Supershear rupture of the 5 January 2013 Craig, Alaska (Mw 7.5) earthquake

Supershear rupture, in which a fracture's crack tip expansion velocity exceeds the elastic shear wave velocity, has been extensively investigated theoretically and experimentally and previously inferred from seismic wave observations for six continental strike-slip earthquakes. We find extensive evidence of supershear rupture expansion of an oceanic interplate earthquake, the 5 January 2013 M-w=7.5 Craig, Alaska earthquake. This asymmetric bilateral strike-slip rupture occurred on the Queen Charlotte Fault, offshore of southeastern Alaska. Observations of first-arriving Sn and Sg shear waves originating from positions on the fault closer than the hypocenter for several regional seismic stations, with path calibrations provided by an empirical Green's function approach, indicate a supershear rupture process. Several waveform inversion and modeling techniques were further applied to determine the rupture velocity and space-time distribution of slip using regional seismic and geodetic observations. Both theoretical and empirical Green's functions were used in the analyses, with all results being consistent with a rupture velocity of 5.5 to 6 km/s, exceeding the crustal and upper mantle S wave velocity and approaching the crustal P wave velocity. Supershear rupture occurred along similar to 100 km of the northern portion of the rupture zone but not along the shorter southern rupture extension. The direction in which supershear rupture developed may be related to the strong material contrast across the continental-oceanic plate boundary, as predicted theoretically and experimentally. The shear and surface wave Mach waves involve strongly enhanced ground motions at azimuths oblique to the rupture direction, emphasizing the enhanced hazard posed by supershear rupture of large strike-slip earthquakes.