Simulation of Postseismic Deformations caused by the 1896 Riku-u Earthquake, Northeast Japan: Re-evaluation of the viscosity in the upper mantle

We examine the postseismic deformations which are induced by an inland earthquake at a subduction zone, and investigate especially the effect of the viscoelastic heterogeneity through Finite Element Method (FEM). We estimate the viscosity of the upper mantle beneath the northeast Japan with realistic structures based on the leveling data which includes the postseismic deformations due to the 1896 Riku-u earthquake. For this purpose, we construct two 3-D FEM models with different viscoelastic structures, namely Layered Model and Plate Model. The results show that spatial patterns of the postseismic deformations differ considerably, depending on the two viscoelastic structures. Postseismic deformations are strongly controlled by the effective thickness of the viscoelastic medium above descending plate. At the surface, if the structure is complicated, the relaxation time is outwardly longer and varies significantly with position. Considering the plate structure, the profile of surface deformation and decaying subsidence rate constrain the Maxwell lime of the upper mantle to be 5 years, which is shorter than the value of 10 years estimated by Thatcher et al. [1980] assuming the layered structures.