Displacement fields due to the 1946 Nankaido earthquake in a laterally inhomogeneous structure with the subducting Philippine Sea plate—a three-dimensional finite element approach

Abstract

The displacement fields associated with the 1946 Nankaido earthquake have been investigated in detail, taking into account the configuration of the Philippine Sea plate subducting beneath southwest Japan. We used a three-dimensional finite element method for the elastic problem to examine the effects of three-dimensionally inhomogeneous structure. Some differences in the displacement patterns between homogeneous and inhomogeneous media have been identified. The amounts of displacements are large in low-rigidity portions such as in the crust and mantle wedge, and are small in a high-rigidity portion composed of the subducting plate. The inhomogeneity affects horizontal displacements at the Earth's surface more than vertical displacements. If the rigidity and the Poisson ratio given here to southwest Japan are appropriate, the fault parameters of the earthquake, which have been estimated from simple dislocation models in a homogeneous semi-infinite medium, need to be modified somewhat. The present computations indicate that larger dip angles and smaller fault slips are more appropriate fault parameters when structural inhomogeneity is taken into account. This suggests that the discrepancy in the obtained total seismic moment between the geodetic and seismic models of the Nankaido earthquake could be reduced.