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Subduction zones are generally characterized by well-defined inclined seismic zones extending, in some cases, down to about 670 km deep beneath the Earth. The Sumatra subduction zone is characterized by the Indo-Australia Plate subducting beneath the Sunda plate and Andaman micro plate about 55 mm/yr, causing seismic activity along the plate boundary. Seismic activity of the shallow part of Sumatra subduction zone near the Sunda and Andaman trenches and outer-rise region was analyzed by using earthquake locations and their focal mechanisms to study the seismic tectonic of the region. The study region was divided into five sub-regions and in each sub region, the focal mechanisms were analyzed according to the depth variation of bathymetry. The distribution of pressure axes, Tensional axes and Null axes of focal mechanisms were investigated. The results of the study can be concluded as given below. Present study shows that normal faulting events are recorded than the reverse faulting events in the outer-rise region. In the near trench of the region, reverse faulting events were observed more than the normal faulting events and more reverse faulting events were observed in the shallow part of the trench. Although only the focal mechanism solutions of large events were used for the analysis which may have location errors, the present study results reasonably agree with the results obtained by the other subduction zones. Patterns of hypocenter distribution and focal mechanisms found in this study are almost the same as that found under the near-trench slopes of other subduction zones by previous investigators. This characteristics of focal mechanisms may be due to the bending of the subdcting plate near the Sunda and Andaman Trenches. KEYWORDS: Seismicity and tectonics, Indo-Australia plate, Sumatra subduction zone, Intra plate process, offshore Sri Lanka

Seismic Activity near the Sunda and Andaman Trenches in the Sumatra Subduction Zone