A MODEL OF PLATE CONVERGENCE IN SOUTHWEST JAPAN, INFERRED FROM LEVELING DATA ASSOCIATED WITH THE 1946 NANKAIDO EARTHQUAKE

Abstract

We propose a kinematic model of plate convergence at the Nankai trough, a convergent boundary between the Philippine Sea and Asian plates. By using a two-dimensional finite element technique, we take account of some structural inhomogeneities of the crust and upper mantle; the Philippine Sea plate is subducting into the asthenosphere underlying the anomalously thin Asian plate, where the plate boundary is coupled tightly in the shallower portion but loosely in the deeper. The model is inferred from preseismic, coseismic, and postseismic changes in surface elevation associated with the 1946 Nankaido earthquake (M=8.2). These changes are precisely estimated from the first-order leveling data (1890-1980) by using an epoch reduction method. The preseismic seaward tilt can be interpreted by a steady state subduction of the Philippine Sea plate with a convergence rate of 4.5 cm/yr. The coseismic surface elevation change is well explained by a low-angle thrust faulting with an average shear stress drop of 2.0 MPa; although the faulting propagates along a lower portion of the plate boundary, it extends upward to the earth's surface branching away from the boundary at a depth of 22 km. The postseismic surface movements are interpreted by superposition of the viscoelastic response to the coseismic faulting and the effect of the steady state subduction of the Philippine Sea plate. In particular, the viscoelastic response of the loosely coupled part of the plate boundary is the underlying mechanism of the postseismic uplift localized in the coseismically subsided region. If the steady state subduction of the Philippine Sea plate continues for 150 yr, the shear stress averaged over the fault plane is found to amount to 2.0 MPa, which is sufficiently high to cause a major subduction earthquake. Hence, we can predict that the cyclic process of stress accumulation and release at the Nankai trough can be repeated every 150 yr.