Location:

Period:

26 Dec 2004 01:17:10 - 29 Dec 2004 07:52:51 (3 days 6 hours 35 minutes)

Volcanoes in 100km radius:

Seulawah Agam(88km)

Earthquakes:

6 swarms found nearby.

2002

PS20020124.1(186.2km)

24 Jan

22 hours

6 earthquakes

2004

PS20041230.1(31.9km)

29 Dec

2 days 18 hours

16 earthquakes

PS20050101.2(172.4km)

31 Dec

21 hours

5 earthquakes

2005

PS20050105.1(71.1km)

4 Jan

1 day 6 hours

9 earthquakes

PS20050113.1(74.5km)

12 Jan

18 hours

5 earthquakes

2015

PS20151108.1(169.6km)

8 Nov

21 hours

11 earthquakes

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Seismic Swarm PS20041226.2: Analysis of Northern Sumatra Earthquake Activity in December 2004

Northern Sumatra, Indonesia, occupies a tectonically active segment of the Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of approximately 5–6 cm per year. This setting produces frequent megathrust earthquakes and associated seismic sequences. The region’s geology features the Sunda Trench to the west, the Sumatran Fault running onshore, and a volcanic arc influenced by ongoing subduction. Historical records document major events, including the 2004 Indian Ocean earthquake, which originated nearby and triggered widespread aftershock sequences.

SeismoSight classified swarm PS20041226.2 as a distinct cluster of 39 earthquakes occurring between 01:17 on 26 December 2004 and 07:52 on 29 December 2004. The sequence unfolded over 78 hours and 35 minutes in northern Sumatra. Magnitudes ranged from 5.0 to 6.3, with focal depths predominantly between 10 km and 51 km. The swarm began with a 5.5 event at 30 km depth, followed within minutes by larger shocks of 6.3 and 6.1. Subsequent activity included multiple events of 5.6–5.8 magnitude clustered around 03:00–05:00 on 26 December, with depths mostly 20–50 km. Later peaks occurred on 27 December, featuring three events above magnitude 6.0 between 09:39 and 10:05, all at shallow to intermediate depths.

Activity declined after 27 December but persisted with isolated 5.0–5.9 events through 28 December, concluding with a final 5.3 shock at 30 km depth on 29 December. Depths showed modest variation, with several events shallower than 20 km and others exceeding 45 km, consistent with activity along both the megathrust interface and subsidiary crustal faults.

Seismic swarms in this region reflect stress redistribution following large subduction-zone ruptures. The 2004 mainshock altered regional stress fields, promoting clusters of moderate-magnitude events. Historical data indicate limited swarm occurrences since 2000, with the preceding swarm recorded in 2002. Such sequences typically last days to weeks and rarely exceed magnitude 6.5 within the swarm itself.

The tectonic framework of northern Sumatra combines oblique subduction and strike-slip faulting along the Sumatran Fault System. This dual mechanism allows for both interplate thrust events and shallower crustal seismicity. Updated geological assessments confirm ongoing strain accumulation along the Sunda megathrust segments north of the 2004 rupture zone, underscoring continued seismic hazard.

USGS Earthquake Catalog
Global Centroid Moment Tensor Project
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports