Seismic Swarm PS20050101.2: Post-Mainshock Activity Near Meulaboh, Indonesia

The seismic swarm designated PS20050101.2 occurred 85 km southwest of Meulaboh, Indonesia, from 07:26 on 31 December 2004 to 04:29 on 1 January 2005. Over 21 hours and 2 minutes, five earthquakes were recorded with magnitudes ranging from 4.9 to 5.7 and focal depths between 24 km and 46 km. This sequence unfolded five days after the great M 9.1 Sumatra-Andaman earthquake of 26 December 2004, whose epicenter lay only 64 km from the swarm centroid, indicating a likely connection to the ongoing aftershock process within the Sunda subduction zone.

The five events displayed a clear temporal clustering. The sequence began with an M 5.2 shock at 46 km depth, followed hours later by a pair of closely spaced events (M 5.4 and M 5.6) at 33 km depth. Activity then continued into the new year with an M 5.7 event at 24 km depth and concluded with an M 4.9 shock at 34 km depth. The relatively shallow depths and moderate magnitudes are consistent with brittle failure along the megathrust interface and adjacent crustal faults that were dynamically stressed by the preceding great earthquake.

The Meulaboh region lies above the Sumatra subduction zone, where the Indo-Australian plate converges with the Eurasian plate at approximately 5–6 cm per year. This tectonic setting produces both megathrust events and shallower crustal seismicity. The December 2004 M 9.1 rupture extended for more than 1,200 km along the plate boundary, releasing enormous strain and triggering widespread aftershock activity that persisted for months. The PS20050101.2 swarm represents a localized pulse within that broader aftershock sequence.

Historical records maintained by SeismoSight document six swarms in the same area since 1 January 2000. Earlier swarms occurred in 2002 (two events) and 2004 (four events). The present swarm, therefore, forms part of an elevated rate of swarm activity observed in the immediate post-2004 period. A subsequent strong earthquake of M 7.2 on 9 May 2010, located 69 km from the swarm center, further illustrates the continued seismic productivity of the region following the 2004 mainshock.

Such swarms are characteristic of subduction-zone environments where stress redistribution after a great earthquake can activate multiple fault patches over short time intervals. Depths in the 24–46 km range place the events near the downdip limit of the seismogenic zone, where temperature and pressure conditions allow both brittle and transitional slip behavior. The absence of larger events within the swarm itself suggests that strain release remained distributed rather than concentrated on a single rupture plane.

Continued monitoring of this segment of the Sunda arc remains essential. The 2004 event released strain accumulated over centuries, yet adjacent segments to the south retain significant locked zones capable of producing future large earthquakes. Swarms such as PS20050101.2 provide valuable data for refining models of post-seismic relaxation and for assessing short-term changes in seismic hazard along the Sumatran margin.

References

• USGS Earthquake Catalog (updated through 2023)
• SeismoSight internal swarm classification database
• Sieh et al., 2008, “Earthquake supercycles inferred from sea-level changes recorded in the corals of west Sumatra,” Science
• Subarya et al., 2006, “Plate-boundary deformation associated with the great Sumatra–Andaman earthquake,” Nature