale much smaller than the yielding zone size, this is not the case at larger scales. For shallow angles Psi between the maximum background compressive stress and the fault, representing thrust faulting, large-scale off-fault synthetic fractures are dominant but there are two conjugate sets of fractures with a typical size comparable to the yielding zone thickness. For smooth rupture propagation with moderate-to-high Psi values representing large strike-slip faults, most of the off-fault fractures that grow across the entire yielding zone are of the synthetic type. The less preferred antithetic set may become more pronounced for rupture propagation encountering fault heterogeneities. In particular, a strong fault barrier promotes antithetic fractures with a comparable size to those of the synthetic type around the barrier, where very high permanent strain is also observed. A consideration of non-local properties of the stress field in space or time can explain the above differences. Our results provide an alternative way of understanding Riedel shear structures and the potentially preferred synthetic shear fractures suggested in previous studies. The examined dynamic processes may be distinguished from quasi-static patterns by the timing, location and inclination angle of characteristic fracture elements. In agreement with other studies, we propose that backward or orthogonally inclined antithetic shear fractures on strike-slip faults and very high permanent strain could be used as signals that reflect abrupt rupture deceleration. On the other hand, relative lack of off-fault yielding at given locations may indicate abrupt rupture acceleration.