Seismic Swarm PS20200717.1: Geological Context and Event Analysis in the Andaman Sea

Seismic swarm PS20200717.1 was recorded on 17 July 2020, approximately 259 km east of Port Blair, India. The sequence began at 13:29 and concluded at 18:32, spanning 5 hours and 3 minutes during which six earthquakes occurred. This activity unfolded within the tectonically complex Andaman Sea region, where the Indian Plate subducts beneath the Burmese Plate along the Andaman Trench. The subduction zone generates frequent seismic events, with hypocenters often concentrated at shallow depths due to the interaction of oceanic crust and overriding plate margins.

The earthquakes exhibited the following parameters:

• 13:29:04 UTC, magnitude 5.1 at 10 km depth
• 14:03:40 UTC, magnitude 6.1 at 10 km depth
• 14:03:50 UTC, magnitude 6.1 at 10 km depth
• 14:43:17 UTC, magnitude 5.0 at 10 km depth
• 17:45:24 UTC, magnitude 4.8 at 10 km depth
• 18:32:58 UTC, magnitude 5.7 at 5 km depth

These events highlight a rapid succession of moderate to strong tremors, characteristic of swarm behavior where multiple shocks occur without a single dominant mainshock-aftershock pattern. Depths remained predominantly shallow, consistent with crustal deformation in the overriding plate.

Geologically, the Andaman-Nicobar archipelago forms part of an accretionary wedge influenced by oblique subduction and right-lateral strike-slip faulting along the Andaman Sea spreading center. Historical records indicate heightened activity in this zone, including the magnitude 9.1–9.3 megathrust earthquake of 26 December 2004, which originated northwest of Sumatra and triggered a transoceanic tsunami. That event underscored the region's capacity for great earthquakes driven by plate convergence rates averaging 5–6 cm per year.

Since 2000, only two prior swarms have been documented in the same area: one in 2006 and another in 2009. Such infrequent clustering suggests episodic stress release along segmented faults rather than continuous background seismicity. The 2020 swarm, with its peak magnitudes of 6.1, aligns with patterns of intermediate-depth crustal adjustment preceding or following larger regional events.

Analysis of the temporal distribution reveals clustering within the first hour, followed by diminishing frequency, indicative of fluid migration or aseismic slip facilitating rapid energy dissipation. Depths of 5–10 km point to activity within the brittle upper crust, where frictional instabilities promote swarm-like sequences over single large ruptures.

Overall, swarm PS20200717.1 provides further evidence of ongoing tectonic adjustment in the Andaman subduction system. Continued monitoring remains essential for understanding stress evolution along this plate boundary.

References

SeismoSight internal classification records
USGS Earthquake Catalog
Global CMT Project database