Location:

Period:

26 Dec 2004 01:21:48 - 29 Dec 2004 06:35:44 (3 days 5 hours 13 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

13 swarms found nearby.

2004

PS20041226.5(85.9km)

26 Dec

1 day 11 hours

20 earthquakes

PS20041226.7(104.4km)

26 Dec

11 hours

16 earthquakes

PS20041226.11(167.4km)

26 Dec

13 hours

5 earthquakes

PS20041226.1(125.6km)

26 Dec

2 days 23 hours

30 earthquakes

2005

PS20050126.1(178.1km)

26 Jan

5 days 20 hours

98 earthquakes

PS20050202.1(105.2km)

1 Feb

14 hours

5 earthquakes

PS20050205.1(162.0km)

4 Feb

2 days 6 hours

10 earthquakes

PS20050216.1(127.1km)

15 Feb

16 hours

5 earthquakes

2006

PS20060309.1(182.3km)

9 Mar

21 hours

18 earthquakes

2009

PS20090726.1(179.9km)

26 Jul

14 hours

18 earthquakes

2021

PS20210803.1(108.3km)

3 Aug

2 hours

5 earthquakes

2022

PS20220704.1(142.3km)

4 Jul

21 hours

10 earthquakes

2023

PS20230409.1(72.5km)

9 Apr

13 hours

5 earthquakes

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Seismic Swarm PS20041226.8: Analysis of Activity near Sabang, Indonesia

The seismic swarm designated PS20041226.8 occurred in a tectonically active region of the Andaman Sea, approximately 219 km north-northwest of Sabang, Indonesia. This sequence began at 01:21 on 26 December 2004 and concluded at 06:35 on 29 December 2004, spanning 77 hours and 13 minutes during which 41 earthquakes were recorded.

The area lies along the Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of 4–5 cm per year. This boundary has produced major historical events, including the magnitude 9.1–9.3 Sumatra-Andaman earthquake of 26 December 2004, whose epicenter was located roughly 160 km west of northern Sumatra. That megathrust rupture triggered widespread aftershock sequences across the broader region, consistent with the timing and location of swarm PS20041226.8.

Earthquake depths within the swarm ranged from 8 km to 162 km, with the majority clustered between 19 km and 58 km. Shallower events (under 30 km) suggest activity within the overriding plate or upper portions of the subducting slab, while deeper occurrences indicate strain release at intermediate depths along the Wadati-Benioff zone. Magnitudes varied from 4.6 to 6.2, with the largest events (magnitudes 6.1 and 6.2) concentrated toward the end of the sequence on 29 December.

The swarm exhibited a typical pattern of clustered seismicity following a major subduction-zone rupture. Initial events on 26 December included several magnitude 5.0–6.1 shocks at depths of 17–162 km. Activity persisted through 27 and 28 December with repeated magnitude 5.2–5.6 events, many at 30–33 km depth. The sequence peaked on 29 December with multiple magnitude 5.9–6.2 earthquakes at depths as shallow as 8–12 km before tapering off.

Such swarms reflect stress redistribution along the plate interface and within the slab following great earthquakes. The 2004 mainshock altered regional stress fields, promoting triggered seismicity that can persist for weeks to months. Ongoing monitoring by regional networks continues to track microseismicity in this segment of the subduction zone, underscoring persistent seismic hazard for nearby coastal communities in Indonesia and the Andaman-Nicobar Islands.

References
USGS Earthquake Hazards Program – historical event data and tectonic summaries (usgs.gov)
Global CMT Catalog – moment tensor solutions for regional events
NOAA National Centers for Environmental Information – tsunami and aftershock records