Rupture Model for the 29 July 2021 Mw 8.2 Chignik, Alaska Earthquake Constrained by Seismic, Geodetic, and Tsunami Observations

Abstract

A great earthquake struck the Semidi segment of the plate boundary along the Alaska Peninsula on 29 July 2021, re-rupturing part of the 1938 rupture zone. The 2021 M_W 8.2 Chignik earthquake occurred just northeast of the 22 July 2020 M_W 7.8 Simeonof earthquake, with little slip overlap. Analysis of teleseismic P and SH waves, regional Global Navigation Satellite System (GNSS) displacements, and near-field and far-field tsunami observations provides a good resolution of the 2021 rupture process. During ∼60-s long faulting, the slip was nonuniformly distributed along the megathrust over depths from 32 to 40 km, with up to ∼12.9-m slip in an ∼170-km-long patch. The 40–45 km down-dip limit of slip is well constrained by GNSS observations along the Alaska Peninsula. Tsunami observations preclude significant slip from extending to depths <25 km, confining all coseismic slip to beneath the shallow continental shelf. Most aftershocks locate seaward of the large-slip zones, with a concentration of activity up-dip of the deeper southwestern slip zone. Some localized aftershock patches locate beneath the continental slope. The surface-wave magnitude MS of 8.1 for the 2021 earthquake is smaller than Mₛ = 8.3–8.4 for the 1938 event. Seismic and tsunami data indicate that slip in 1938 was concentrated in the eastern region of its aftershock zone, extending beyond the Semidi Islands, where the 2021 event did not rupture.

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