Supershear transition in long-term simulations of slip on 2D rate-and-state faults: favorable heterogeneity, dynamical effects, and spatial resolution

Mary Agajanian, & Nadia Lapusta

Submitted December 15, 2025, SCEC Contribution #15008

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. For linear slip-weakening friction, this is synonymous with high prestress on the fault. 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 fault model to achieve supershear transition. First, a patch of lower direct effect lowers the peak stress and decreases the local seismic ratio, allowing ruptures to transition to 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 sequences simulations, incorporating at least partial effects of dynamic stress concentrations, and adding expected long-term stresses to single dynamic simulations.

Citation
Agajanian, M., & Lapusta, N. (2025). Supershear transition in long-term simulations of slip on 2D rate-and-state faults: favorable heterogeneity, dynamical effects, and spatial resolution. Journal of Geophysical Research: Solid Earth, (submitted).