Rupture of the 2020 MW 7.8 Earthquake in the Shumagin Gap Inferred From Seismic and Geodetic Observations

The eastern portion of the Shumagin gap along the Alaska Peninsula ruptured in an M_W 7.8 thrust earthquake on 22 July 2020. The megathrust fault space-time slip history is determined by joint inversion of regional and teleseismic waveform data along with co-seismic static Global Navigation Satellite System (GNSS) displacements. The rupture expanded westward and along-dip from the hypocenter, located adjacent to the 1938 M_W 8.2 Alaska earthquake, with slip and aftershocks extending into the gap about 180 to 205 km, respectively, at depths from 15 to 40 km. The deeper half of ~75% of the Shumagin gap experienced faulting. However, the patchy slip is significantly less than possible accumulated slip since the region's last major rupture in 1917, compatible with geodetic seismic-coupling estimates of 10-40% beneath the Shumagin Islands. The rupture terminated in the western region of very low seismic coupling. There was a regional decade-scale decrease in b-value prior to the 2020 event.