Abstract

The country of Japan and the surrounding area has been divided into 12 seismogenic sources, which belong to five belts with an almost uniform orientation of the stress field. In each one of these sources, the active crustal deformation has been determined, using the summation of the seismic moment tensors of mainly post-1964 earthquakes. The analysis showed that due to the subduction of the Philippine Sea plate beneath the Eurasian plate, the deformation is expressed as compression at a mean direction of N53°W and an average rate of 1.5 cm/yr. The fault-plane solution corresponding to the summed tensor indicates low-angle thrust faulting with the following parameters: strike 226°, dip 21°, and rake 98°. Along the Sagami trough, dextral strike-slip motion is predominant at an average rate of 6 cm/yr. The summed tensor corresponds to a fault-plane solution with strike 291°, dip 73°, and rake 180°. Along the districts of Fukushima, Tohoku, and the Hokkaido Island, the motion of the Pacific plate toward Eurasia causes compression at a mean direction of N65°W and an average rate of 4 cm/yr. The summed tensor corresponds to a fault-plane solution with strike 199°, dip 18°, and rake 81°, that is, low-angle thrust faulting dipping toward the land. This pattern resembles the Hellenic arc, where the deformation is also taken up by motion on low-angle (18°) thrust faults dipping to the north. In the Tohoku district, the rate of motion reaches a value of 8 cm/yr, which is the highest calculated in the whole Japan area. In the inner part of the Japanese islands (along western Honshu and Niigata), the mean P axis is almost horizontal, trending nearly E-W. In western Honshu, the deformation is mainly expressed as dextral strike-slip motion at an average rate of 5 cm/yr in a N59°E direction. The representative fault-plane solution for this area has strike 240°, dip 81°, and rake 173°. At Niigata, the western coast of Hokkaido, and further north at Okushiri, the deformation is taken up as compression at a mean direction of N96°E at an average rate of 2 cm/yr. The summed tensor corresponds to a fault-plane solution with strike 5°, dip 41°, and rake 90°. The calculated directions and rates of motion along the Japanese islands are in good agreement with the expected values from plate-motion models.

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