Location:

Period:

26 Dec 2004 01:40:07 - 27 Dec 2004 20:37:20 (1 day 18 hours 57 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

6 swarms found nearby.

2004

PS20041226.3(135.7km)

26 Dec

2 days 1 hours

25 earthquakes

PS20050101.1(157.3km)

31 Dec

23 hours

5 earthquakes

2005

PS20050101.3(153.6km)

1 Jan

1 day 1 hours

6 earthquakes

PS20050105.1(164.9km)

4 Jan

1 day 6 hours

9 earthquakes

PS20050113.1(152.8km)

12 Jan

18 hours

5 earthquakes

2015

PS20151108.1(192.8km)

8 Nov

21 hours

11 earthquakes

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Seismic Swarm PS20041226.9: Analysis of Activity West of Banda Aceh

A seismic swarm designated PS20041226.9 occurred from 01:40 on 26 December 2004 to 20:37 on 27 December 2004, approximately 209 km west-southwest of Banda Aceh, Indonesia. The sequence lasted 42 hours and 57 minutes and included 14 earthquakes with magnitudes ranging from 5.0 to 5.7. All events were recorded at depths between 16 km and 46 km, consistent with activity along the subduction interface.

The swarm took place in a tectonically active region where the Indo-Australian plate subducts beneath the Eurasian plate along the Sunda Trench. This convergent margin forms part of the broader Pacific Ring of Fire and has produced repeated large-magnitude earthquakes throughout recorded history. The trench segment near northern Sumatra is characterized by oblique subduction, with the Indo-Australian plate moving northward at roughly 5–6 cm per year relative to the overriding plate. Strain accumulation along the megathrust has repeatedly led to both great earthquakes and associated aftershock sequences.

The timing of the swarm coincides with the immediate aftermath of the magnitude 9.1 Sumatra–Andaman earthquake of 26 December 2004. That event ruptured approximately 1,600 km of the plate boundary and generated a devastating trans-oceanic tsunami. The subsequent swarm events are interpreted as aftershocks occurring on or near the same megathrust surface. Focal depths in the 16–46 km range align with the expected seismogenic zone for this subduction segment. Magnitudes remained moderate, with the largest event reaching 5.7, indicating distributed failure rather than a single large rupture.

Geological records show that northern Sumatra has experienced multiple great earthquakes prior to 2004, including events in 1861 and 1907. Paleoseismic studies of coral microatolls and tsunami deposits confirm recurrence intervals on the order of 200–400 years for magnitude-9-class ruptures in this sector. The 2004 mainshock and its aftershock sequence, including swarm PS20041226.9, fit within this long-term pattern of strain release along the Sunda megathrust.

Since 1 January 2000, only one swarm has been documented in the immediate area, occurring in 2004. This rarity underscores the exceptional nature of the post-2004 aftershock activity. Depths and magnitudes recorded during the swarm are typical for aftershocks in this tectonic setting, where brittle failure occurs within the locked portion of the subduction interface.

Further monitoring by regional seismic networks continues to track residual strain adjustment in the years following the 2004 rupture. The geological framework of the Sunda Trench remains a key control on future seismic hazard for coastal communities in northern Sumatra and the Andaman Sea region.

References

USGS Earthquake Catalog
Global CMT Project
Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) annual reports