Seismic Swarm S20050128.1: Earthquake Activity Northwest of Sabang, Indonesia

Seismic swarm S20050128.1 was recorded northwest of Sabang, Indonesia, beginning at 02:25 on 27 January 2005 and concluding at 07:45 on 29 January 2005. The sequence lasted 53 hours and 19 minutes, during which 44 earthquakes were registered at a location 267 km NNW of Sabang.

Magnitudes ranged from 4.1 to 4.9, with the majority of events clustered between 4.3 and 4.8. Depths were predominantly 30 km, though several events occurred at shallower levels down to 16 km and one deeper event reached 43 km. Activity was most intense on 27 January, with 19 events distributed throughout the day and evening. Frequency decreased on 28 January, with 23 events recorded, before tapering to a single event on 29 January that marked the swarm’s end.

This swarm represents the sole seismic swarm documented in the region since 1 January 2000. No prior swarms have been identified in the available historical record for the area.

The swarm occurred within the tectonically active Sumatra-Andaman subduction zone, where the Indo-Australian plate converges with and subducts beneath the Eurasian plate. This setting produces frequent seismicity along the Sunda Trench, including megathrust earthquakes and associated aftershock sequences. The region’s geological history is dominated by oblique subduction, which also drives strike-slip faulting along the Great Sumatran Fault on land. High seismic hazard characterizes the offshore domain north of Sumatra, with instrumentally recorded events extending back over a century.

The timing of swarm S20050128.1, occurring one month after the 26 December 2004 Mw 9.1–9.3 Sumatra-Andaman earthquake, places it within the extensive aftershock zone of that event. The 2004 rupture extended northward along the trench into the Andaman Sea, altering regional stress conditions and triggering prolonged seismic sequences at distances comparable to the swarm’s location.

References

SeismoSight internal classification records for swarm parameters and event list.
USGS Earthquake Hazards Program summaries of regional tectonics and 2004 mainshock rupture characteristics.