Mechanism of the post-seismic deformation due to the 2021 Chignik <i>M</i>w8.2 earthquake and its implication for regional rheology

SUMMARY The widespread, multiyear crustal deformation induced by megathrust earthquakes (Mw8+) is primarily controlled by the combined effects of continuous aseismic slip on the fault plane (afterslip) and viscoelastic relaxation driven by coseismic stress perturbations in the upper mantle. However, till today it remains a considerable challenge to separate these two mechanisms in geodetic observations. We derived the first 3-yr Global Navigation Satellite System (GNSS) observations following the 2021 Chignik Mw8.2 earthquake to investigate the mechanisms of post-seismic deformation. We established a model capable of simultaneously simulating afterslip and viscoelastic relaxation, and constrained the upper-mantle rheology beneath the Alaska Peninsula. The best-fitting model effectively reproduces the GNSS observations and reveals a notable viscosity difference between the mantle wedge and the oceanic asthenosphere, with steady-state viscosities of $3 \times {{10}^{18}}$ and $4 \times {{10}^{19}}$ Pa s, respectively. The inferred mantle wedge viscosity beneath the Alaska Peninsula is lower than the values reported for south-central and southeastern Alaska, suggesting an eastward increase in viscosity along the subduction zone. Two main patches of afterslip are identified during the first 3 yr. The patch of up-dip afterslip overlaps with the 1938 Chignik Mw8.3 earthquake rupture zone, and demonstrates a close spatial correlation with the slow slip event in 2018. The above new results enhance our insights into the spatial variability of regional rheology and slip behaviour along the Alaska–Aleutian subduction zone.