Aseismicity of the Andaman Subduction Zone and Recent Volcanism

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.