Strain accumulation process and interplate coupling in the Nankai region, Southwest Japan

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The Nankai region is located in one of the most active plate-subduction zones in Japan, where earthquake deformation cycle caused by the subduction of the Philippine Sea plate beneath the Eurasian plate is very clear as shown by the occurrence of great interplate earthquake within the short span of 100-130 years. We deployed GPS observation network to clarify strain accumulation process and discuss dynamics of the interplate interaction.Results obtained from three-year GPS measurements are as follows : 1. All stations have been displaced toward the west-northwest to northwest direction, 2. Displacement velocity is maximum at the southernmost site and rapidly dicreases with increasing distance from the plate boundary, and 3. Large displacement gradient exists in a narrow region extending parallel to the strike of the plate boundary. These results imply that current interplate coupling at the Nankai trough is strong ; much of the relative plate motion has been taken up by deformation of the leading edge of the Eurasian plate.Using an inversion technique of GPS-derived displacement velocity, we estimated strength of interplate coupling at the Nankai trough. Strong coupling thus estimated is in striking contrast to the weak interaction in the Tohoku region where considerable part of relative plate motion may be taken up by aseismic "slow" or "silent" slip. Furthermore, we discussed more regional deformation field based on the results by the nationwide GPS network by Geographical Survey Institute. We pointed out eastward motion in Chugoku region, which is small but systematically contradictory to the subduction of the Philippine Sea plate. One of the interpretations is that southwest Japan belongs to the "Amurian" microplate which is considered to have a slight eastward velocity and eastward motion of southwest Japan is constrained to reduce by the collision with northeast Japan and/or by the interaction with the Philippine Sea plate.