Supershear transition due to a free surface in 3-D simulations of spontaneous dynamic rupture on vertical strike-slip faults

► A free surface promotes supershear rupture transition and propagation. ► Minor near-surface supershear rupture can occur due to a supershear loading field. ► Dominant near-surface supershear rupture can occur due to SV-P phase conversion. ► Local near-surface supershear transition occurs regardless of the level of prestress. ► Local supershear rupture causes global supershear transition for larger prestress.