Location:

Period:

26 Dec 2004 01:59:13 - 29 Dec 2004 01:39:41 (2 days 23 hours 40 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.2; Nicobar Islands, India region; (24 Jul 2005) (81km from the swarm center)

14 swarms found nearby.

2004

PS20041226.3(155.0km)

26 Dec

2 days 1 hours

25 earthquakes

PS20041226.8(125.6km)

26 Dec

3 days 5 hours

41 earthquakes

PS20041226.5(154.1km)

26 Dec

1 day 11 hours

20 earthquakes

PS20050101.1(149.8km)

31 Dec

23 hours

5 earthquakes

2005

PS20050101.3(134.6km)

1 Jan

1 day 1 hours

6 earthquakes

PS20050126.1(157.6km)

26 Jan

5 days 20 hours

98 earthquakes

PS20050205.1(158.1km)

4 Feb

2 days 6 hours

10 earthquakes

PS20050216.1(130.8km)

15 Feb

16 hours

5 earthquakes

PS20050725.1(91.9km)

24 Jul

11 hours

6 earthquakes

2010

PS20100613.1(123.8km)

12 Jun

11 hours

6 earthquakes

2014

PS20140321.1(195.7km)

21 Mar

7 hours

7 earthquakes

2019

PS20190401.1(195.8km)

31 Mar

8 hours

12 earthquakes

2021

PS20210803.1(163.4km)

3 Aug

2 hours

5 earthquakes

2023

PS20230409.1(151.5km)

9 Apr

13 hours

5 earthquakes

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Seismic Swarm PS20041226.1: Insights into Activity in the Nicobar Islands Region

The seismic swarm designated PS20041226.1 was recorded in the Nicobar Islands, India region, beginning at 01:59 on 26 December 2004 and concluding at 01:39 on 29 December 2004. Over 71 hours and 40 minutes, the sequence included 30 earthquakes, providing a detailed record of clustered seismic energy release along this tectonically active margin.

The Nicobar Islands lie within the Andaman-Nicobar archipelago, positioned at the convergent boundary where the Indian Plate subducts beneath the Burma Plate. This subduction zone drives frequent seismicity, with the islands situated near the trench axis and associated forearc structures. Crustal deformation here reflects oblique convergence, producing both thrust and strike-slip faulting. Depths of events in the swarm ranged primarily from 2 km to 41 km, consistent with activity in the upper plate and along the plate interface.

The swarm initiated shortly after the great 26 December 2004 megathrust earthquake, which ruptured a vast segment of the Sunda megathrust extending northward into the Andaman-Nicobar region. Magnitudes during the swarm reached a peak of 6.6, with multiple events of 5.5–6.0 occurring in the initial hours. Notable pairs include the 6.0 and 6.6 events at 09:20 on 26 December, separated by one second and located at depths of 2 km and 16 km respectively. Subsequent activity showed a gradual decline in frequency and magnitude, with the final recorded event measuring 5.9 at 34 km depth.

Event distribution indicates a concentration of moderate-magnitude shocks at intermediate depths around 30–33 km, interspersed with shallower occurrences. This pattern suggests stress redistribution following the mainshock, possibly involving afterslip or triggered slip on subsidiary faults. No events exceeded magnitude 6.6, distinguishing the swarm from the primary rupture while highlighting continued seismic hazard in the days immediately afterward.

Regional historical data since 2000 show this as the first swarm in the Nicobar Islands area. A subsequent strong earthquake of magnitude 7.2 occurred on 24 July 2005, centered 81 km from the swarm epicentral zone, underscoring the persistent seismic potential along the margin.

Analysis of the temporal sequence reveals an initial burst of activity within the first 12 hours, followed by episodic clusters on 27 and 28 December. Depths remained broadly stable, with minor variations that may reflect heterogeneous fault properties or fluid involvement at depth. Such swarms contribute to understanding post-megathrust relaxation processes in subduction settings.

References

United States Geological Survey Earthquake Catalog
SeismoSight internal swarm classification records