The Paucity of Supershear Earthquakes on Large Faults Governed by Rate and State Friction

Supershear earthquakes are rare compared to their subshear counterparts, but the cause for their paucity remains to be understood. We investigate for the first time the prevalence of supershear ruptures across multiple earthquake cycles on long faults using rate-and-state friction and a 2.5D approximation that accounts for the finite seismogenic width W. We find supershear events occur only in a narrow range of friction parameters that is not commonly observed in laboratory experiments, which may explain its rarity in nature. Particularly, the ratio between direct and evolution effects of rate-and-state friction needs to be low (a/b < 0.4) and the nucleation length has to be sufficiently large compared to W, but not too large that it causes a transition from seismic to aseismic slip. Our numerical and analytical developments contribute fundamentally to understanding the state of stress on long faults over multiple earthquake cycles and their potential for hosting supershear earthquakes.