Rheological Structure and Lithospheric Stress Interaction in the Alaska Subduction Zone Gleaned From the 2018 Mw 7.9 Oceanic Crustal Earthquake

Abstract

AbstractPostseismic deformation at convergent margins is controlled mainly by continuous slip on the fault (afterslip) and relaxation of the earthquake‐induced stress in the viscoelastic upper mantle (viscoelastic relaxation). Study of these deformation processes provides insight into the rheological properties of upper mantle and slip behavior of the fault. We have constructed a three‐dimensional finite element model to investigate the postseismic deformation of the 2018 Mw 7.9 Kodiak earthquake. We derived the first 2‐year postseismic Global Positioning System observations to constrain afterslip and upper mantle rheology in the south‐central Alaska. The upper mantle is separated into the mantle wedge and oceanic upper mantle topped by an 80‐km thick asthenosphere layer by the subducting slab. Results show that afterslip generally occurred in areas adjacent to the rupture zone and has a small magnitude of a few tens of millimeters. The viscosities of the asthenosphere and mantle wedge are determined to be in a range of 1–4 × 1018 and 0.5–5 × 1019 Pa s with an optimal value of 2 × 1018 and 2 × 1019 Pa s, respectively. Model results reveal a localized weak mantle wedge of ∼1018 Pa s beneath Lower Cook Inlet that may be due to the fluids dehydrated from the slab. Coulomb stress changes show that the earthquake enhanced coseismic and postseismic stress loading of up to 0.9 and 0.1 bar, respectively, on the shallow subduction interface near Kodiak Island, but there is no obvious triggered seismicity, probably due to the low stress status already released by the 1964 Mw 9.2 Alaska earthquake.