Location:

Period:

28 Dec 2004 21:09:05 - 29 Dec 2004 15:33:07 (18 hours 24 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

5 swarms found nearby.

2002

PS20020124.1(151.6km)

24 Jan

22 hours

6 earthquakes

2004

PS20041226.6(100.7km)

26 Dec

2 days 1 hours

24 earthquakes

PS20050101.2(129.4km)

31 Dec

21 hours

5 earthquakes

2012

PS20120411.3(162.3km)

11 Apr

1 day 4 hours

10 earthquakes

PS20120421.1(59.9km)

20 Apr

15 hours

5 earthquakes

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Seismic Swarm PS20041229.2: Analysis of Activity West of Meulaboh, Indonesia

The seismic swarm designated PS20041229.2 occurred in the waters west of Sumatra, Indonesia, beginning at 21:09 on 28 December 2004 and ending at 15:33 on 29 December 2004. The events were centered 222 km west-southwest of Meulaboh in Aceh province. Over 18 hours and 24 minutes, five earthquakes were recorded, with magnitudes ranging from 4.4 to 5.5 and focal depths between 26 km and 33 km.

The sequence initiated with a magnitude 5.1 event at 21:09:05 on 28 December at 30 km depth. This was followed 42 minutes later by a magnitude 5.5 earthquake at 21:51:53 (33 km depth) and an immediate magnitude 5.0 aftershock at 21:51:54 (30 km depth). Activity then paused until 14:03:22 on 29 December, when a magnitude 5.1 event occurred at 26 km depth. The swarm concluded with a magnitude 4.4 earthquake at 15:33:07 on 29 December at 30 km depth.

This region forms part of the Sunda subduction zone, where the Indo-Australian Plate converges with and subducts beneath the Eurasian Plate at rates of approximately 5–6 cm per year. The resulting megathrust fault system produces frequent moderate to great earthquakes along the trench west of Sumatra. Meulaboh lies near the northern segment of this margin, which has hosted multiple historical ruptures, including events in 1861 and the great 2004 Sumatra-Andaman earthquake of magnitude 9.1–9.3 on 26 December.

Earthquake swarms in subduction settings often reflect transient stress changes, fluid migration, or aftershock triggering following a major rupture. The timing of PS20041229.2, two days after the 2004 mainshock, is consistent with such post-seismic processes. Historical records since 1 January 2000 indicate only two swarms have been identified in the catalog for this area: one in 2002 and the present event in 2004.

Such low-frequency swarm occurrences underscore the predominantly mainshock-aftershock character of seismicity in the Sunda Trench while highlighting occasional clustered activity that warrants continued monitoring for changes in background rates.

References

USGS Earthquake Hazards Program – Sumatra-Andaman subduction zone tectonics
Global CMT Catalog – focal mechanisms and depths for Sumatran events
IRIS Data Management Center – regional seismicity summaries since 2000