Seismic Swarm PS20041229.1: Analysis of Activity in the Andaman Subduction Zone

The seismic swarm designated PS20041229.1 occurred between 22:16 UTC on 28 December 2004 and 11:58 UTC on 29 December 2004, approximately 137 km north of Bamboo Flat in India’s Andaman and Nicobar Islands. Within a span of 13 hours and 41 minutes, six earthquakes were recorded, with magnitudes ranging from 4.6 to 5.7 and focal depths between 20 km and 33 km. This sequence represents the sole swarm event documented in the region since 1 January 2000.

The Andaman Islands lie along the convergent boundary where the Indian Plate subducts beneath the Burma Plate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent seismicity, including megathrust events and associated aftershock sequences. The December 2004 swarm took place two days after the Mw 9.1–9.3 Sumatra–Andaman earthquake of 26 December 2004, whose rupture extended northward along the plate interface into the Andaman segment. The swarm’s timing and location are consistent with post-mainshock stress redistribution in the overriding plate and along subsidiary faults.

Earthquake depths recorded during the swarm fall within the typical range for crustal and upper-plate events in this arc system. The largest event reached magnitude 5.7 at 33 km depth, while shallower events near 20 km suggest activity on structures above the main subduction interface. Such swarms can reflect fluid migration, aseismic slip, or triggered failure on secondary faults following a major rupture.

Historical context underscores the region’s elevated seismic hazard. The 2004 megathrust event remains the dominant modern benchmark, having generated widespread coseismic deformation and a devastating tsunami across the Indian Ocean basin. A subsequent strong earthquake of magnitude 7.5 occurred on 10 August 2009, centered 266 km north of Bamboo Flat and approximately 64 km from the 2004 swarm centroid. This event further illustrates ongoing strain accumulation and release along the arc.

No additional swarms have been identified in the local catalog since 2000, indicating that clustered activity of this type is infrequent relative to typical aftershock decay patterns. Continued monitoring by regional and global networks remains essential for distinguishing between isolated sequences and potential precursors to larger events in this tectonically complex margin.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Indian Meteorological Department Seismological Reports