Crustal structure beneath the Alcock Seamount in the Andaman Sea

Alcock and Sewell seamounts are important geomorphic features in the Andaman Sea due to their association with the tectonic history of the Sunda subduction zone. Despite the availability of global geophysical data, the nature of crust beneath these seamounts remains debated due to the paucity of local geophysical data. Therefore, a passive ocean bottom seismometer (OBS) experiment was conducted by deploying broadband OBSs in the Andaman region from 25th December 2013 to 10th May 2014. Based on the local earthquakes recorded by OBS, we attempted to analyse the waveforms of the events using the state-of-the-art transfer function, and estimated the crustal velocities beneath the Alcock seamount. The inferred P wave velocity increases from 5.7 km/s to 7.3 km/s at a depth of 9 km and from 7.3 km/s to 8.1 km/s at a depth of 15 km. We validated the upper velocity model using an active OBS record obtained from a concurrent multichannel seismic survey acquired onboard RV Samudra Ratnakar. Further, we carried out traveltime streamer tomography by downward continuing the shot gathers of MCS data and obtaining high-resolution velocity-depth models over two shallow ridges on the Alcock Seamount. Tomography-derived velocity-depth model matches with that estimated from the active and passive OBS records. The presence of high subsurface velocities beneath the Alcock Seamount suggests the occurrence of extensive volcanoclastic deposits, possibly formed as a result of the magmatic intrusion. Higher velocities for deep crustal layers suggest an oceanic or island arc crust beneath the Alcock Seamount, which has probably evolved due to backarc volcanism in the Andaman Sea.