Seismo-tectonics of the Andaman-Nicobar Islands

Abstract

The epicentres of 345 earthquakes that occurred between January 1967 and December 1982 in a region bounded by 4°N to 16°N latitudes and 90°E to 98°E longitudes were plotted in order to study the seismicity of this region. Most of the earthquakes in this region are shallow focus, the greatest focal depth being about 250 km. In the northern part of this region, north of 10.5°N, tne earthquake foci are aligned along two branches, which strike northeastwards. No intermediate depth earthquakes are associated with the eastern branch. South of 10.5°N, the foci are located along a zone striking north-northwest. Focal mechanism solutions are reported for 24 earthquakes that occurred in this region during this period. The first motions of P and PKP phases were used to determine the mechanism solutions. Five of these events occurred in northern Sumatra and the rest occurred in the Andaman-Nicobar Islands. The mechanism solutions of the shallow focus earthquakes in northern Sumatra are characterized by a predominant component of thrust and strike-slip faulting. The inferred orientation of slip vectors deduced from mechanisms of shallow focus events is nearly northeast. Mechanism solutions of 13 shallow focus events that occurred in the Andaman-Nicobar Islands are characterized by thrust, normal and strike-slip faulting. The orientation of slip vectors deduced from thrusting mechanisms is consistent with the underthrusting of the Indian Ocean under the Andaman-Nicobar Islands in a northeasterly direction. Normal faulting mechanism solutions of these islands suggest that they are tectonically similar to island arcs. The results reported in this study show for the first time, using mechanism solutions, the existence of underthrusting along the Andaman-Nicobar Islands, for which there is ample geological evidence.