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Location:
off the west coast of northern Sumatra
Period:
11 Apr 2012 08:38:36 - 12 Apr 2012 13:09:45 (1 day 4 hours 31 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
10
M 7.0+:
M 8.6; 2012 Wharton Basin Earthquake; (11 Apr 2012) (28km from the swarm center)
M 7.2; off the west coast of northern Sumatra; (10 Jan 2012) (26km from the swarm center)
2 swarms found nearby.
2004
PS20041229.2(162.3km)
28 Dec
18 hours
5 earthquakes
2012
PS20120421.1(109.3km)
20 Apr
15 hours
5 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm PS20120411.3: Analysis of Events Off Northern Sumatra

A seismic swarm designated PS20120411.3 was recorded off the west coast of northern Sumatra, commencing at 08:38 on 11 April 2012 and concluding at 13:09 on 12 April 2012. Over this 28-hour and 31-minute period, ten earthquakes were detected. The sequence began with a magnitude 8.6 event at a depth of 20 km, followed by nine aftershocks ranging from magnitude 5.0 to 5.4 at depths between 7 km and 15 km.

The full sequence of events is as follows:

  • 11 Apr 2012 08:38:36, magnitude 8.6, depth 20 km
  • 11 Apr 2012 10:10:04, magnitude 5.0, depth 10 km
  • 11 Apr 2012 10:21:15, magnitude 5.3, depth 13 km
  • 11 Apr 2012 12:00:35, magnitude 5.1, depth 10 km
  • 11 Apr 2012 12:37:48, magnitude 5.1, depth 10 km
  • 11 Apr 2012 14:18:44, magnitude 5.0, depth 7 km
  • 11 Apr 2012 15:46:51, magnitude 5.2, depth 13 km
  • 11 Apr 2012 16:04:23, magnitude 5.1, depth 15 km
  • 11 Apr 2012 18:54:45, magnitude 5.4, depth 10 km
  • 12 Apr 2012 13:09:45, magnitude 5.2, depth 10 km

This swarm represents the only such cluster recorded in the region since 2000. The prior swarm occurred in 2004. The mainshock aligns closely with the 2012 Wharton Basin earthquake (magnitude 8.6), centered 28 km from the swarm epicenter. An additional notable event was a magnitude 7.2 earthquake on 10 January 2012, located 26 km from the swarm center.

Geological Context of the Region

Northern Sumatra lies at the convergent margin where the Indo-Australian Plate subducts beneath the Eurasian Plate along the Sunda Trench. This tectonic setting produces frequent megathrust earthquakes and associated tsunamis. The 26 December 2004 Sumatra-Andaman earthquake (magnitude 9.1) originated nearby and generated a devastating Indian Ocean tsunami. Intraplate strike-slip faulting within the Wharton Basin, east of the trench, also contributes to seismic activity, as evidenced by the 2012 mainshock, recognized as one of the largest strike-slip events instrumentally recorded.

The swarm's shallow focal depths and rapid succession of moderate aftershocks reflect stress redistribution following the initial rupture. Historical records indicate elevated seismicity in the area since the 2004 event, underscoring the persistent influence of plate boundary dynamics.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)