Seismic Swarm PS20041226.4 Near Bamboo Flat, India

A seismic swarm designated PS20041226.4 was recorded northwest of Bamboo Flat in India's Andaman Islands. The sequence began at 02:15 on 26 December 2004 and concluded at 14:08 on 28 December 2004, spanning 59 hours and 52 minutes. During this interval, 14 earthquakes were registered at a location 61 km northwest of Bamboo Flat.

The events exhibited magnitudes between 5.1 and 5.8, with focal depths ranging from 5 km to 38 km. Earliest shocks reached 5.7 and 5.8 within the first hour, followed by additional events of 5.3–5.8 through 26 December. Activity continued at lower frequency on 27 December with magnitudes of 5.1–5.6, and the final event of 5.3 occurred on 28 December. Depths remained predominantly in the upper to mid-crust, consistent with regional tectonic patterns.

This swarm represents the first of two sequences documented since 1 January 2000 in the broader Andaman region. The 2004 activity occurred within a convergent margin where the Indian Plate subducts beneath the Burma Plate along the Andaman Trench. The islands form part of an accretionary complex shaped by long-term oblique subduction, producing thrust faults, strike-slip structures, and volcanic arcs. Historical seismicity reflects both great megathrust ruptures and moderate crustal events at depths typically less than 40 km.

The December 2004 timing aligns with heightened regional strain release following the magnitude 9.1–9.3 Sumatra-Andaman earthquake of 26 December 2004. Aftershock sequences and triggered swarms commonly occur along adjacent segments of the plate boundary in response to static and dynamic stress changes. Depths recorded in the swarm fall within the expected range for interplate and intraplate aftershocks in this setting.

Geological mapping of the Andaman Islands shows exposed ophiolites, sedimentary basins, and coral reefs uplifted by repeated coseismic deformation. Paleoseismic studies indicate recurrence intervals of several hundred years for great earthquakes, with the 2004 event representing one of the largest instrumentally recorded ruptures. Post-2004 geodetic measurements continue to document viscoelastic relaxation and afterslip along the megathrust.

The PS20041226.4 swarm illustrates typical swarm characteristics: clustered occurrence without a single dominant mainshock, rapid onset, and gradual decay. Such sequences provide data on fault interaction and stress transfer in subduction zones. Continued monitoring supports improved understanding of seismic hazard in the Andaman-Nicobar region, where population centers remain exposed to both strong ground shaking and tsunami risk.

References

United States Geological Survey Earthquake Catalog
SeismoSight internal swarm classification records
Andaman and Nicobar Islands geological survey reports