Okinawa, which is located in the southwest part of Japan, is tectonically very active. Ryukyu subduction zone is characterized by the subducting Philippine Sea plate beneath the Eurasian plate and by crustal extension at the back of the arc in the Okinawa Trough. Historically, Okinawa region have been ex-perienced the severe damage of earthquake and tsunami several times. At the present day, a great numbers of seismographs are deployed on Ryukyu arc islands by Japan Meteorological Agency (JMA) and other au-thorities to monitor the seismic activities for the purpose of disaster mitigation and various researches.
To comprehend the topographic feature in Okinawa, it is essential to elucidate the tectonics of the Ry-ukyu subduction zone. In this paper, we describe the overview of the seismic activity associated with the tectonics and the stress field in the Ryukyu subduction zone briefly.
Seismic activity in Okinawa region is very active over the whole area. More than 15,000 earth-quakes occurred in or around the Okinawa region in 2017 according to Japan Meteorological Agency (JMA) and it accounted for about 10% of total number of earthquake occurred in Japan (Fig.1). The distribution of hypocenters demonstrate that the earthquakes occurred the shallow portion (depth<30km) are located toward the Ryukyu trench side. On the other hand, the earthquakes occurred the considerably deep portion is visible as going to continental side. It suggests that the Philippine Sea plate is subducting beneath the Eurasian Plate and the earthquakes occur along the subducting plate. The seismic stations in the Okinawa region are limited on the islands arc due to the topographical features. Considering that large numbers of earthquakes are determined in the Okinawa region is fully related to extreme seismic activities in spite of poor detection capability.
