Seismic Swarm PS20050205.1 in the Northern Sumatra Region

A notable seismic swarm, designated PS20050205.1, occurred approximately 277 km north-northwest of Sabang, Indonesia, between 4 and 6 February 2005. The sequence began at 09:44 on 4 February and concluded at 16:25 on 6 February, encompassing 10 earthquakes over 54 hours and 40 minutes. This activity took place in a tectonically complex offshore zone influenced by the ongoing subduction of the Indo-Australian Plate beneath the Sunda Plate along the Sunda Trench.

The events ranged in magnitude from 5.0 to 5.9, with focal depths varying between 10 km and 79 km. The sequence included multiple closely timed pairs, such as those on 4 February at 09:44 and on 5 February at 08:00 and 17:35, reflecting clustered rupture processes typical of swarm behavior rather than a single mainshock-aftershock pattern. Depths generally clustered in the upper 30 km, consistent with crustal and shallow subduction interface seismicity.

This swarm formed part of heightened seismic activity following the magnitude 9.1 Sumatra-Andaman earthquake of 26 December 2004, which ruptured a vast segment of the subduction zone and triggered widespread aftershocks and secondary sequences across the northern Sumatra and Andaman regions. The 2005 swarm location lies along the northern extension of the megathrust, where stress redistribution from the great earthquake likely promoted swarm initiation through dynamic and static triggering mechanisms.

Regional geology features a convergent margin characterized by oblique subduction, back-arc spreading in the Andaman Sea, and the presence of the Sumatra Fault System onshore. Historical records indicate that the area experiences frequent moderate-to-large earthquakes, with the 2004 event marking the largest in the instrumental era and altering regional stress fields for years afterward. Post-2004 data show an elevated rate of earthquake swarms in this sector, underscoring the role of fluid migration and aseismic slip in modulating swarm occurrence along the plate interface.

Since 1 January 2000, ten swarms have been identified in the broader region under consistent classification criteria. These include three swarms in 2004 and seven in 2005, illustrating a clear temporal clustering around the great 2004 rupture. Such patterns highlight how major subduction-zone events can sustain elevated swarm productivity for extended periods through viscoelastic relaxation and pore-pressure changes.

The PS20050205.1 swarm exemplifies the transition from aftershock decay to more diffuse swarm activity in the months following a megathrust earthquake. Monitoring of similar sequences contributes to improved understanding of subduction-zone dynamics and long-term seismic hazard assessment in one of the world's most active tectonic settings.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Global Centroid Moment Tensor (GCMT) Project
Scientific literature on the 2004 Sumatra-Andaman earthquake and its aftershock sequences (e.g., publications in Science and Nature, 2005–2007)