Seismic Swarm PS20050126.1: Analysis of Activity Northwest of Sabang, Indonesia

The earthquake swarm designated PS20050126.1 occurred in a tectonically active segment of the Andaman-Sumatra subduction zone, approximately 259 km northwest of Sabang, Indonesia. This region lies along the convergent boundary where the Indo-Australian plate subducts beneath the Eurasian plate at rates of 4–5 cm per year, generating frequent seismic events. The swarm initiated at 17:30 UTC on 26 January 2005 and concluded at 13:41 UTC on 1 February 2005, spanning 140 hours and 11 minutes during which 98 earthquakes were recorded.

Magnitudes ranged from 5.0 to 5.7, with the largest events reaching 5.7 on 28 and 30–31 January. Depths predominantly clustered between 20 and 40 km, consistent with the seismogenic zone of the subducting slab, though several events occurred at shallower depths under 10 km and a few exceeded 80 km. Activity showed temporal clustering, with peak rates on 26–27 January and renewed bursts on 30–31 January. Multiple events occurred within minutes of one another, indicating rapid stress release along interconnected fault segments.

Geologically, the swarm location aligns with the northern extension of the Sunda Trench system, where oblique subduction and associated strike-slip faulting, including the Great Sumatran Fault, accommodate plate motion. This setting has produced major historical earthquakes, such as the magnitude 9.1 event of 26 December 2004 located to the southeast. Post-2004 aftershock sequences commonly feature swarm-like behavior due to viscoelastic relaxation and fluid migration within the accretionary prism.

Historical records from the SeismoSight database indicate seven swarms in the broader region since 1 January 2000. These include four swarms in 2004 and three in 2005, underscoring elevated seismicity following the 2004 mainshock. Such swarms typically exhibit b-values near 1.0, reflecting heterogeneous stress conditions rather than single-fault rupture.

Insights from the event catalog reveal a predominance of thrust and strike-slip mechanisms inferred from depth distributions and temporal patterns. Shallow events (<10 km) may relate to upper-plate deformation, while deeper activity (>30 km) corresponds to intraslab seismicity. The absence of a clear mainshock-aftershock decay suggests swarm dynamics driven by pore-pressure diffusion or aseismic slip transients common in subduction forearcs.

This swarm contributed to ongoing monitoring of post-seismic deformation in the Andaman Sea region. Continued observation remains essential given the zone’s capacity for great earthquakes.

References
SeismoSight internal swarm classification database
USGS Earthquake Catalog (historical context)
Global CMT Project (regional tectonics)