Location:

Period:

26 Dec 2004 01:21:20 - 28 Dec 2004 03:20:18 (2 days 1 hour 58 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.2; 284 km WNW of Sabang, Indonesia; (26 Dec 2004) (17km from the swarm center)

10 swarms found nearby.

2004

PS20041226.9(135.7km)

26 Dec

1 day 18 hours

14 earthquakes

PS20041226.1(155.0km)

26 Dec

2 days 23 hours

30 earthquakes

PS20050101.1(35.0km)

31 Dec

23 hours

5 earthquakes

2005

PS20050101.3(22.4km)

1 Jan

1 day 1 hours

6 earthquakes

PS20050126.1(187.8km)

26 Jan

5 days 20 hours

98 earthquakes

PS20050725.1(143.4km)

24 Jul

11 hours

6 earthquakes

2010

PS20100613.1(134.1km)

12 Jun

11 hours

6 earthquakes

2014

PS20140321.1(169.5km)

21 Mar

7 hours

7 earthquakes

PS20141111.1(192.5km)

11 Nov

3 hours

5 earthquakes

2019

PS20190401.1(175.8km)

31 Mar

8 hours

12 earthquakes

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Seismic Swarm PS20041226.3: Analysis of Activity Near Sabang, Indonesia

A seismic swarm designated PS20041226.3 was recorded between 01:21 on 26 December 2004 and 03:20 on 28 December 2004, approximately 284 km west-northwest of Sabang, Indonesia. Over 49 hours and 58 minutes, 25 earthquakes were detected, with magnitudes ranging from 5.0 to 7.2 and focal depths between 13 km and 48 km. This sequence occurred within the tectonically active Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate along the Sunda Trench.

The swarm initiated with a magnitude 6.1 event at 30 km depth, followed by multiple events of magnitude 5.0–5.6 within the first few hours. A prominent magnitude 7.2 earthquake occurred at 04:21 on 26 December at 39 km depth, positioned 17 km from the swarm centroid. Subsequent activity included events such as magnitude 5.8 at 33 km and magnitude 5.5 at 30 km, maintaining a pattern of moderate-to-strong shaking without a single dominant mainshock-aftershock cascade typical of isolated large ruptures. Depths remained predominantly in the upper 50 km, consistent with crustal and upper-mantle processes along the subduction interface.

Geologically, the region lies at the northern termination of the Sumatra segment of the Sunda Arc, characterized by oblique subduction and rapid plate motion exceeding 50 mm per year. The trench accommodates both thrust and strike-slip deformation, contributing to elevated seismicity. Historical records document recurrent large earthquakes, including the magnitude 9.1–9.3 event of 26 December 2004 whose epicenter was located roughly 200 km southeast of the swarm area. That megathrust rupture triggered widespread aftershock sequences across the arc, altering local stress fields and potentially influencing swarm nucleation through dynamic and static triggering.

Swarm characteristics observed here—clustered timing, variable magnitudes without clear foreshock-mainshock progression, and sustained activity over two days—align with stress redistribution along segmented faults or fluid migration within the accretionary prism. Depths cluster around 30 km for many events, suggesting involvement of the plate interface or overlying crustal blocks. No volcanic association is indicated for this offshore location, pointing instead to purely tectonic drivers.

Post-2000 instrumental data confirm the magnitude 7.2 event as the strongest recorded within 50 km of the swarm center. Regional monitoring networks have since improved resolution of similar clusters, aiding in distinguishing swarm behavior from classic aftershock decay. Continued subduction and locking of the megathrust maintain the potential for future seismic sequences in this corridor.

References

SeismoSight internal swarm classification PS20041226.3
USGS Earthquake Catalog (strong events since 2000)