Seismic Swarm PS20041226.5: Analysis of Activity in the Nicobar Islands Region

The Nicobar Islands lie along the convergent boundary between the Indian Plate and the Burma Plate, where oblique subduction drives frequent seismic activity. This tectonic setting forms part of the larger Andaman-Sumatra subduction zone, characterized by thrust faulting and occasional strike-slip motion. The region has experienced repeated large-magnitude events due to the accumulation and release of strain along the plate interface.

Seismic swarm PS20041226.5 began at 01:22 on 26 December 2004 and concluded at 12:48 on 27 December 2004, lasting 35 hours and 25 minutes. During this interval, 20 earthquakes were recorded, with magnitudes ranging from 5.0 to 6.1 and focal depths between 12 km and 47 km. The sequence occurred entirely within the Nicobar Islands, India region.

The initial event reached magnitude 6.0 at a depth of 30 km. Subsequent shocks included a magnitude 6.1 event at 02:00 on 26 December at 33 km depth, followed by multiple magnitude 5.4–5.5 events clustered between 01:33 and 10:32 that same day. Later activity on 26 December featured events at shallower depths, including a magnitude 5.3 shock at only 12 km. On 27 December, three additional events occurred, the final one at magnitude 5.3 and 33 km depth.

This swarm represents the sole documented swarm in the Nicobar Islands since 1 January 2000. No prior swarms appear in the historical record for the region after that date, underscoring the unusual nature of the December 2004 sequence within the available instrumental catalog.

Geological context indicates that the Nicobar Islands sit atop the Andaman-Nicobar ridge, an accretionary complex shaped by long-term subduction. Historical seismicity includes great earthquakes in 1881 and 1941, both of which generated tsunamis affecting the islands. The 2004 swarm timing coincides with the immediate post-rupture phase of the Mw 9.1 Sumatra-Andaman earthquake, though the events are classified here strictly as swarm activity under internal SeismoSight criteria.

Swarm sequences such as PS20041226.5 typically reflect fluid migration or localized stress adjustments rather than mainshock-aftershock patterns. Depths predominantly between 20 km and 35 km align with the expected range for interface and intraslab events in this subduction environment. The concentration of 20 events within 35 hours illustrates a rapid energy release rate compared with background seismicity.

Continued monitoring of the Nicobar segment remains essential given the subduction zone's capacity for Mw 8+ earthquakes. Updated catalogs from global networks confirm ongoing low-level activity, yet no additional swarms matching the 2004 parameters have been identified since.

References

USGS Earthquake Catalog
Global CMT Project
Indian Meteorological Department Seismological Reports