Global frequency of oceanic and continental supershear earthquakes

Earthquakes are supershear when their rupture speed is faster than that of the seismic shear waves produced. These events are rare, but they can be highly destructive owing to the associated strong ground shaking, and understanding why they occur may provide insights into fault mechanics. Only a few supershear earthquakes have been reported previously, most of which were continental. Here we perform a systematic global search for supershear earthquakes by analysing seismic data from all large ( M w ≥ 6.7) shallow strike-slip earthquakes occurring between 2000 and 2020. Based on the rupture speeds determined by slowness-enhanced back-projection, and the identification of Rayleigh Mach waves, we identify four oceanic earthquakes consistent with supershear events. We find that at least 14.0% of large earthquakes during the study period were supershear, with oceanic events occurring as frequently as continental ones. We further observe a wider range of stable rupture speeds during supershear events than predicted by two-dimensional fracture mechanics theory, which we attribute to the presence of fault damage zones or slip obliqueness. The transition to and propagation of supershear earthquakes may be promoted in oceanic settings due to the thicker crustal seismogenic zones and the material contrast at oceanic–continental boundaries. Supershear earthquakes occur more frequently than previously thought, as suggested by the identification of four oceanic events from a global analysis of large shallow strike-slip earthquakes.