PLATE-TECTONICS AND CRUSTAL DEFORMATION AROUND THE JAPANESE ISLANDS

We analyze over a century of geodetic data to study crustal deformation and plate motion around the Japanese Islands, using the block-fault model for crustal deformation developed by Matsu'ura et al. (1986). In this model, crustal deformation corresponds to the sum of relative movement among designated blocks and displacement due to dislocations along the locked parts of designated faults. We model the area including the Japanese Islands with 19 crustal blocks and 104 faults based on the distribution of active faults and seismicity. Three kinds of data are used; rates of change of angles and lengths, and average velocities of very long baseline interferometry stations. With the inversion of these geodetic data we obtain block motions and average slip rates of faults. This geodetic model predicts that the Pacific plate moves N69-degrees +/- 2-degrees-W at about 80 +/- 3 mm/yr relative to the Eurasian plate which is much lower than that predicted in geologic models. Substantial aseismic slip occurs on the subduction boundaries. The block containing the Izu Peninsula may be separated from the rigid part of the Philippine Sea plate. The faults on the coast of Japan Sea and the western part of the Median Tectonic Line have slip rates exceeding 4 mm/yr, while the Fossa Magna does not play an important role in the tectonics of the central Japan. The geodetic model requires the division of northeastern Japan, contrary to the hypothesis that northeastern Japan is a part of the North American plate. Owing to rapid convergence, the seismic risk in the Nankai trough may be larger than that of the Tokai gap.