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. Plain Language Summary A large M W 7.8 underthrusting earthquake ruptured on the plate boundary between the Pacific and North American Plates along the Alaska Peninsula on 22 July 2020. The fault slipped in the region between large plate boundary earthquakes in 1938 and 1946 that has been called the Shumagin seismic gap. The last major earthquake on this part of the plate boundary occurred in 1917, with both that event and the 2020 earthquake rupturing the eastern half of the Shumagin gap. The slip was concentrated in several patches, with the largest slip patch located directly below the Shumagin Islands. Global Positioning Satellite (GPS) stations on the islands recorded the ground displacement clearly. Using GPS displacements along with seismic wave ground motions recorded by regional strong motion and global broadband stations, the slip space-time history during the faulting is determined. The coseismic slip is significantly less than the amount that could have accumulated from relative plate motions since 1917, which is consistent with geodetic estimates of low seismic coupling on this portion of the plate boundary. There was a regional decrease in the slope (b-value) of the earthquake magnitude distribution over the decade before the event.

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On November 14th 2001, a Mw 7.8 earthquake occurred in the Kokoxili region of northern Tibet. The earthquake ruptured more than 400 km along the western part of the Kunlun fault with a maximum of 8 m left‐lateral slip. In this paper, we use a multitemporal Interferometric SAR (InSAR) time series technique to map the postseismic motion following the large Kokoxili event. SAR data from Envisat descending orbits along five adjacent tracks covering almost the entire ruptured fault length are used to calculate the displacement time series for a period between 2 and 6 years after the earthquake. A peak‐to‐trough signal of 8 cm in the radar line of sight is observed during the period between 2003 and 2008. Two different mechanisms are employed to explain the observed surface displacements, namely afterslip and viscoelastic relaxation. The observations inverted for afterslip on and below the coseismic rupture plane shows that the maximum slip in the afterslip model is 0.6 m. The position of the maximum postseismic slip is located in the middle of two relatively high coseismic slip patches, which suggests that afterslip is a plausible mechanism. Models of viscoelastic stress relaxation in a Maxwell half‐space give a best fitting viscosity for the mid‐to‐lower crust of 2–5 × 1019 Pa s, and the principal postseismic relaxation process is due to viscous flow in the lower crust to upper mantle. However, the InSAR observations are incapable of distinguishing between localized (afterslip) and distributed (viscoelastic relaxation) deformation. And the lowest misfits are produced by mixed models of viscoelastic relaxation in the mantle below 70 km and afterslip in the crust. Modeling of viscoelastic relaxation in a Maxwell half‐space, and also a mixed mechanism model, enables us to place an effective viscosity of 2 × 1019 Pa s on the lower crust to mantle of northern Tibet.