The 19 October 2020 M w 7.6 Earthquake in Shumagin, Alaska: An Unusual Dextral Strike-Slip Event

On 19 October 2020, an extremely rare event occurred near the Alaska Peninsula: a strike-slip earthquake with a magnitude of M w 7.6 struck the southern coast of Alaska. In order to explore the cause of the earthquake, the focal mechanism and rupture process of this earthquake were inverted using far-field seismic waveform data and coseismic displacements derived from near-field GPS records. The results show that the focal mechanism was mainly dextral strike-slip, with a small thrust component, with a focal depth of 28 km, and a fault dip of 60°. The seismic moment of this earthquake was 3.05 × 10 20 Nm, and the magnitude was M w 7.6. The rupture mainly propagated southwards and was concentrated in its deeper part, below the initial rupture location. An earlier 22 July M w 7.8 megathrust earthquake released nearly half of the energy accumulated in the region over the past 100 years, while the October M w 7.6 earthquake did not occur on the megathrust, so the seismic risk in the region is still worthy of attention. The difference in the locking of the subducting fault from east to west resulted in a dextral torque on the oceanic crust, accompanied by the triggering effect of the July earthquake, which may be the cause of the October event.