Seismic Swarm PS20041226.11: Analysis of Activity South of Port Blair

Seismic swarm PS20041226.11 was recorded on 26 December 2004 in the region 121 km south of Port Blair, India. The sequence began at 01:52 and concluded at 15:23, encompassing five earthquakes over a period of 13 hours and 30 minutes. This event provides insight into clustered seismic behavior along the Andaman segment of the Sunda subduction zone.

The Andaman-Nicobar archipelago lies at the convergent boundary where the Indian Plate subducts beneath the Burma Plate. This tectonic setting produces frequent seismicity, with the Sunda Trench marking the primary interface. Depths of the recorded events ranged between 12 km and 30 km, consistent with activity in the upper portion of the subducting slab and overlying crust. The magnitudes varied from 4.6 to 5.5, indicating moderate energy release distributed across multiple faults rather than a single large rupture.

The sequence opened with a 5.2 magnitude event at 12 km depth, followed by a 5.3 magnitude shock at 29 km. Subsequent events included magnitudes of 5.4 at 26 km and 5.5 at 23 km, before the final 4.6 magnitude earthquake at 30 km depth. Such temporal clustering without a dominant mainshock followed by clear aftershock decay aligns with swarm characteristics, where fluid migration or stress redistribution along the plate interface may trigger successive failures.

The Andaman region has experienced significant historical earthquakes, including the great 2004 Sumatra-Andaman event of magnitude 9.1 that initiated only hours earlier on the same day. The swarm location sits within the rupture zone of that megathrust earthquake, suggesting the activity reflects localized adjustment along the plate boundary. Since 1 January 2000, only two swarms have been documented in the broader area according to SeismoSight internal classification, with this 2004 sequence representing the first.

Geological studies confirm that the Andaman Islands formed through accretionary processes associated with prolonged subduction since the Mesozoic. The current convergence rate of approximately 5 cm per year sustains elevated seismic hazard. Depths in the 12–30 km range for the swarm events correspond to the seismogenic zone where brittle failure predominates before transition to ductile behavior at greater depths.

Analysis of swarm patterns in subduction zones indicates they often occur in areas of heterogeneous stress or fluid presence. The five events of PS20041226.11 illustrate this distributed release, with no single shock exceeding 5.5. Continued monitoring of similar sequences contributes to refined understanding of preparatory processes ahead of larger earthquakes along the Sunda margin.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog for regional tectonic parameters.
Global CMT catalog for focal mechanism context in the Andaman Sea.