Seismological evidence of active faults in the offshore Nicobar region, Andaman Sea: insights from ocean-bottom seismometer data

After the 26 December 2004 catastrophic megathrust earthquake, the offshore Nicobar region experienced recurring earthquake swarms in 2005, 2014, 2015, and 2019, highlighting persistent tectonic instability and episodic fault reactivation. To better understand the seismically active faults and magma dynamics beneath the region, we conducted a passive Ocean Bottom Seismometer (OBS) experiment in the eastern Nicobar offshore area of the Andaman Sea, from 10 October 2023 to 20 January 2024. Eight broadband OBS instruments were deployed, each comprising four sensors (X, Y, Z, and pressure), along with five short-period Ocean Bottom Geophones (OBG). In this study, we present seismicity and seismological evidence of active faults in the offshore Nicobar region using local earthquake data from the OBS network. We located 588 earthquakes (ML 1.0–4.5), and the spatial distribution of relocated epicenters indicates that the following faults are active: the nearly north–south-trending Andaman Nicobar Fault (ANF), the northwest–southeast-oriented Seulimeum Fault (SLF), a northern strand of the Great Sumatra Fault, and a north–south-trending fault (T1). The N–S fault (T1) is associated with a small pull-apart basin. This basin may have formed due to the right-stepping geometry of the SLF. Seismicity patterns reveal that the ANF remains active from east of the Nicobar Islands to the Andaman Islands, and the SLF extends northward to about 8°N, coinciding with the volcanic arc. Hypocentral depth distribution indicates that lithospheric-scale ANF and SLF collectively accommodate tectonic strain and govern the regional deformation regime.