CHARACTERISTICS OF EARTHQUAKE DISTRIBUTION AND THE MECHANISM OF EARTHQUAKES IN THE BOUNDARY AREA BETWEEN BURMA, INDIA AND CHINA

The spatial distribution was studied of the earthquakes of mb4.0 which oceured in the region from 20Nto 30Nand from 90Eto 104Efor the period from January 1965 to October 1981. It is seen that the earthquakes mainly oceured in the regions near the northwest, northeast and southeast boundaries of the Assam massif and along some faults which are concave towards the Assam massif, forming some belts or clusters of earthquakes. Among them there is the highest densty of earthquake in the Burmese arc, southeast side of the Assam massif, nearly all of the earthquakes of intermediate focus oceured in this area. The focuses of the earthquakes become deeper gradually from west to east forming an obliqae zone located from 20 Nto 26 Nbeneath the Burmese central belt. South of 24 Nthe zone dips due east at an angle of about 30; north of 24 Nthe oblique zone dips in the direction S70Eat an angle of about 50, the thickness of the oblique zone is 20-30km. It seems that the oblique zone becomes two thin layers each with depths varying from 50 km to 110 km and separation of about 30 km.Fault plane solutions for 38 earthquakes occurring in the same period have been determined. The fault plane solutions are mainly strike-slip in the Burmese are; the fault plane solutions showing thrusts with strike-slip component appear in the area near the northwest and the northwest and the northeast boundaries of the Assam massif. On the oblique zone of earthquakes the fault plaice solutions seem to vary with depth: They are strike-slip above the depth of 50 km; in the depths from 50 km to 70 km the fault plane solutions show mainly normal fault, but most of them are strike-slip with thrust or normal compoment in the deoths from 50 km to 120 km. Fault plane solutions of only two earthquakes with focuses below 120km have been determined, they mainly represent thrust faults. The pressure axes of the fault plane solutions are mainly in the direction NEand nearly horizontal in the nqrthern part of Burma and the vicinity of the Assam massif; it indicates that in this area the Indian plate collides with the Eurasian plate in the direction toward NE.