The unsustained and sustained free-surface-induced supershear rupture, and their effects on near-field strong ground motions

Supershear earthquake rupture speed is typically discussed in the context of a stress threshold for the Burridge-Andrews transition mechanism, which explains how faults that are close to failure may support rupture propagation faster than the S wave speed. However, even in cases in which the initial shear stress is lower than the critical value of the Burridge-Andrews mechanism, the rupture may transition to supershear speed as it reaches the free surface on a strike-slip fault. This effect is termed the free-surface-induced supershear rupture. In our numerical simulations of dynamic rupture processes on strike-slip faults, we find that the free-surface-induced supershear rupture may return to sub-Rayleigh speed before the rupture reaches the end of the fault. We denote this kind of supershear rupture as the unsustained free-surface-induced supershear rupture. We furthermore find that a low normal stress, a large strength parameter, and a shallow hypocenter favor the unsustained free-surface-induced supershear. By simulating the near-field ground motion of three cases with the same initial stresses and different hypocenter depths, we show that the near-field peak ground velocity of the unsustained free-surface-induced supershear rupture mainly displays sub-Rayleigh characteristics, such as the stronger peak ground velocity beyond the end of the fault, and no observable Mach cone as shown in the sustained free-surface-induced supershear rupture. However, the merger of the sub-Rayleigh crack and the formerly supershear crack in unsustained cases may generate a large pulse in the near-field peak ground velocity. Our work helps to elucidate the differences between unsustained and sustained free-surface-induced supershear ruptures, and provides a possible method to identify the two kinds of ruptures in the near field.