Seismic Swarm PS20050202.1: Analysis of February 2005 Activity Southeast of Port Blair

A seismic swarm designated PS20050202.1 was recorded southeast of Port Blair in the Andaman Sea, beginning at 10:35 on 1 February 2005 and concluding at 01:30 on 2 February 2005. Over 14 hours and 55 minutes, five earthquakes were registered at depths ranging from 10 to 43 km, with magnitudes between 5.3 and 5.5. This sequence provides insight into clustered seismicity within a tectonically complex region.

The events unfolded as follows. The initial pair occurred within seconds of each other on 1 February at 10:35, both registering magnitude 5.4 at depths of 12 km and 32 km. A third event followed at 13:54 with magnitude 5.3 at 43 km depth. Activity resumed early on 2 February with two closely timed shocks at 01:30, magnitudes 5.4 and 5.5 at depths of 10 km and 39 km. Such temporal clustering, with events separated by minutes to hours, characterizes swarm behavior distinct from typical mainshock-aftershock sequences.

The swarm location lies approximately 230 km southeast of Port Blair, India, within the Andaman Sea. This area forms part of the Andaman-Nicobar accretionary complex at the convergent boundary where the Indian Plate subducts beneath the Burma Plate. The subduction zone generates frequent seismicity, with the oceanic crust descending at rates of several centimeters per year. The regional geology features a volcanic arc, back-arc spreading centers, and active fault systems that accommodate oblique convergence. Historical records document elevated activity following the magnitude 9.1 Sumatra-Andaman earthquake of 26 December 2004, which ruptured a vast segment of the same plate boundary and triggered widespread aftershocks.

Since 1 January 2000, four swarms have been identified in the broader region, with the earliest occurring in 2004. Swarm PS20050202.1 represents the second such episode in early 2005. These clusters often reflect fluid migration or stress redistribution along the subduction interface and overlying crustal faults. Depths between 10 and 43 km align with the seismogenic zone where brittle failure predominates.

Analysis of this swarm underscores the persistent seismic hazard in the Andaman Sea. Continued monitoring of similar sequences aids in refining models of plate boundary behavior and assessing potential links to larger tectonic events.

References

SeismoSight internal swarm classification records for PS20050202.1.
USGS Earthquake Catalog for regional tectonic context.
Global CMT Project for subduction zone parameters.