Seismic Swarm PS20140321.1 in the Andaman Sea: Geological Context and Event Analysis
On 21 March 2014, a seismic swarm designated PS20140321.1 was recorded approximately 250 km north-northwest of Sabang, Indonesia. The sequence began at 13:41 UTC and concluded at 21:03 UTC, encompassing seven earthquakes over a duration of 7 hours and 22 minutes. This activity occurred within a tectonically complex portion of the Andaman Sea, where the Indo-Australian Plate subducts beneath the Eurasian Plate along the Sunda Trench system.
The regional geology features a convergent margin characterized by oblique subduction, strike-slip faulting, and back-arc spreading. The Andaman Sea hosts active spreading centers and transform faults that accommodate the relative plate motion. Historical seismicity in this zone reflects both megathrust events along the trench and shallower crustal earthquakes associated with the Andaman Sea rift system. The 2004 Indian Ocean earthquake and tsunami originated nearby, underscoring the area's capacity for large-magnitude events, although swarm sequences typically involve moderate magnitudes without a dominant mainshock.
The March 2014 swarm events unfolded as follows. The initial earthquake registered magnitude 6.4 at a depth of 21 km. Subsequent events included a magnitude 4.7 at 10 km depth, followed by a magnitude 5.5 at 10 km, a magnitude 5.1 at 10 km, another magnitude 5.2 at 10 km, a magnitude 5.1 at 9 km, and a final magnitude 5.2 at 9 km. Depths remained predominantly shallow, consistent with activity along upper-crustal structures in the Andaman back-arc region.
Swarm sequences such as this one differ from typical aftershock series by lacking a clear principal shock and instead exhibiting clustered, similar-magnitude events over short time intervals. In the Andaman Sea, such swarms may arise from fluid migration, stress triggering along interconnected faults, or episodic slip on transform segments. Since 1 January 2000, eleven swarms have been identified in the broader study area under the SeismoSight classification. Earlier documented swarms occurred in 2004 with three events and in 2005 with eight events, indicating recurrent episodic behavior in this segment of the plate boundary.
These observations contribute to understanding short-term seismic patterns in subduction-related back-arc settings. Continued monitoring supports improved characterization of fault interactions and potential precursory signals in the region.