Implications of the 2020–2021 Earthquakes for Megathrust Coupling and Seismicity in the Vicinity of the Shumagin Gap

Before 2020, the last major earthquakes in the Alaska–Aleutian subduction zone near the Shumagin Islands were the great (M_w 8.0+) 1938 and 1946 earthquakes. These events occurred on either side of the Shumagin Gap, a region without a historical great megathrust earthquake. A sequence of recent large earthquakes—the July 2020 M_w 7.8 Simeonof megathrust event, the October 2020 M_w 7.6 Sand Point intraslab (strike-slip) event, and the July 2021 M_w 8.2 Chignik megathrust event—drew attention to this region. The 2020 events were previously shown to be consistent with a transition from high plate interface coupling east of the Shumagin Gap to low coupling in the Shumagin Gap. The 2021 Chignik earthquake occurred in a seismogenic section of the subduction zone, with the most recent large earthquake in 1938. This region was brought closer to failure by stress changes from the 2020 events. Analyses of the Chignik sequence provide additional support that this region straddles the transition from coupled to uncoupled. The evolution of seismicity is compatible with stresses produced by such a coupling transition, and the Chignik event was able to undergo greater strain release because of the uncoupled nature of the megathrust in the Shumagin Gap.