Seismic Swarm PS20020124.1: January 2002 Events near Meulaboh, Indonesia

The region surrounding Meulaboh on the western coast of Sumatra lies within one of the world's most active subduction zones. The Indo-Australian Plate converges with the Eurasian Plate along the Sunda Trench, generating frequent seismic activity at varying depths. This tectonic setting has produced multiple large earthquakes in recent decades, underscoring the area's persistent seismic hazard.

Swarm PS20020124.1 was recorded approximately 107 km south-southwest of Meulaboh. The sequence began at 15:24 on 24 January 2002 and concluded at 14:05 on 25 January 2002, spanning 22 hours and 41 minutes. During this interval, six earthquakes were detected, all occurring at a focal depth of 33 km. The events registered the following parameters:

• 24 January 2002 15:24:05, magnitude 5.4
• 24 January 2002 17:52:25, magnitude 5.4
• 24 January 2002 17:56:21, magnitude 5.2
• 24 January 2002 18:12:05, magnitude 5.7
• 24 January 2002 23:46:42, magnitude 4.3
• 25 January 2002 14:05:58, magnitude 5.0

These closely spaced events reflect typical swarm behavior, characterized by multiple shocks of similar magnitude without a single dominant mainshock. Such patterns often arise from fluid migration or localized stress adjustments along fault segments within the subduction interface.

The swarm center lies near the rupture zone of several significant earthquakes since 2000. Notable events include the magnitude 9.1 Sumatra-Andaman Islands earthquake of 26 December 2004, located 40 km from the swarm epicenter, and the magnitude 7.2 event of 9 May 2010 situated 52 km away. Additional strong shocks comprise the magnitude 7.4 earthquake of 20 February 2008, 84 km distant, and the magnitude 7.4 event of 2 November 2002, 85 km from the swarm location. These occurrences highlight the repeated release of strain accumulated along the megathrust.

Historical records indicate that the Sunda subduction zone has generated great earthquakes at intervals of several decades to centuries. The 2004 event remains the largest instrumentally recorded in the region and triggered a devastating tsunami across the Indian Ocean. Post-2004 aftershock sequences and subsequent moderate swarms continue to delineate active segments of the plate boundary.

Ongoing monitoring by regional seismic networks contributes to refined hazard assessments for coastal communities in Aceh Province. Continued analysis of swarm sequences aids in distinguishing background seismicity from potential precursory activity ahead of larger ruptures.

References:
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification data