SEISMICITY AND STRESS STATE IN THE SOUTH CHINA SEA, INDOCHINA AND THEIR VICINITY

The distribution of earthquakes from 1973 to 1982 in the South China Sea, Indochina and their vicinity was studied using the data from I.S.C. It was found that the earthquakes are mainly concentrated along the boundaries of plates. Beside that, some shallow earthquakes are distributed in the west part of Burma and along the boundary between Burma and China, with a few earthquakes occurring in the South China Sea. The features of Benioff zone along theboudaries between the Indian plate, Philippine Sea plate and Eurasian plate were studied. The plates are not coupled well under the Java trench and the Philippine trench. The subducting Indian plate under the Burmese mountain and, Andaman-Nicobar arc moves NNE. The fault plane solutions of earthquakes were studied using the first motions of P waves. The stress state on the subduction zones and within the area were deduced from the fault plane solutions and the fault movement. It was found that the direction of principal compression axis of stress is in the NNE direction in west parts of Burma and Thailand, and in the NNW or N-S direction in the South China Sea. It was also found that the stress state has close relation with the interaction between plates.