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

Supershear rupture propagation has been inferred from seismic observations for natural faults and observed in laboratory experiments. We study the effect of the free surface on the transition of earthquake rupture from subshear to supershear speeds using simulations of spontaneous dynamic rupture on vertical strike-slip faults. We find that locally supershear rupture near the free surface can occur due to (i) the generalized Burridge-Andrews mechanism, that is, a supershear loading field between P- and SV-wave arrivals generated by the main rupture front at depths, and (ii) the phase conversion of SV to P-diffracted waves at the free surface. Weaker supershear slip due to the generalized Burridge-Andrews mechanism is caused by the low strength at shallow portions of the fault relative to deeper ones. Dominant supershear rupture is supported by the additional supershear loading field produced by phase conversion. Locally supershear propagation at the free surface occurs regardless of the level of prestress and can cause transition to supershear propagation over the entire seismogenic depth. Such global supershear transition, which depends on prestress. can occur under prestress levels lower than the theoretical estimates for models with no free surface. Although the effectiveness of supershear transition due to the free surface can be diminished by several potentially important factors, it may play an important role on natural faults, at least in those strike-slip earthquakes that accumulate significant surface slip. (C) 2010 Elsevier B.V. All rights reserved.