Fast rupture propagation for large strike-slip earthquakes

•Systematically estimated average rupture speeds of recent large earthquakes.•Dip-slip events have slow rupture speed.•Large strike-slip earthquakes appear to have significant supershear rupture speed.

Studying rupture speeds of shallow earthquakes is of broad interest because it has a large effect on the strong near-field shaking that causes damage during earthquakes, and it is an important parameter that reflects stress levels and energy on a slipping fault. However, resolving rupture speed is difficult in standard waveform inversion methods due to limited near-field observations and the tradeoff between rupture speed and fault size for teleseismic observations.Here we applied back-projection methods to estimate the rupture speeds of 15 Mw≥7.8 dip-slip and 8 Mw≥7.5 strike-slip earthquakes for which direct P waves are well recorded in Japan on Hi-net, or in North America on USArray. We found that all strike-slip events had very fast average rupture speeds of 3.0-5.0 km/s, which are near or greater than the local shear wave velocity (supershear). These values are faster than for thrust and normal faulting earthquakes that generally rupture with speeds of 1.0-3.0 km/s.