Seismic Swarm S20050129.1: Earthquake Activity Northwest of Sabang, Indonesia

The seismic swarm designated S20050129.1 occurred 252 km northwest of Sabang, Indonesia, in a tectonically active segment of the Andaman Sea. This region forms part of the Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of approximately 5–6 cm per year. The interaction produces the Sunda Trench and generates frequent earthquakes through thrust faulting and strike-slip motion along the Sumatran fault system.

The swarm initiated at 10:19 on 28 January 2005 and concluded at 23:11 on 29 January 2005, spanning 36 hours and 51 minutes. During this interval, 33 earthquakes were recorded, with magnitudes ranging from 3.9 to 4.7. Focal depths clustered between 18 km and 30 km, consistent with intermediate-depth seismicity within the subducting slab or overlying crust. The largest event reached magnitude 4.7 at 17:14 on 29 January.

Earthquake swarms in subduction settings often reflect transient stress perturbations rather than a single dominant mainshock-aftershock sequence. Here, the events displayed a diffuse temporal distribution without clear foreshock-mainshock progression, suggesting possible contributions from fluid migration along the plate interface or post-seismic relaxation following the magnitude 9.1–9.3 Sumatra-Andaman earthquake of December 2004. Depths remained stable near 30 km for most events, with only isolated shallower occurrences at 18–23 km.

Historical records since 1 January 2000 indicate just two swarms in this sector, with S20050129.1 representing the first documented cluster. The second swarm occurred later, underscoring the episodic nature of swarm activity amid ongoing background seismicity driven by plate convergence.

Such moderate-magnitude swarms contribute to strain release along the megathrust but rarely trigger larger events when confined to small rupture areas. Continued monitoring of the Sunda Trench remains essential given the zone’s capacity for great earthquakes and associated tsunami hazards.

References

USGS Earthquake Catalog (events 2005)
Global CMT Project focal mechanisms
Subduction zone parameters from Hayes et al. (2018) plate model