Location:

Period:

20 Feb 2008 08:08:30 - 21 Feb 2008 03:47:43 (19 hours 39 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.4; 56 km NW of Sinabang, Indonesia; (20 Feb 2008) (24km from the swarm center)
M 9.1; 2004 Sumatra - Andaman Islands Earthquake; (26 Dec 2004) (83km from the swarm center)
M 7.4; 50 km NW of Sinabang, Indonesia; (2 Nov 2002) (32km from the swarm center)

11 swarms found nearby.

2002

PS20020124.1(107.7km)

24 Jan

22 hours

6 earthquakes

PS20021102.1(26.6km)

2 Nov

8 hours

6 earthquakes

2004

PS20041226.6(104.7km)

26 Dec

2 days 1 hours

24 earthquakes

PS20050101.2(123.3km)

31 Dec

21 hours

5 earthquakes

2005

PS20050328.1(189.3km)

28 Mar

3 days 8 hours

24 earthquakes

PS20050328.3(54.3km)

28 Mar

21 hours

14 earthquakes

PS20050401.1(58.0km)

1 Apr

5 hours

5 earthquakes

PS20050406.1(107.4km)

5 Apr

16 hours

5 earthquakes

PS20050411.1(85.9km)

11 Apr

3 hours

5 earthquakes

2010

PS20100407.1(99.0km)

6 Apr

6 hours

5 earthquakes

2011

PS20110115.1(34.5km)

15 Jan

5 hours

5 earthquakes

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Seismic Swarm Near Sinabang, Indonesia: February 2008 Event Analysis

A seismic swarm designated PS20080220.1 was recorded 27 km NNW of Sinabang, Indonesia, beginning at 08:08 on 20 February 2008 and concluding at 03:47 on 21 February 2008. Over 19 hours and 39 minutes, seven earthquakes were registered, providing a clear snapshot of clustered seismic activity along the Sunda subduction zone.

The sequence opened with a magnitude 7.4 event at 26 km depth, followed by six aftershocks ranging from magnitude 5.0 to 5.4 at depths between 22 km and 35 km. The full progression included events at 08:28 (M5.4, 35 km), 08:36 (M5.2, 35 km), 09:05 (M5.1, 35 km), 09:11 (M5.3, 24 km), 09:32 (M5.0, 22 km), and a final M5.0 at 03:47 on 21 February at 35 km depth. This pattern reflects typical swarm behavior in which an initial large rupture triggers subsequent smaller releases along adjacent fault segments without a single dominant mainshock-aftershock decay.

The location lies on the western edge of Simeulue Island, directly above the Sunda Trench where the Indo-Australian Plate converges with the Eurasian Plate at approximately 5–6 cm per year. Subduction in this segment produces both megathrust and intraslab seismicity, with hypocenters commonly occurring between 20 km and 40 km depth. The February 2008 swarm occurred within a zone still experiencing post-seismic relaxation following the great 2004 Sumatra-Andaman earthquake.

Regional records since 2000 document nine distinct swarms in the immediate area. Earlier episodes occurred in 2002 (two swarms), 2004 (two swarms), and 2005 (five swarms), indicating recurrent swarm-type activity along this portion of the margin. Three notable strong earthquakes have also been recorded since 2000 within tens of kilometers of the 2008 swarm center: the M9.1 Sumatra-Andaman mainshock of 26 December 2004 (83 km distant), an M7.4 event on 2 November 2002 (32 km distant), and the M7.4 shock of 20 February 2008 itself (24 km from swarm centroid).

These observations underscore the persistent seismic hazard along the northwestern Sumatran margin, where strain accumulation and release continue to produce both isolated large events and episodic swarm sequences.

References:
SeismoSight internal swarm classification PS20080220.1
USGS earthquake catalog (events 2000–2008)
Global CMT catalog for moment-tensor solutions