Seismic Swarm S20050127.2: Analysis of Activity Northwest of Sabang, Indonesia

A notable earthquake swarm, designated S20050127.2, occurred 271 km northwest of Sabang, Indonesia, from 15:34 on 26 January 2005 to 09:47 on 30 January 2005. Over 90 hours and 13 minutes, 105 earthquakes were recorded. This sequence unfolded in a tectonically active offshore region and provides insight into post-mainshock stress adjustments along a major subduction boundary. The swarm's first 100 events exhibited magnitudes between 3.9 and 5.5, with the majority clustered around 4.5–4.8. Depths predominantly remained near 30 km, though several shallower events reached 16–22 km. The sequence initiated with a magnitude 4.2 shock at 30 km depth, followed by a rapid increase in activity featuring multiple 4.6–4.8 events within the first 24 hours. Peak magnitudes of 5.5, 5.3, and 5.4 appeared on 28 and 29 January, interspersed with sustained moderate shaking. Event frequency declined toward the swarm's conclusion, consistent with typical swarm decay patterns. Geologically, the location lies along the Sunda Trench, where the Indo-Australian Plate subducts beneath the Eurasian Plate at rates of approximately 5–6 cm per year. This convergent margin generates frequent seismicity, including both interplate thrust events and intraslab activity. The swarm's depth distribution aligns with the shallow portion of the subduction interface and overlying crustal stresses. The broader region experienced heightened seismic rates following the magnitude 9.1–9.3 Sumatra–Andaman earthquake of 26 December 2004, whose rupture extended northward past the swarm area. Postseismic relaxation and triggered slip on adjacent fault segments contributed to aftershock sequences and swarms throughout early 2005. Historical records document repeated great earthquakes along this margin, including events in 1797, 1833, and 1861, underscoring a long-term cycle of strain accumulation and release. Analysis of the initial 100 events reveals a compact spatial and temporal clustering without clear mainshock-aftershock pairing, typical of swarm behavior driven by fluid migration or aseismic slip. Most activity concentrated at mid-crustal depths, suggesting involvement of the plate interface and upper mantle wedge.

References

USGS Earthquake Catalog (historical and regional data)
Global CMT Catalog (moment tensor solutions for regional events)
Subduction Zone Observatory reports on Sunda Trench tectonics