Location:

Period:

20 Apr 2012 19:34:06 - 21 Apr 2012 11:10:50 (15 hours 36 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

4 swarms found nearby.

2004

PS20041226.6(154.2km)

26 Dec

2 days 1 hours

24 earthquakes

PS20041229.2(59.9km)

28 Dec

18 hours

5 earthquakes

PS20050101.2(189.1km)

31 Dec

21 hours

5 earthquakes

2012

PS20120411.3(109.3km)

11 Apr

1 day 4 hours

10 earthquakes

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Seismic Swarm PS20120421.1: Analysis of April 2012 Activity Off Western Sumatra

A seismic swarm designated PS20120421.1 was recorded between 19:34 on 20 April 2012 and 11:10 on 21 April 2012, approximately 284 km west-southwest of Meulaboh, Indonesia. The sequence lasted 15 hours and 36 minutes and comprised five earthquakes. All events occurred in a tectonically active offshore 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 individual events are listed below in chronological order:

20 April 2012, 19:34:06 UTC – magnitude 5.2 at 19 km depth
20 April 2012, 22:19:46 UTC – magnitude 5.7 at 24 km depth
20 April 2012, 22:28:59 UTC – magnitude 5.9 at 21 km depth
21 April 2012, 11:04:30 UTC – magnitude 5.2 at 10 km depth
21 April 2012, 11:10:50 UTC – magnitude 4.6 at 10 km depth

The largest event reached magnitude 5.9, with focal depths ranging from 10 km to 24 km, consistent with shallow seismicity near the plate interface or within the overriding plate. Such swarms typically reflect episodic stress release along fault segments without a single dominant mainshock–aftershock pattern.

Western Sumatra lies above the Sunda megathrust, one of the most seismically active subduction zones on Earth. The 26 December 2004 Mw 9.1–9.3 Sumatra–Andaman earthquake nucleated roughly 200 km northwest of this swarm location and generated a trans-oceanic tsunami. Post-2004 aftershock sequences and subsequent moderate swarms have been documented along the same trench segment, illustrating continued strain accumulation and release.

Since 1 January 2000, four swarms have been identified in the broader region according to internal SeismoSight classification. Three occurred in 2004 and one in 2012. The 2012 swarm represents the most recent cluster in this record. These episodes underscore the area’s tendency toward clustered moderate-magnitude activity superimposed on the background rate of great subduction earthquakes.

From a monitoring perspective, the rapid succession of five events within less than 16 hours indicates localized fault interaction rather than widespread aftershock migration. Depths clustered near 10–24 km suggest activity at or immediately above the seismogenic portion of the megathrust. Continued surveillance of this segment remains important given its proximity to the 2004 rupture area and the potential for future large events.

References

USGS Earthquake Hazards Program – Tectonic Summary of the Sumatra Region
Global CMT Catalog – Moment Tensor Solutions for Sumatran Subduction Zone Events
SeismoSight Internal Swarm Database (classification data only)