Seismic Swarm PS20230409.1 in the Andaman Sea: Geological Context and Event Analysis

A seismic swarm designated PS20230409.1 was recorded on 9 April 2023, approximately 279 km south-southeast of Port Blair in India's Andaman and Nicobar Islands. The sequence began at 07:46 UTC and concluded by 21:00 UTC, encompassing five earthquakes over 13 hours and 13 minutes. All events occurred at a focal depth of 10 km, with magnitudes ranging from 4.1 to 5.7.

The individual events unfolded as follows: a magnitude 5.3 quake at 07:46:14, followed by a 5.5 event at 10:31:20, a 5.7 shock at 12:17:33, a magnitude 5.2 at 20:55:58, and a final 4.1 at 21:00:08. Such swarms represent clusters of earthquakes without a single dominant mainshock, often linked to fluid migration or stress adjustments along fault systems.

The Andaman Sea region lies at the convergent boundary where the Indian Plate subducts beneath the Burma Plate along the Sunda Trench. This tectonic setting produces frequent seismicity, with the Andaman-Nicobar ridge marking an active accretionary complex. Historical records show elevated activity, including the devastating 2004 Mw 9.1 Sumatra-Andaman earthquake, whose rupture extended northward into this zone and triggered widespread aftershocks.

Since 2000, fifteen seismic swarms have been documented in the area. These occurred in 2004 (four swarms), 2005 (four), 2006 (one), 2009 (one), 2014 (two), 2019 (one), 2021 (one), and 2022 (one). The 2023 swarm aligns with this pattern of episodic clustering, reflecting ongoing plate-boundary deformation.

Monitoring by regional networks continues to track such sequences for potential escalation. The shallow depths observed here indicate activity within the upper crust, consistent with the region's subduction-related faulting.

References

USGS Earthquake Catalog
Indian National Centre for Ocean Information Services (INCOIS) reports
Global CMT Project focal mechanism data