The postseismic deformation of the 2018 Mw 7.9 Alaska offshore earthquake

We have constructed a three-dimensional finite element model to investigate the viscoelastic postseismic deformation of the 2018 Mw7.9 Kodiak earthquake. We derived the first 2-year postseismic GPS observations to constrain the 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 takes place in areas adjacent to the rupture zone and have a smaller magnitude of a few tens of millimeters. The steady-state 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 suggest that a localized weak mantle wedge of ~10¹⁸ Pa s near Lower Cook Inlet is required to reproduce observations. 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 are no obvious triggered megathrust afterslip and seismicity, probably due to the low stress status already released by the 1964 Mw 9.2 great Alaska earthquake.

Research members: Jian Zhang, Dr.Yan Hu