Seismic Swarm PS20050725.1: Nicobar Islands, India Region

The Nicobar Islands lie along the convergent margin where the Indian Plate subducts beneath the Burma Plate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes and occasional earthquake swarms within the overriding plate and along the megathrust interface. The islands form part of the Andaman–Nicobar ridge, an accretionary complex shaped by long-term subduction and occasional back-arc extension.

Between 15:42 UTC on 24 July 2005 and 03:26 UTC on 25 July 2005, a swarm of six earthquakes was recorded in the Nicobar Islands region. The sequence began with a magnitude 7.2 event at 16 km depth, followed within minutes by two magnitude 5.1 shocks at depths of 19 km and 30 km. Roughly ten hours later, three additional events occurred: a magnitude 5.4 at 33 km, a magnitude 5.0 at 30 km, and a magnitude 5.6 at 31 km. All six events were located within a compact area, consistent with a short-lived swarm rather than a classic mainshock–aftershock sequence.

Since 1 January 2000, four swarms have been documented in the same region, with three occurring in 2004 and one in 2005. The 2005 swarm coincided temporally with the magnitude 7.2 main event and was situated 21 km from the epicenter of that shock. A separate magnitude 7.5 earthquake struck 17 km from the swarm center on 12 June 2010, underscoring the persistent seismic productivity of the area.

The Nicobar Islands experienced elevated seismicity following the great 2004 Sumatra–Andaman earthquake, whose rupture extended southward into the Nicobar segment of the megathrust. Subsequent stress redistribution likely contributed to the swarm activity observed in 2004 and 2005. Depths of the 2005 swarm events ranged from 16 km to 33 km, placing them within the seismogenic zone of the subducting slab and the overlying crust.

Continued monitoring by regional and global networks remains essential for understanding swarm recurrence and potential links to larger megathrust events in this tectonically active corridor.

References

• USGS Earthquake Catalog
• Incorporated Research Institutions for Seismology (IRIS) Event Search
• Tectonic framework descriptions from peer-reviewed literature on the Andaman–Nicobar subduction zone