Seismic Swarm PS20060309.1 Near Port Blair: Geological Context and Event Analysis

The seismic swarm designated PS20060309.1 occurred approximately 196 km east-southeast of Port Blair in the Andaman Islands, India. Registered between 14:57 on 9 March 2006 and 12:03 on 10 March 2006, the sequence comprised 18 earthquakes within a span of 21 hours and 6 minutes. Magnitudes ranged from 5.0 to 5.4, with focal depths predominantly between 10 km and 30 km, consistent with activity along crustal fault segments in a subduction-influenced setting.

This event unfolded in a tectonically complex region where the Indian Plate converges with the Burma Plate along the Andaman-Nicobar segment of the Sunda subduction zone. The Andaman Islands occupy an accretionary prism formed by oblique subduction, which generates both megathrust earthquakes and shallower strike-slip faulting within the overriding plate. Historical records indicate recurrent seismic swarms in this zone since at least 2000, with six documented episodes through 2006. Earlier swarms occurred in 2004 (four events) and 2005 (one event), underscoring a pattern of clustered seismicity that may reflect episodic stress release along segmented faults.

The 2006 swarm initiated with two magnitude-5.1 events at 15 km depth, followed rapidly by events of 5.2 at 20 km and 21 km. Subsequent activity included a magnitude-5.4 shock at 10 km depth at 18:21, accompanied by several magnitude-5.0 to 5.3 events clustered within the same shallow crustal layer. Later phases featured slightly deeper events, such as a magnitude-5.4 at 23 km and a magnitude-5.0 at 30 km, before concluding with a magnitude-5.2 at 21 km. The tight temporal grouping and limited magnitude range suggest a swarm driven by fluid migration or aseismic slip rather than a classic foreshock-mainshock-aftershock sequence.

Geological studies of the Andaman region highlight its position within a back-arc basin influenced by the Andaman Sea spreading center to the east. This setting promotes normal and strike-slip faulting superimposed on the regional compression. Depths recorded during the swarm align with the brittle-ductile transition zone in the overriding plate, where strain accumulation from plate convergence is periodically relieved through swarm-like activity.

Post-2004 great earthquake effects likely influenced the 2006 swarm. The Mw 9.1–9.3 Sumatra-Andaman event of December 2004 altered regional stress fields, potentially triggering delayed swarm activity through viscoelastic relaxation or pore-pressure changes. The observed concentration of events between 10 km and 22 km depth supports models of shallow crustal readjustment following major subduction-zone ruptures.

Analysis of swarm statistics reveals consistent recurrence in the Andaman area, with the 2006 episode marking the sixth since 2000. Such patterns aid in refining probabilistic seismic hazard assessments for the region, where proximity to population centers like Port Blair necessitates ongoing monitoring of microseismic clusters.

References

USGS Earthquake Catalog (historical data 2000–2006)
Global CMT Project focal mechanism database
Andaman-Nicobar tectonic summaries, Geological Survey of India