Gas migration signatures over the volcanic cratered seamount, off the Nicobar Islands in the Andaman Sea

Abstract

The submarine volcanic arc off-Nicobar is seismologically very active with numerous earthquake swarms reported after the tsunamigenic 26th December 2004 earthquake in the Andaman-Nicobar-Sumatra subduction zone. Therefore, we carried out multiple high-resolution multibeam echo-sounding (MBES) surveys along with water column imaging (WCI) onboard RV Sindhu Sadhana along the volcanic arc to detect water column anomalies related to volcanism. We observed for the first time two gas flares originating from the outer flanks of cratered seamount (CSM) in the WCI data during the 2018 expedition. The flare on the SE flank of CSM originated at a depth of 710 m and rises to a depth of ~225 m from the ocean surface, whereas the flare on the NW flank of CSM originates at 400 m and rises to ~150-100 m from the ocean surface. We repeated the experiment in November 2021 and remapped the flare on the NW flank of CSM. Furthermore, a new gas flare was observed in a southward direction at an offset of 215 m. This flare which originated at 380 m and rises to 150 m from the ocean surface was imaged during multiple passes along different radial directions at different times. Geological sampling over the gas venting location shows a large rock sample with a couple of live Bathymodiolus spp., a chemosynthetic species typically observed at methane seeps and/or hydrothermal vent sites. These observations confirm the volcanic activity at the cratered seamount located off the Nicobar region in the Andaman Sea.