THE DISTRIBUTION OF EARTHQUAKES, BEHAVIOR OF THE SUBDUCTION ZONE AND STRESS STATE IN THE RYUKYU ISLAND ARC

The distribution and focal mechanisms of earthquakes from 1966 to 1982 in the Ryukyu island arc and the Okinawa trough were studied using the data from I. S. C. and the network of Chinese seismological stations. The behavior of the subduction zone and the stress state were discussed. The earthquakes of mb4.0 distribute mainly in an arc-like narrow belt in the west side of the Ryukyu trench and form a clear Benioff zone beneath the island arc. To the north of the Tokara channel, the subduction zone curves heavily, the dip angle is steep in the lower part, about 72; The axes of the principal tension stress are in the subduction plane, mainly coincide with the subducting direction and the axes of the principal compression stress are nearly horizontal below 70 km. To the south of the Tokara channel, the dip angle of the subduction zone is smaller, about 55, in the lower part; The axes of the principal compression stress are mostly in the subducting plane and most of the axes of the principal tension stress are nearly horizontal below 90 km. It seems that there is a double seismic zone on the subduction zone.In the Okinawa trough, the axes of principal tension stress are in the NNW direction and nearly horizontal, the axes of the principal compression stress appear to be in the NEE direction. The stress state in northern part of China is similar to that in the Okinawa trough, there is no compression effect from the Philipping sea plate on the northern part of China.