Numerical analysis of postseismic displacement of infer the lithospheric strength beneath the East Sea (Sea of Japan)

The viscoelastic relaxation, in which viscoelastic lower crust and mantle relieve the stress caused by a sudden slip of earthquake, leads to surface deformation in the postseismic period. The geodetic data during the postseismic has been adopted for constraining the strength of the lithosphere. The 2011 Mw 9.0 Tohoku earthquake at the plate boundary induced coseismic and postseismic horizontal displacements (5.4 cm and 2.2 cm, respectively) of the Ulleung Basin in the East Sea located ~1000 km far from the epicenter. These geophysical observations provide a unique chance to quantitatively constrain the strength of the lithosphere beneath the East Sea if adequate numerical modeling is applied. To calculate coseismic slip and postseismic deformation associated with the Tohoku earthquake, we developed 2D and 3D subduction zone models using open-source finite element software Pylith and slab geometry inferred from Slab 1.0. The numerical domain is divided into the elastic upper crust, viscoelastic lower crust, and viscoelastic upper mantle. The viscoelastic layers are assigned as Maxwell or Kelvin bodies. We adopted a wide range of viscosity for simulating the model of viscoelastic relaxation. The models showed the evolution of the viscoelastic relaxation in the entire domain with a maximum length of ~3000 km. The early and late deformations in the geodetic data correspond to Kelvin and the Maxwell viscoelasticity models, respectively. The numerical model, which well-constrains the GPS data over the East Sea and Korea Peninsula, infers that the mantle viscosity beneath the East Sea is lower than the average mantle viscosity of the Glacial Isostatic Adjustment. Thus, we argue that the mantle and lithosphere of the East Sea are warm and mechanically weak.