Seismic Swarm S20060309.1 Near the Andaman Islands

Seismic swarm S20060309.1 occurred approximately 221 km east-southeast of Port Blair, India, in the Andaman Sea. The sequence began at 14:47 on 9 March 2006 and concluded at 10:52 on 10 March 2006, spanning 20 hours and 4 minutes. During this period, 28 earthquakes were recorded, with magnitudes ranging from 3.8 to 4.9 and focal depths between 10 km and 26 km. The majority of events clustered at depths of 10–24 km, consistent with shallow crustal activity within the overriding plate.

The Andaman region lies along the convergent boundary between the Indian Plate and the Burma Plate, part of the broader Sunda subduction zone. Here, the Indian Plate subducts obliquely beneath the Burma Plate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent moderate earthquakes and occasional great events. The 2004 Sumatra-Andaman earthquake (magnitude 9.1–9.3) originated nearby and triggered widespread stress redistribution, elevating aftershock rates across the Andaman-Nicobar segment for years afterward.

Swarm activity in subduction forearcs often reflects transient processes such as fluid migration along faults or aseismic slip that perturbs nearby locked patches. The 2006 swarm fits this pattern, occurring two years after the great megathrust rupture. Event magnitudes remained below 5.0, and no damage or tsunami generation was reported. The temporal clustering—most events occurring within the first eight hours—illustrates the typical rapid onset and decay of swarm sequences compared with classical aftershock decay governed by Omori’s law.

Historical records maintained by SeismoSight indicate that only two prior swarms have been identified in the same source region since 1 January 2000: one in 2004 and another in 2005. These earlier sequences likewise featured moderate magnitudes and shallow depths, underscoring the episodic nature of swarm seismicity in the post-2004 stress field.

Ongoing monitoring by regional networks continues to track microseismicity along the Andaman trench. Such data refine models of plate coupling and help assess the likelihood of future moderate events that could influence stress transfer toward the Nicobar segment. While the 2006 swarm did not herald a larger rupture, it exemplifies the persistent seismic unrest that characterizes this tectonically active margin.

References

• USGS Earthquake Catalog (historical events near Andaman Islands)
• Indian Meteorological Department Seismological Bulletin (regional monitoring data)
• Tectonic framework studies published in Journal of Geophysical Research: Solid Earth (Andaman subduction zone)