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

Postseismic 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 x 1018 and 0.5-5 x 1019 Pa s with an optimal value of 2 x 1018 and 2 x 1019 Pa s, respectively. Model results reveal a localized weak mantle wedge of similar to 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. Large earthquakes cause prolonged postseismic surface deformation in areas hundreds of kilometers from the rupture zone that can be recorded by geodetic methods. The postseismic deformation is mainly due to continuous aseismic slip on the fault (afterslip) and viscoelastic relaxation of the earthquake-induced stresses in the upper mantle. On 23 January 2018, an Mw 7.8 strike-slip earthquake ruptured multiple faults in the Pacific plate southwest of the Alaska subduction zone. Global Positioning System (GPS) stations west and northwest of the Gulf of Alaska have continuously recorded the postseismic surface deformation that is up to 16 mm near Kodiak Island within the first 2 years. We have constructed a three-dimensional finite element model to study the stress-driven afterslip and viscoelastic relaxation of upper mantle following this event. The rheological properties of the upper mantle in the Alaska subduction zone are well constrained by GPS observations. Model results reveal a localized weak mantle wedge near Lower Cook Inlet that is consistent with a low velocity zone at depths of similar to 60-250 km from tomography studies. We also estimate coseismic and postseismic stress change over the subduction interface due to the 2018 Kodiak earthquake. Postseismic deformation of the 2018 Kodiak earthquake is controlled by afterslip and viscoelastic relaxation of the upper mantleUnderestimation of the observations near Lower Cook Inlet may indicate locally weakened mantle wedgeThe event promotes strain accumulation on the shallow megathrust near Kodiak Island where has no obvious triggered seismicity