Supershear Transition in Long‐Term Simulations of Slip on 2D Rate‐and‐State Faults: Favorable Heterogeneity, Dynamical Effects, and Spatial Resolution

Abstract Supershear ruptures propagate with speeds exceeding the bulk shear wave speed. The traditional Burridge‐Andrews mechanism of supershear transition requires faults to have a low seismic ratio. Liu and Lapusta (2008) used a single‐rupture model of a 1D fault to show that supershear transition can be triggered by favorable heterogeneity even when shear stresses are lower than required by the Burridge‐Andrews mechanism. Our study extends their work to rupture sequences on 2D faults governed by rate‐and‐state friction. We identify a parameter regime in which ruptures are all sub‐Rayleigh and demonstrate two mechanisms for this model to achieve supershear transition. First, introducing a patch of lower direct effect lowers the peak stress and decreases the local seismic ratio, allowing ruptures to become supershear. Then, modifying the parameters of the velocity‐strengthening loading region increases the stressing on the seismogenic zone, promoting supershear transition. In both cases, the supershear transition first occurs within the higher‐stressed bands at the boundary of the seismogenic region. We also investigate the role of inertial effects and show that supershear transition cannot occur in quasi‐dynamic sequences. We find that incorporating even limited stress‐concentrating dynamic effects ensures realistic rupture speeds. Finally, our simulations show that insufficient spatial resolution can change the response from supershear to sub‐Rayleigh. Our findings highlight the importance of studying 2D faults in long‐term sequence simulations, incorporating at least partial effects of dynamic stress concentrations, and adding expected long‐term stresses to single‐rupture dynamic simulations.