JOURNAL GEOLOGICAL SOCIETY OF INDIA Vo1.49. May 1997, pp. 513-521 Aseismicity of the Andaman Subduction Zone and Recent Volcanism SUJlT DASGUPTA· AND MAN01 MUKHOPADHYAY·· "Geological Survey of India, 27, J.L. Nehru Road, Calcutta - 700 016 ··Department of Applied Geophysics, Indian School of Mines, Dhanbad - 826 004 Abstract: The Barren Island volcano in central Andaman Sea erupted afresh on March 29, 1991, since its last eruptions in 1789 and 1832. Tectonic framework of the Barren Island volcano is delineated by a set of seismically mapped faults, including the West Andaman Fault, which. connects to the Semangko Fault in Sumatra. The morphology of Andaman Benioff zone as seen on a vertical section reveals a seismic gap at 90-110 km depth. Spatially it relates to the zone of current volcanism of the Barren and its neighbouring Alcock Seamount in a structurally depressed area. The seismic gap in the Benioff zone is interpreted as a zone of partial melting in the descending Indian Ocean lithosphere which prevents stress accumulation owing to the decreased viscosity. Best double couple solutions for the Barren Island earthquakes of October 10 and December 25, 1990, which preceded the volcanism suggest that rupturing through normal dip-slip faulting prevailed in the overriding Burma plate prior to the recent volcanism. Pronounced back-arc seismicity at shallower depths is seen further to the east and southeast of the Alcock Seamount; this relates to back-arc rifting under the Andaman Sea. Keywords: Seismicity, Andaman, Barren Island, Volcanism. INTRODUCTION Subduction zone morphology. if detailed through distribution of earthquakes located by teleseismic, regional or local network can provide valuable additional informations on the subduction process (Isacks et al. 1968; Hanus and Vanek, 1978; Dasgupta and Mukhopadhyay, 1993. Protti et al. 1994), although only seismically active parts of a consumptive margin are investigated by this· method. A more complete infonnation about the shape of subducting margin or on the characteristics of magmatic source zones can be obtained through seismic wave propagation studies whereby it is possible to investigate the volcanism-related low-velocity anomalous zones in the mantle-wedge to compliment the knowledge on the general seismological char- acter of subduction zones (cf. Zhao and Hasegawa, 1994, Zhao et al. 1994). The Andaman arc in northeastern Indian Ocean defines a nearly 1100 km long active plate margin, where, the Western Pacific arc system through its continuation into the Sunda arc gradually transists northward along the strike to young fold belts ofB urma (Sheinmann, 1971; Katili, 1975; Hamilton, 1979). In the present work we examine the morphology of the Andaman Benioff zone in order to correlate the current seismicity with the recent Barren Island volcanism in central Andaman Sea. Results for some 17 earthquake focal mecha- nisms (Centroid moment tensorlbest double couple solutions) are reviewed to investigate the faulting and stress pattern operative in the . overriding and downgoing plates in the region at the confluence of the Andaman magmatic arc. PLATE TECTONIC SETTING Currently active subduction of the Indian Ocean lithosphere below the Andaman arc is primarily documented by the presence of : (i) an east-dipping Benioff zone down to nearly 200 km focal depth (Mukhopadhyay, 1984); 0016-7622/97-49-5-513/$ 1.00 © GEOL. soc. INDIA