A SEISMOTECTONIC STUDY OF THE BURMA AND ANDAMAN ARC REGIONS USING CENTROID-MOMENT TENSOR DATA

The concept of a ''mean slip angle'' is introduced. This enables a cla ssification of focal mechanisms in any region into predominantly strik e-slip, thrust and normal categories. Based on this concept, the Harva rd Centroid Moment Tensor (CMT) data, in the Burma and Andaman are reg ions from 1977 to 1992, comprising 167 focal mechanism solutions, are examined and categorized. Distinct trends on the surface and in depth sections emerge on examination of these categories. For instance, in t he Burmese are there is a clear segregation along the slab between str ike-slip type mechanisms down to a depth of about 90 km and thrust eve nts which occur exclusively below this depth. In addition, a study of P- and T-axis orientations indicates that the stress pattern in the su bducted slab is different from that farther east. Whereas the P axes o f the thrust and strike-slip mechanisms in the slab show a predominant ly NNE trend, commensurate with the direction of motion of the Indian plate with respect to the Eurasian plate, those in the region to the e ast, show, interestingly, an average E-W-oriented compressive regime. The northern and southern parts of the Andaman are region exhibit dist inct tectonic patterns. Whereas in the southern part the disposition o f focal mechanisms along the slab and the P- and T-axis orientations i ndicate active subduction, the mechanisms in the northern part exhibit a peculiar segregation into clusters of thrust, normal and strike-sli p types, without conforming to the local trend of the are. A similar s tudy in the western Aleutian trench region indicates a much simpler su bduction pattern.