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

南海地区位于日本最活跃的板块俯冲带之一，菲律宾海板块向欧亚板块俯冲所引起的地震形变周期十分明显，在短短的100-130年内就发生了多次板间大地震。通过GPS观测网的布设，阐明了应变积累过程，探讨了板块间相互作用的动力学机制。所有站点均已向西北偏西至西北方向移位，2.位移速度在最南端最大，并随着距板块边界距离的增加而迅速减小。大的位移梯度存在于平行于板块边界走向的狭窄区域内。这些结果表明，目前在南开海槽的板间耦合是强的，大部分的相对板块运动已被采取的变形的前缘欧亚板块。由此估计的强耦合与东北地区的弱相互作用形成鲜明对比，在东北地区，相当大一部分相对板块运动可能被缓慢或无声的滑动所占据。并根据地调院全国GPS网观测结果，讨论了更多区域的形变场。我们指出，中国地区的东向运动，这是小的，但系统的矛盾，菲律宾海板块的俯冲。一种解释是，日本西南部属于“Amurian”微板块，该板块被认为具有轻微的向东运动的速度，日本西南部的向东运动受到与日本东北部碰撞和/或与菲律宾海板块相互作用的限制而减弱。

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.