Postseismic Deformation and Stress Perturbation in NE China due to the 2011 Tohoku Earthquake

Giant subduction earthquakes produce significant coseismic and postseismic crustal deformation that can be geodetically detected thousands of kilometers from the epicenter. The observed postseismic deformation has been widely used to constrain rheological properties of the upper mantle, but mostly only for regions in the near field. Here we study far-field postseismic deformation of the 2011 Mw 9.0 Tohoku earthquake by analyzing data from more than 300 continuous and campaign-mode GPS stations in Korea, China and Russia. Postseismic velocities in NE China changed from ~10 mm/yr in the first two years after the earthquake to ~3 mm/yr in 2013-2015. Building on previous studies, we have constructed spherical-Earth viscoelastic finite element models to study how the deformation is controlled by changes in rheological properties of the upper mantle with distance from the subduction zone. Our model includes the elastic continental and oceanic plates, the elastic subducting slab, the viscoelastic continental and oceanic upper mantle. The viscoelastic relaxation of the upper mantle is simulated with the bi-viscous Burgers rheology. The continuous aseismic afterslip of the megathrust is modeled using a thin and weak viscous shear zone along the fault. Our model indicates that the steady-state viscosity of the upper mantle in NE China, ~1500 km from the rupture zone, has to be about one order of magnitude higher than that of the mantle wedge in the arc and backarc region. This lateral viscosity change appears to take place where the subducting slab encounters the mantle transition zone and turns flat. Assuming a friction coefficient of 0.4, we estimated the coseismic and postseismic (0-2 yrs) Coulomb stresses on active crustal faults in NE China to be no more than 5 kPa and 2 kPa, respectively.