Location:

Period:

26 Dec 2004 02:34:52 - 28 Dec 2004 03:52:59 (2 days 1 hour 18 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.4; 56 km NW of Sinabang, Indonesia; (20 Feb 2008) (96km from the swarm center)

10 swarms found nearby.

2002

PS20020124.1(86.4km)

24 Jan

22 hours

6 earthquakes

PS20021102.1(117.1km)

2 Nov

8 hours

6 earthquakes

2004

PS20041229.2(100.7km)

28 Dec

18 hours

5 earthquakes

PS20050101.2(77.6km)

31 Dec

21 hours

5 earthquakes

2005

PS20050328.3(141.3km)

28 Mar

21 hours

14 earthquakes

PS20050401.1(139.1km)

1 Apr

5 hours

5 earthquakes

PS20050411.1(190.5km)

11 Apr

3 hours

5 earthquakes

2008

PS20080220.1(104.7km)

20 Feb

19 hours

7 earthquakes

2011

PS20110115.1(138.9km)

15 Jan

5 hours

5 earthquakes

2012

PS20120421.1(154.2km)

20 Apr

15 hours

5 earthquakes

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Seismic Swarm PS20041226.6: Analysis of Post-Mainshock Activity off Western Sumatra

A notable seismic swarm designated PS20041226.6 occurred in the waters 265 km west-southwest of Meulaboh, Indonesia, between 02:34 on 26 December 2004 and 03:52 on 28 December 2004. Over this 49-hour period, 24 earthquakes were recorded, with magnitudes ranging from 5.0 to 6.0 and focal depths primarily between 17 km and 47 km. The sequence began with two closely spaced events of M5.7 and M5.9, followed by additional shocks that peaked at M6.0 on 26 December at 15:06:33.

This swarm unfolded within the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. The regional tectonics feature a megathrust interface capable of producing great earthquakes, as demonstrated by the M9.1–9.3 Sumatra-Andaman event that struck just hours earlier on 26 December 2004. The swarm’s timing and location align with the aftershock zone of that megathrust rupture, reflecting stress redistribution along the plate boundary and adjacent crustal faults.

Seismic activity in the area has remained elevated since 2000, with only two documented swarms recorded up to the present. The earlier swarm occurred in 2002, indicating that clustered moderate-magnitude sequences are infrequent but recurrent features of the margin. A subsequent strong earthquake of M7.4 struck on 20 February 2008, centered 56 km northwest of Sinabang and roughly 96 km from the swarm centroid, further illustrating the persistent seismic hazard.

The 24 events of swarm PS20041226.6 exhibited a typical aftershock-like decay, with the majority occurring within the first 24 hours. Depths clustered around 30–33 km, consistent with slip on or near the subduction interface, while shallower events near 17–24 km suggest involvement of the overriding plate. No events exceeded M6.0, yet the cumulative energy release contributed to ongoing hazard assessment in the immediate post-2004 period.

Geological monitoring of this segment of the Sunda trench continues to emphasize its potential for both megathrust and intraslab seismicity. The 2004 mainshock and its associated sequences, including PS20041226.6, have provided critical data for refining rupture models and tsunami-generation scenarios along the western Sumatran margin.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Incorporated Research Institutions for Seismology (IRIS) Event Search
Global Centroid Moment Tensor (GCMT) Project
SeismoSight internal swarm classification database