Seismic Swarm PS20021102.1: Analysis of Activity Near Sinabang, Indonesia

On 2 November 2002, a seismic swarm designated PS20021102.1 was recorded approximately 66 km west-southwest of Sinabang, Indonesia. The sequence began at 01:26 UTC and concluded at 09:46 UTC, encompassing six earthquakes over a span of eight hours and twenty minutes. This activity occurred within the tectonically dynamic Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates exceeding 5 cm per year, generating frequent intermediate-depth seismicity.

The swarm events unfolded as follows. The initial shock at 01:26:10 registered magnitude 7.4 at a depth of 30 km. Subsequent events included a magnitude 5.2 at 01:38:20 (23 km depth), another magnitude 5.2 at 01:45:35 (33 km depth), a magnitude 5.0 at 01:56:53 (33 km depth), a magnitude 4.6 at 02:20:02 (23 km depth), and a final magnitude 6.3 at 09:46:46 (27 km depth). These depths align with typical slab seismicity in the region, reflecting brittle failure within the subducting oceanic lithosphere.

Geologically, the swarm center lies near the northern extension of the Sumatran fault system and the Sunda megathrust. Historical records indicate heightened activity in this sector, with the 26 December 2004 M9.1 Sumatra-Andaman earthquake occurring 66 km from the swarm epicenter and producing widespread co-seismic deformation. An additional M7.4 event on 20 February 2008 was located 56 km northwest of Sinabang, underscoring the persistent strain accumulation along the plate interface.

Since 1 January 2000, only one prior swarm has been documented in the immediate vicinity, occurring in 2002. The 2 November 2002 sequence itself featured a separate M7.4 earthquake 50 km northwest of Sinabang, just 13 km from the swarm centroid, suggesting possible stress triggering or shared fault segments. Such swarms often arise from fluid migration or aseismic slip transients rather than classical mainshock-aftershock patterns, consistent with observations in subduction environments.

The 2004 megathrust rupture, which nucleated nearby, released immense energy and altered regional stress fields, potentially influencing later sequences like this swarm. Depths between 23 and 33 km indicate events within the Wadati-Benioff zone, where phase changes and dehydration reactions can promote clustered seismicity. Updated monitoring by global networks continues to refine models of slip partitioning in this area, emphasizing the interplay between thrust and strike-slip mechanisms.

This swarm provides valuable data on short-term clustering in a high-hazard corridor. Ongoing subduction drives recurrent large events, necessitating sustained seismic surveillance for hazard mitigation along the western Sumatran margin.

References:
USGS Earthquake Catalog
Global CMT Project
SeismoSight internal swarm classification PS20021102.1