Seismic Swarm PS20041226.7: Analysis of Activity in the Andaman Sea Region

On 26 December 2004, a seismic swarm designated PS20041226.7 was recorded approximately 291 km south-southeast of Port Blair, India, in the Andaman Sea. The sequence began at 01:30 and concluded at 12:52 local time, encompassing 16 earthquakes over 11 hours and 22 minutes. This event occurred within a tectonically active zone characterized by ongoing subduction along the Sunda Trench.

The Andaman Sea lies at the convergent boundary between the Indian plate and the Burma microplate, part of the broader Sunda subduction system. Here, the Indian plate descends beneath the overriding plate at rates of several centimeters per year, generating frequent seismic activity at intermediate depths. The regional geology features a complex arrangement of back-arc spreading centers, strike-slip faults, and volcanic arcs that accommodate oblique convergence. Historical records indicate that this setting has produced great earthquakes, with the 2004 Sumatra-Andaman mainshock representing one of the most significant releases of strain in the modern instrumental era.

All events in the swarm exhibited magnitudes between 4.8 and 5.7 and focal depths ranging from 22 to 38 km. The sequence opened with a magnitude 5.5 event at 30 km depth, followed by additional shocks clustered tightly in both time and space. Notable entries include a magnitude 5.7 earthquake at 06:02 and two closely spaced events near 06:59 reaching 5.4 and 5.6. Later activity featured a magnitude 5.5 at 10:56 before the final magnitude 5.1 at 12:52. Depths remained relatively consistent, suggesting rupture within the subducting slab or along the plate interface rather than shallow crustal faults.

Such swarm behavior reflects rapid stress redistribution following a major rupture. The tight temporal clustering and moderate magnitudes indicate a cascade of afterslip and triggered failure rather than a single large mainshock-aftershock pattern. Depths in the 20–40 km range align with the expected seismogenic zone for this segment of the subduction interface, where hydrated oceanic crust facilitates brittle failure.

Since 1 January 2000, only two swarms have been identified in the broader region, underscoring the relative rarity of this type of clustered activity. The 2004 sequence marked the first such episode in the catalog. These statistics highlight the dominant role of isolated large events over prolonged swarm activity in the seismic budget of the Andaman segment.

The swarm provides valuable insight into post-rupture processes along the Sunda margin. Continued monitoring of similar sequences can improve understanding of how strain is accommodated after great earthquakes and refine probabilistic assessments for the area.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (historical regional events).
Global CMT Project (focal mechanism context for Andaman Sea).