Hypocenter determination by Japan-Taiwan collaborative research to presume Philippine sea plate zone in the vicinity of Ryukyu Arc to Eastern Taiwan region.

琉球弧西南部至台湾东部地区的地质构造特征是菲律宾海板块与欧亚板块的俯冲或碰撞。台湾现今的碰撞边界位于沿着花莲-台东的纵谷，但台湾东北部与琉球弧西南部之间的板块边界尚不清楚，仍有争议。为了弄清该地区的板块边界或构造格局，必须研究准确的地震活动，即精确的震源分布、震源机制等，从而获得该地区更精确或更详细的震源分布。结果表明，该地区的震源位置与日本气象厅和台湾中央气象局的定位结果有较大差异，这可能是由于该地区测站网络的偏差和岩石圈结构的复杂性等原因造成的。最近，在半年内，与那国岛附近发生了四次大地震。（2001年12月18日，M7.3），石垣岛附近。（2002年3月15日，M6.6）和台湾东海岸（2002年3月31日，M7.2和2002年5月15日，M6.8）。这些地震震源带似乎沿着板块边界排列，其震源机制反映在该地区复杂的构造上。给出了精确的震源分布和地震断层面模型。并对活动的成因和构造进行了探讨。

Geology and tectonics of the region between the southwestern Ryukyu arc and the eastern Taiwan are characterized by subduction or collision of the Philippine Sea plate and Eurasian plate. The present collision boundary in Taiwan is located along the Longitudinal Valley between Hualien and Taitung, but the plate boundary of the region between the northeast off Taiwan and the southwestern Ryukyu arc is not yet clear and still controversial. In order to make clear the plate boundary or configuration and tectonics in the region, it is indispensable to study the accurate seismic activities, in other words, precise hypocenter distributions, focal mechanisms, and so on.We, therefore, have been able to obtain more accurate or detailed hypocenter distributions in this region. As the results, our hypocenter locations show a large discrepancy with those of Japan Meteorological Agency and Taiwan Central Weather Bureau, which may be due to biased station networks, complex lithospheric structures and others, in this area. Recently, within a half year, four large earthquakes occurred near Yonaguni Is. (Dec.18 2001, M7.3), near Ishigaki Is. (Mar.15 2002, M6.6), and off east coast of Taiwan (Mar.31 2002, M7.2, and May.15 2002, M6.8). These earhtquake focal zones seem to align along the plate boundary, and their focal mechanisms are reflected on a complicated tectonics in the region. We describe the precise hypocentral distribution, and fault plane model of earthquake. Also we make consideration about the cause of the activity and the tectonics.