Seismic Swarm PS20090726.1 Near the Andaman Islands

On 26 July 2009, a seismic swarm designated PS20090726.1 was recorded 197 km east-southeast of Port Blair, India, in the Andaman Sea. The sequence began at 07:12 UTC and concluded at 21:49 UTC, encompassing 18 earthquakes over 14 hours and 37 minutes. Magnitudes ranged from 4.4 to 5.2, with the majority occurring at depths of approximately 10 km and two events recorded at greater depths of 16–17 km and one at 71 km.

The events unfolded in rapid succession, beginning with a 5.0-magnitude shock at 07:12, followed by additional tremors at 07:40 (5.1), 08:11 (5.2), and continuing through the afternoon. Later activity included a pair of closely spaced events at 15:26–15:27 and a final 5.1-magnitude earthquake at 21:49. Depths remained predominantly shallow, consistent with activity along the shallow portion of the plate boundary.

This swarm occurred within the tectonically active Andaman subduction zone, where the Indian Plate converges with the Burma Plate at rates of 5–6 cm per year. The Andaman Islands sit atop the Burma Plate, and the region experiences frequent seismicity due to oblique subduction and associated strike-slip faulting along the Andaman Sea spreading center. Historical records document major events, including the 2004 Mw 9.1–9.3 Sumatra–Andaman earthquake, which ruptured a lengthy segment of the megathrust immediately to the south.

Swarm activity in this setting often reflects fluid migration or stress transfer along the subduction interface and nearby transform faults rather than a single dominant mainshock-aftershock sequence. The 2009 swarm fits this pattern, with closely timed events of similar magnitude and no clear foreshock–mainshock progression.

Regional statistics indicate that nine such swarms have been identified since 1 January 2000. Earlier episodes occurred in 2004 (five swarms), 2005 (one swarm), and 2006 (three swarms). These recurrent clusters underscore the persistent seismic productivity of the Andaman segment.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Incorporated Research Institutions for Seismology (IRIS) event data
• Geological Survey of India – Seismotectonic Atlas of India