Plain Language Summary
Determining where and how much slip occurs during an earthquake allows us to estimate the remaining earthquake hazard potential. Models of earthquake slip can vary from each other a lot when the data are sparse, because of assumptions such as the geometry and spatial extent of the rupture plane and the roughness of the slip distribution. The early postseismic process is dominated by afterslip on the rupture plane, which is sensitive to the slip distribution of the coseismic event, under the model of stress-driven afterslip. Postseismic GPS displacements are a completely different dataset from the coseismic observations, and provide new and independent information about the earthquake rupture. We test how a range of coseismic slip models for the July 29, 2021 Mw8.2 Chignik earthquake, all of which can fit the coseismic data well, predict postseismic deformation over three months and compare that with GPS measurements. We find that the postseismic data provides a good constraint on the spatial distribution of coseismic slip, especially at the downdip (deeper) end of the rupture. A more spatially compact coseismic rupture is required to generate the stress-driven afterslip that can fit the data, no matter what viscoelastic relaxation contribution considered.
1 Introduction
The Alaska-Aleutian subduction zone has hosted several great megathrust earthquakes in the last century, including the 1938 Mw8.2, 1946 Mw8.6, 1957 Mw8.6, 1964 Mw9.2 and 1965 Mw8.7 earthquakes (Nishenko and Jacob, 1990). Recently, the Mw7.8 Simeonof earthquake (e.g., Xiao et al., 2021 struck the Shumagin islands on July 21, 2020 followed by the Mw 7.6 strike-slip Sand Point earthquake on October 19. On July 29, 2021 — advanced by the Simeonof earthquake — the Mw8.2 Chignik earthquake Elliott et al., 2022 struck the adjacent Semidi segment, to the NE of the 2020 Mw7.8 event. The Chignik earthquake partially re-ruptured the 1938 Mw8.2 coseismic rupture zone (Figure 1). The availability of multiple forms of geodetic and seismic data provides us with a great opportunity to fully assess the coseismic slip and the post-seismic processes that followed, helping us to quantify the slip budget and earthquake potential of this section of the Alaska-Aleutian subduction zone.