Location:

Period:

7 Apr 2005 02:21:24 - 8 Apr 2005 05:48:47 (1 day 3 hours 27 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

8 swarms found nearby.

2005

PS20050328.2(50.7km)

28 Mar

1 day 2 hours

28 earthquakes

PS20050328.1(44.2km)

28 Mar

3 days 8 hours

24 earthquakes

PS20050329.1(91.9km)

28 Mar

12 hours

5 earthquakes

S20050329.1(37.6km)

28 Mar

3 days 4 hours

56 earthquakes

PS20050406.1(134.3km)

5 Apr

16 hours

5 earthquakes

PS20050411.1(153.6km)

11 Apr

3 hours

5 earthquakes

PS20050730.1(13.1km)

29 Jul

22 hours

5 earthquakes

2010

PS20100407.1(154.6km)

6 Apr

6 hours

5 earthquakes

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Seismic Swarm PS20050407.1 Near Singkil, Indonesia

Seismic swarm PS20050407.1 occurred in a tectonically active zone 139 km SSW of Singkil, Indonesia, along the western margin of Sumatra. This region lies above the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. The resulting megathrust interface produces frequent large earthquakes and associated aftershock sequences.

The swarm began at 02:21 on 7 April 2005 and concluded at 05:48 on 8 April 2005, spanning 27 hours and 27 minutes. Seven events were recorded during this interval, with magnitudes ranging from 4.1 to 5.7 and focal depths between 24 km and 60 km. Event details include a magnitude 5.0 quake at 02:21 on 7 April at 24 km depth, followed by a 5.6 event at 11:46 and a 5.2 event at 11:50, both near 26–30 km depth. Additional events occurred at 15:23 (M5.0, 24 km), 16:34 (M4.1, 30 km), and two final magnitude 5.7 shocks on 8 April at 01:51 (30 km) and 05:48 (60 km).

Swarm sequences such as this lack a dominant mainshock–aftershock pattern and instead feature clustered events of comparable size. In subduction settings, they may reflect stress redistribution along the plate interface or within the overriding plate following major ruptures. The April 2005 swarm took place roughly three months after the magnitude 9.1 Sumatra–Andaman earthquake of December 2004 and one month after the magnitude 8.6 Nias event of March 2005, both of which altered regional stress fields.

Since 1 January 2000, five earthquake swarms have been identified in the broader study area, with PS20050407.1 representing the earliest recorded instance. Subsequent swarms illustrate the persistent seismic productivity of the Sumatran margin, where moderate-magnitude clusters recur amid ongoing plate convergence.

Geological context for the Singkil region includes the offshore trench, forearc basins, and the onshore Sumatran Fault System, a right-lateral strike-slip feature that accommodates oblique convergence. Depths of the recorded events place them within the seismogenic zone of the megathrust and adjacent crustal volumes, consistent with typical subduction-zone seismicity.

References

United States Geological Survey Earthquake Catalog
Incorporated Research Institutions for Seismology (IRIS) Event Search
SeismoSight internal swarm classification records