FINITE ELEMENT MODELING OF THE THREE-DIMENSIONAL TECTONIC FLOW AND STRESS FIELD BENEATH THE KYUSHU ISLAND, JAPAN

Abstract

Three-dimensional patterns of tectonic deformations and stresses are calculated to simulate the tectonics of Kyushu and its surrounding regions in southwestern Japan using a finite element method. The configuration of the subducted Philippine Sea plate is modeled by means of three-dimensional finite elements. Viscosities in the crust and upper mantle assigned are taken from the data of postglacial or postseismic rebounds. Several possible loads are taken into consideration: a slab pull force arising from the density contrast between the subducted slab and the surrounding asthenosphere; a ridge push force due to evolution of oceanic plates; a traction due to flows in the asthenosphere; a crustal buoyancy acting on a low-density crust; a buoyant force in the back-arc region which may simulate the spreading of a back-arc basin; a positive buoyancy acting on the aseismic ridge; compressive forces in the E-W direction exerting on the crust and subducted slab.
The results suggest that the tensile stress field observed in the crust of Kyushu may be generated by the interaction between the slab pull force, crustal buoyancy, and flows in the asthenosphere. It may also be concluded that the stress field of down-dip extension in the Wadati-Benioff zone can be explained mainly by the slab pull force. The asthenospheric flows might have a significant effect on the genesis of near-horizontal extension which yields normal faulting in the uppermost mantle, and such stresses would be more likely to appear on the oceanic side than on the continental side. In order to explain the dip angle of the subducted slab, if it is a manifestation of the flow pattern at a subduction zone, a traction due to asthenospheric flow of about 200 bar is required in the asthenosphere. The predicted values of velocities of tectonic flows and of maximum shear stresses are a few cm/yr and 500 bar at most in the crust and the subducted slab beneath Kyushu, though these are dependent on viscosity.