Seismotectonic evaluation of off Nicobar earthquake swarms, Andaman Sea

Abstract

The Andaman Sea has experienced a number of earthquake swarms as a result of complex tectonics arising from oblique subduction, active volcanic arc, the backarc spreading center, and sliver fault systems. One of the most energetic offshore earthquake swarms occurred in the off Nicobar region of the Andaman Sea in January 2005, following the 26th December 2004 (9.1 Mw) Sumatra megathrust event. After a brief quiescence, this region got reactivated again after the 21st March 2014 (6.5 Mw) event, followed by the occurrence of earthquake swarms in March 2014, October 2014, November 2015, and April 2019. In the present study, we analysed the temporal variation of the b-value of these swarms using the global network data and the Ocean Bottom Seismometer data to understand their genesis. Temporal variation of b-value suggests that b-values are larger than unity for earthquake swarms, indicating volcanic origin. Bimodal distribution of frequency magnitude relation suggests that the earthquake swarms have occurred due to complex seismic processes controlled by both tectonic and volcanic activities. We propose that a combination of magmatic pulsation in the arc volcanism in response to 2004 and 2005 megathrust events and the 6.5 Mw magnitude 21st March 2014 event in off Nicobar region, and reactivation of sliver fault systems, are the dominant mechanisms for the observed frequent earthquake swarms.