Seismic Swarm PS20050411.1 Near Sinabang, Indonesia

On 11 April 2005, a seismic swarm designated PS20050411.1 was recorded 28 km SSW of Sinabang, Indonesia. The sequence began at 06:11 UTC and concluded at 09:53 UTC, encompassing five earthquakes over 3 hours and 42 minutes. Event parameters include a magnitude 6.1 shock at 24 km depth at 06:11:11, followed by a magnitude 5.8 event at 30 km depth at 06:11:14. Subsequent activity comprised a magnitude 5.5 earthquake at 26 km depth at 09:04:30, a magnitude 4.6 event at 30 km depth at 09:15:00, and a magnitude 5.0 shock at 28 km depth at 09:53:36.

This swarm occurred within the tectonically active Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates exceeding 5 cm per year. The region, centered on Simeulue Island, experiences frequent seismicity due to megathrust faulting along the Sunda Trench. Historical records since 2000.01.01 document nine swarms in the area, with two occurring in 2002, one in 2004, and six in 2005, highlighting episodic clustering of moderate events.

Notable larger earthquakes near the swarm center include the magnitude 8.6 event 78 km WSW of Singkil on 28 March 2005, the magnitude 7.8 shock 75 km east of Sinabang on 6 April 2010, and the magnitude 7.4 earthquake 50 km northwest of Sinabang on 2 November 2002. These events underscore the persistent seismic hazard in the vicinity, driven by strain accumulation and release along the plate interface.

Swarm activity such as PS20050411.1 typically reflects fluid migration or stress triggering in the overriding plate or along subsidiary faults, distinct from mainshock-aftershock sequences. The shallow focal depths (24–30 km) align with the regional seismogenic zone, where brittle failure predominates above the deeper aseismic slip.

References

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