Seismic Swarm Activity in the Nicobar Islands Region: Analysis of the August 2021 Event

The Nicobar Islands, situated in the northeastern Indian Ocean as part of the Andaman and Nicobar archipelago, lie within a tectonically active subduction zone where the Indian Plate converges with the Burma Plate. This setting produces frequent seismicity, including earthquake swarms—clusters of events occurring closely in time and space without a single dominant mainshock. The region’s geology features a complex system of thrust faults and volcanic arcs, contributing to its history of significant seismic episodes, such as the 2004 Sumatra-Andaman earthquake that generated a devastating tsunami.

On 3 August 2021, a seismic swarm designated PS20210803.1 was recorded in the Nicobar Islands, India region. Beginning at 01:51 and concluding by 03:54, the swarm encompassed five earthquakes within a span of just over two hours. The sequence included events with the following parameters: a magnitude 5.3 earthquake at 01:51:14 (depth 10 km), followed by a magnitude 5.3 at 03:42:07 (depth 10 km), a magnitude 6.1 at 03:42:38 (depth 10 km), a magnitude 5.8 at 03:43:53 (depth 10 km), and a magnitude 4.3 at 03:54:54 (depth 10 km). All events occurred at shallow depths of 10 km, consistent with activity along the plate interface.

This swarm provides insight into localized stress accumulation and release along the subduction boundary. The rapid succession of moderate-magnitude events, clustered temporally, suggests episodic fault slip possibly influenced by fluid migration or aseismic creep in the overriding plate. Such patterns align with the region’s historical behavior, where swarm activity often precedes or accompanies larger tectonic adjustments.

Historical records since 2000 indicate 14 documented swarms in the area, distributed across specific years as follows: four in 2004, five in 2005, one in 2006, one in 2009, two in 2014, and one in 2019. These occurrences underscore the recurrent nature of swarm seismicity in this subduction environment, reflecting ongoing plate motion at rates of approximately 5–6 cm per year.

Monitoring of such events supports improved understanding of seismic hazards in the Nicobar Islands, aiding regional preparedness efforts amid the potential for both local shaking and tsunami generation.

References

SeismoSight internal swarm classification data (PS20210803.1).
USGS Earthquake Catalog for regional tectonic context.
Global CMT Project for subduction zone parameters.