The Sustainability of Free‐Surface‐Induced Supershear Rupture on Strike‐Slip Faults

While prior numerical simulations indicate that the Earth's free surface can induce supershear propagation on strike‐slip faults, copious observations of strike‐slip earthquakes have produced only a few instances of such supershear rupture. Our dynamic rupture simulations with varying initial normal stresses and fault strengths show that free‐surface‐induced supershear rupture on strike‐slip faults may return to sub‐Rayleigh speed as the rupture progresses. Such a phenomenon is defined as unsustained free‐surface‐induced supershear rupture, indicating that a rupture on a strike‐slip fault, even if it encounters the free surface, may not generate supershear rupture that is sustained long enough to be observable. The shape of the rupture front contour on the fault may also lead to a false impression about the rupture velocity, since the daughter crack may also propagate at sub‐Rayleigh speed, with near‐field fault‐perpendicular ground motion. Shallower hypocenter depths increase the likelihood of an unsustained free‐surface‐induced supershear rupture. Plain Language Summary Supershear ruptures is an essential part of our understanding of rupture dynamics and near‐field ground motion. By performing dynamic rupture simulations on strike‐slip faults, we show that the sustainability of free‐surface‐induced supershear ruptures depends on the initial normal stresses, fault strengths, and hypocenter depths in a homogeneous medium. Our work implies that unsustained free‐surface‐induced supershear may help explain the relative paucity of supershear rupture observations in natural earthquakes. Key Points Free‐surface‐induced supershear rupture could be unsustained, even on a homogeneous strike‐slip fault The daughter crack ahead of the main crack could also propagate at sub‐Rayleigh speed The hypocenter depth could also determine the sustainability of free‐surface‐induced supershear ruptures