Seismic Swarm Activity in the Nicobar Islands Region: January 2005

The Nicobar Islands lie along the active Sunda subduction zone, where the Indian plate converges with the Burma microplate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes and occasional seismic swarms as stress is redistributed along the plate interface and associated crustal faults.

Between 14:29 UTC on 1 January 2005 and 15:35 UTC on 2 January 2005, a swarm of six earthquakes was recorded in the Nicobar Islands region. The sequence lasted 25 hours and 6 minutes and comprised the following events:

• 1 January 2005, 14:29:11 UTC – magnitude 5.5 at 27 km depth
• 1 January 2005, 22:23:15 UTC – magnitude 5.4 at 33 km depth
• 1 January 2005, 22:23:18 UTC – magnitude 5.1 at 48 km depth
• 1 January 2005, 22:28:13 UTC – magnitude 5.5 at 10 km depth
• 1 January 2005, 22:28:15 UTC – magnitude 5.6 at 33 km depth
• 2 January 2005, 15:35:58 UTC – magnitude 5.9 at 33 km depth

All events occurred within a compact area near the island chain, consistent with post-mainshock adjustment following the great 26 December 2004 Sumatra–Andaman earthquake (Mw 9.1–9.3). A magnitude 7.2 event located 284 km west-northwest of Sabang, Indonesia—approximately 32 km from the swarm centroid—occurred on the same day as the mainshock and likely contributed to local stress changes.

Since 1 January 2000, four seismic swarms have been documented in the broader region, with the first recorded in 2004. Such swarms are characteristic of the Nicobar segment, where the subduction interface transitions from fully locked to partially creeping behavior. Depths between 10 km and 48 km place the activity within both the overriding plate and the upper portion of the subducting slab, typical for this margin.

The January 2005 swarm illustrates the prolonged aftershock phase that followed the 2004 megathrust rupture. Clusters of events with magnitudes 5.1–5.9 over short time windows reflect brittle failure on subsidiary faults as the crust accommodates residual strain. No surface rupture or significant tsunami was associated with this particular sequence.

References
USGS Earthquake Catalog (2005 events and regional tectonics)
Global CMT Catalog (focal mechanisms and depths)
India Meteorological Department Seismological Bulletin (Nicobar regional monitoring)