Stress Triggering and Megathrust Cascades Along the Eastern Alaska Subduction Margin

During 2020-2021, a M7.8 and M8.2 earthquake sequence ruptured two neighboring segments of the Alaska-Aleutian subduction zone offshore of the Alaska Peninsula. This complex system displays highly variable coupling as well as distinct changes in structural properties along the plate interface. The spatial and temporal proximity of these two earthquakes and their relation to previous events provide opportunities to investigate stress transfer and earthquake triggering, conditions required for rupture propagation, and why some areas may be more prone to generating predominately M7-8 rather than larger M8.5-9+ earthquakes. We present a coseismic slip model for the 2021 M8.2 Chignik earthquake constrained by static and high-rate GNSS, InSAR, and seismic waveform data. The Chignik event originated at the northeast edge of the 2020 M7.8 Simeonof rupture zone and propagated to the northeast. The bulk of the energy release occurred near the hypocenter with maximum slip amplitudes of ~6 m between 20-30 km depths. Almost all of the rupture was contained within the Semidi segment of the megathrust while the Simeonof rupture largely stayed within the neighboring Shumagin segment to the west, with little significant overlap between the two earthquakes. This suggests that lateral coupling boundaries may influence earthquake rupture nucleation and arrest. The west-to-east increase in coupling and changes in interface properties may encourage partial interface ruptures. Coulomb stress changes show that the Simeonof event increased the stress in the hypocentral region of the Chignik event. The influence of previous earthquakes on the Simeonof event is less clear. A re-evaluation of the M8.3 1938 earthquake suggests that the majority of the slip occurred closer to the trench, outboard of the Chignik and Simeonof ruptures. Depending on how far west the slip extended, this rupture would have induced positive Coulomb stress changes either in the vicinity of the hypocenter or in the area of major energy release of the Simeonof earthquake, or both. Given these stress changes and the fact that the 2020-2021 ruptures did not substantially overlap with the updated 1938 rupture zone, the Simeonof and Chignik earthquakes may be part of a cascade of megathrust earthquakes that ruptured almost the entire margin during the 20th century.