Three-Dimensional Stress Distribution in Southwestern Japan as Expected from the Configuration of the Subducting Philippine Sea plate (Part I)

It has been inferred [SHIONO (1977)] from the focal mechanism of earthquakes that the stress distribution in Southwestern Japan has three major characteristics;(1) a NW-SE compression along the Nankai trough, (2) a WNW-ESE compression in the inland region, (3) an extension parallel to the leading edge of the Philippine Sea plate at subcrustal depths.In order to investigate possible sources of these complex features of the stress state, a three-dimensional finite element method is applied to model the configuration and elastic properties of the Philippine Sea plate subducting under this region. In the present paper, three different types of forces and their combinations are considered;(i) a negative buoyancy due to the density contrast between the subducting Philippine Sea plate and surrounding mantle, (ii) a northwestward compressional force generated by the underthrusting of the plate, (iii) a westward compressional force due to the subducting Pacific plate.The results suggest that the parallel extension at subcrustal depths is closely associated with the configuration of the plate, and that normal faulting at these depths is mainly caused by negative buoyancy. The shear stress in the continental crust and in the subducting plate is estimated to be of the order of several hundreds bars.