著者

渡部 豪 田部井 隆雄

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.57, no.1, pp.1-10, 2004-08-25 (Released:2010-03-11)

参考文献数

23

被引用文献数

1 4

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The Ryukyu Arc is a NE-SW trending island arc that connects southwest Japan and Taiwan. The Philippine Sea plate subducts at the Ryukyu Trench, the southeastern boundary of the Ryukyu Arc. While the plate model predicts a rather high subduction rate (60-80mm/yr), plate coupling has been estimated so small from the earthquake data. Another factor characterizing tectonic features of the region is an active backarc opening at the Okinawa Trough that forms the northwestern boundary of the region. Japanese nationwide continuous GPS array has illustrated a trenchward motion of the Ryukyu Arc. However, it is difficult to quantify the effects of the subduction and the backarc opening because there is little deployment of the GPS network in the direction perpendicular to the strikes of those two boundaries. To model tectonic movement of the region, at first we divide the region into four crustal blocks based on horizontal GPS velocities and geological conditions. As the first approximation, plate coupling is neglected. Crustal velocities predicted from the block motions are in good agreement with the observed ones with an average discrepancy of about 3mm/yr. Next, we introduce moment tensor data of shallow earthquakes to calculate crustal strain rates in several segments and further estimate spatial variation of the plate coupling. The results show that the estimated plate coupling is smaller than 10% in most segments except 50% in the northernmost part. We interpret that the plate coupling at the Ryukyu Trench is so small and the trench can behave as a free boundary. Thus if a backarc opening occurs at the Okinawa Trough, the Ryukyu Arc can easily move trenchward without significant internal deformation. However, detailed mechanism of the backarc opening remains still unknown and is an important problem to be resolved in the future.