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