The 29 July 2021 M_W 8.2 Chignik, Alaska Peninsula Earthquake Rupture Inferred From Seismic and Geodetic Observations: Re‐Rupture of the Western 2/3 of the 1938 Rupture Zone

Abstract

The Pacific plate underthrusts the North America plate along the Alaska-Aleutian subduction zone, producing great earthquake ruptures along most of the arc (e.g., Sykes et al., 1981). Southwest of the great 1964 Alaska rupture zone, the Semidi segment offshore of the Alaska Peninsula (Figure 1) has experienced several great earthquakes. On 29 July 2021, an M W 8.2 thrust-faulting earthquake ruptured this region. This is the largest earthquake to strike U.S. territory in 55 years. The 2021 aftershock distribution overlaps the rupture zone of an M W 8.2 event in 1938, which had an aftershock zone and tsunami source region extending approximately 270-300 km along the megathrust (e.g.,

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

“…Similar reductions are needed for the seismo‐geodetic inversion model of Liu et al. (2022) (Figure S3 in Supporting Information S1). We first summarize the final result and then discuss how it was arrived at.…”

“…A standard inversion with only teleseismic body waves and surface waves, for a planar‐fault model with ϕ = 243° and δ = 14° yielded M 0 = 2.88 × 10 21 N m ( M W 8.24), with a 100‐s long duration, and peak slip of 2.9 m. An inversion for the same fault orientation using the same seismic waves plus 1 strong motion recording, 6‐hr‐GNSS time series, and 16 static GNSS observations yielded M 0 = 2.89 × 10 21 N m ( M W 8.24), with ∼70‐s duration and peak slip 4.8 m. Liu et al. (2022) jointly inverted regional and teleseismic data along with coseismic static and hr‐GNSS displacements for a slip model extending ∼175 km northeastward with peak slip of ∼8.6 m in the depth range of 20–40 km (Figure S3 in Supporting Information S1). These models have somewhat deeper slip in near the hypocenter, and a distribution of slip along‐strike that shifts to shallower depths to the northeast as in our preferred model (Figures 2 and 3).…”

“…Due to having further slip that extends up‐dip to near the continental shelf break, the computed tsunamis from the Liu et al. (2022) model are too early for all DART stations and overpredict amplitudes (Figure S3 in Supporting Information S1; Mulia et al., 2022).…”

“…For this event, the primary perturbations to our starting model (Figure S2 in Supporting Information S1) involve the reduction of the up-dip extent of the model as required to fit the tsunami observations. Similar reductions are needed for the seismo-geodetic inversion model of Liu et al (2022) (Figure S3 in Supporting Information S1). We first summarize the final result and then discuss how it was arrived at.…”

Features of the Largest Earthquake Seismic Cycles in the Western Part of the Aleutian Subduction Zone

“…The length of the Simeonof earthquake fault zone was about 180 km [40], which only partially filled the Shumagin gap. A year later, on 29 July 2021, the fault zone of the 1938 earthquake was partially ruptured by the M8.2 Chignik earthquake having 175 km-long fault zone [32]. Historical documents indicate that great earthquakes ruptured at least a 500-km-long segment of the plate boundary near the Alaska Peninsula in 1788 and 1847 [37].…”