Seismic Swarm S20050127.1: Analysis of Activity Northwest of Sabang, Indonesia

An earthquake swarm designated S20050127.1 occurred 254 km northwest of Sabang, Indonesia, between 14:36 on 26 January 2005 and 13:14 on 31 January 2005. Over 118 hours and 38 minutes, 168 events were recorded. The first 100 events, spanning the initial phase of activity, exhibited magnitudes primarily between 3.9 and 5.9, with the majority clustered around 4.5–5.3. Depths were predominantly near 30 km, though several events reached depths of 16–43 km and one outlier at 98 km.

Temporal distribution showed rapid onset on 26 January, with events occurring at intervals of minutes to hours. Early activity included multiple magnitude 4+ shocks within the first 12 hours, escalating to a peak magnitude of 5.7 at 06:56 on 27 January. Subsequent hours featured repeated events above magnitude 5.0, including a 5.9 event at 16:58 on 27 January. Depths remained consistent near 30 km for most events, suggesting a concentrated source volume within the crust.

Geologically, the swarm region lies along the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent intermediate-depth seismicity and occasional large megathrust earthquakes. The area northwest of Sabang, near the Andaman Sea transition, has experienced elevated activity following the magnitude 9.1 Sumatra-Andaman earthquake of 26 December 2004, which ruptured over 1,200 km of the plate boundary and triggered widespread aftershock sequences.

Historical records indicate sparse swarm-type activity in the region since 2000, with only one prior swarm documented in 2004. Such clusters differ from typical aftershock decay by maintaining relatively steady event rates without a single dominant mainshock. Depths around 30 km align with the expected range for events occurring within the subducting slab or overlying plate interface in this segment of the margin.

The swarm’s magnitude distribution, with numerous events exceeding 4.5, reflects sustained stress release along a localized fault network. No events in the initial 100 surpassed magnitude 6.0, consistent with swarm behavior rather than a foreshock-mainshock-aftershock sequence. Depths showed limited variation, supporting the interpretation of a compact seismogenic volume.

Further monitoring of similar swarms contributes to understanding stress transfer following great subduction earthquakes in the Sunda Arc. Updated regional seismic catalogs continue to refine models of slab geometry and coupling in the northern Sumatra–Andaman sector.

References
USGS Earthquake Catalog (historical seismicity data)
Global CMT Catalog (event parameters)
NOAA National Centers for Environmental Information (tectonic summaries)