Sub-surface magma movement inferred from low-frequency seismic events in the off-Nicobar region, Andaman Sea

Monitoring volcanic activity along submarine volcanoes is challenging. We present Ocean Bottom Seismometer (OBS) data that provide geophysical evidence of subsurface magmatism along submarine volcanoes off Nicobar, Andaman Sea. For the first time in this region, hybrid very long-period earthquakes (VLPEs) were documented by a passive OBS experiment. These events initiate with high-frequency (5–10 Hz) signals with a clear P-phase onset, followed by low-frequency (0.01–0.5 Hz) oscillations lasting 300–600 s, and include a prominent high-frequency (10–40 Hz) hydro-acoustic phase. A total of 141 high-frequency events were detected on 21–22 March 2014, of which 71 exhibited low-frequency oscillations. Events are distributed NW–SE along the submarine volcanic arc and the Seulimeum strand of the Great Sumatra Fault. The off Nicobar region has experienced frequent earthquake swarms since the 26 December 2004 tsunamigenic Sumatra earthquake (January 2005, March and October 2014, November 2015, March 2019). The occurrence of low-frequency earthquakes with prominent hydro-acoustic phases suggests subsurface tectonic and magmatic influences. We propose that upward migration of magma pulses from a deeper reservoir to a shallow magma chamber activated strike-slip movement on sliver faults, inducing earthquake swarms in the off Nicobar region.