Investigation of Andaman Sea using seismic data and gravity modelling

On December 26, 2004, a 9.1 Mw megathrust earthquake struck along a 1300 km rupture extending from Sumatra to the Andaman-Nicobar region. This event occurred along the Sunda subduction zone, where the Indo-Australian Plate subducts obliquely beneath the Southeast Asian Plate. The oblique convergence has resulted in a sliver fault system comprising the Sagaing Fault, Andaman Sea Transform Fault (ASTF), Andaman Sea Spreading Center (ASSC), Andaman Nicobar Fault (ANF), West Andaman Fault (WAF), and Great Sumatra Fault (GSF). Key morphotectonic features in this region include the volcanic arc hosting Barren Island (BI) and Narcondam Island (NI) and the volcanic-origin Alcock Rise (AR). Additional significant faults include the Diligent Fault (DF), East Marginal Fault (EMF), and Cocos Fault (CF). The ANF, an active strike-slip fault north of the WAF, significantly influences basin morphology and generates earthquakes above 10°N latitude. This study focused on (1) analyzing the geometry and impact of ANF branches on basin morphology and (2) understanding the crustal architecture and the role of underplating in the Andaman volcanics. Three 2D seismic reflection lines between AR and NI revealed a positive flower structure in the basin, indicating the presence of an ANF branch. Fluid evidence was identified within a ~90 km² area at ~650 m depth below the seafloor through velocity, polarity, Q attenuation, and AVA analyses, although well data is unavailable to confirm the fluid type. The findings suggest that fluid migration is influenced by the crustal-scale ANF and associated depocenter variations.

To further explore the crustal architecture beneath NI, BI, and AR, four gravity profiles were extracted from satellite-derived free-air gravity data, followed by forward gravity modeling. The Moho depths beneath BI and NI were found to be ~17.67 km and ~17.58 km, respectively. Beneath AR, the Moho depth varies from 16.4 km to 17 km, reaching 19.4 km north of AR and Narcondam, connecting to the Burma region. The thickness of the underplated layer ranges from 1.5 to 2.7 km beneath AR and is less than 2 km beneath NI. This underplated layer beneath AR likely originates from the magma chamber associated with the Andaman Sea Spreading Center.