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.