Crustal Deformation Following Great Subduction Earthquakes Controlled by Earthquake Size and Mantle Rheology
After a great subduction earthquake, viscoelastic stress relaxation causes opposing motion of Earth's surface in the strike‐normal direction, with the dividing boundary located roughly above the downdip termination of the rupture. As the effect of the viscoelastic relaxation decays with time, the effect of the relocking of the megathrust becomes increasingly dominant to cause the dividing boundary to migrate away from the rupture zone, eventually leading to wholesale landward motion. The evolution of the postseismic deformation is controlled not only by mantle viscosity but also by the size of the earthquake. Large coseismic fault slip induces greater stress perturbation that takes a longer time to relax, and a greater rupture length along‐strike results in a pattern of postseismic viscous mantle flow less efficient for stress relaxation. Here we employ spherical‐Earth finite element models of Burgers rheology to quantify postseismic deformation processes for ten 8.0 ≤ Mw ≤ 9.5 subduction earthquakes. Using geodetic data as constraints, we reconstruct spatiotemporally continuous evolution of the postseismic deformation following each earthquake. We comparatively examine the “reference time” when the dividing boundary of the opposing motion passes through the map view location of the 50‐km depth contour of the subduction interface. Our results suggest a positive dependence of the reference time on earthquake size, although site‐ and/or event‐specific factors such as subduction rate, afterslip, and postseismic locking state of the megathrust also affect the evolution. Upper mantle viscosities constrained by available geodetic observations show somewhat different values between subduction zones located far from one another.
Publisher URL: https://onlinelibrary.wiley.com/doi/abs/10.1029/2017JB015242