Location:

Period:

28 Mar 2005 18:03:41 - 31 Mar 2005 22:58:53 (3 days 4 hours 55 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

6 swarms found nearby.

2005

PS20050328.2(86.5km)

28 Mar

1 day 2 hours

28 earthquakes

PS20050328.1(21.6km)

28 Mar

3 days 8 hours

24 earthquakes

PS20050329.1(93.0km)

28 Mar

12 hours

5 earthquakes

PS20050406.1(111.7km)

5 Apr

16 hours

5 earthquakes

PS20050407.1(37.6km)

7 Apr

1 day 3 hours

7 earthquakes

PS20050730.1(39.6km)

29 Jul

22 hours

5 earthquakes

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Seismic Swarm S20050329.1 Near Singkil, Indonesia: Geological Context and Event Analysis

Seismic swarm S20050329.1 occurred in the waters southwest of Sumatra, Indonesia, centered approximately 128 km SSW of Singkil in Aceh province. The sequence began at 18:03 on 28 March 2005 and concluded at 22:58 on 31 March 2005, spanning 76 hours and 55 minutes. During this period, 56 earthquakes were recorded, with magnitudes ranging from 4.1 to 4.9 and focal depths predominantly between 18 km and 30 km.

This activity unfolded in a tectonically active region along the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of 5–6 cm per year. The zone has produced major megathrust events, including the 26 December 2004 Mw 9.1–9.3 Sumatra-Andaman earthquake and the 28 March 2005 Mw 8.6 Nias-Simeulue earthquake. The latter occurred hours before the swarm onset and likely influenced local stress conditions through afterslip and dynamic triggering.

Event timings show a dense cluster on 28 March following the mainshock, with subsequent activity distributed over the following days. Magnitudes remained moderate, peaking at 4.9 on multiple occasions, while depths clustered around 21–30 km, consistent with intraslab or interface seismicity in the subduction environment. No events exceeded magnitude 5.0, indicating a swarm rather than a typical foreshock-mainshock-aftershock sequence.

Since 1 January 2000, three such swarms have been documented in the broader region, with this event representing the first. The concentration of activity at intermediate depths suggests involvement of the subducting slab, possibly modulated by fluid migration or stress transfer from the recent great earthquake.

The swarm provides insight into post-seismic relaxation processes in the Nias segment of the subduction interface. Continued monitoring in this area remains essential given its history of large-magnitude ruptures.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification records