Seismic Swarm PS20110115.1 Near Sinabang, Indonesia: Event Analysis and Regional Context

Seismic swarm PS20110115.1 occurred on 15 January 2011, centered 31 km southwest of Sinabang, Indonesia. The sequence began at 11:23 local time and concluded at 17:20, encompassing five earthquakes over a period of five hours and fifty-six minutes. This activity unfolded in a tectonically dynamic setting along the Sunda subduction zone, where the Indo-Australian Plate subducts beneath the Eurasian Plate at rates of approximately 5–6 cm per year.

The swarm events exhibited the following parameters: a magnitude 5.7 earthquake at 11:23:52 with a focal depth of 17 km; a magnitude 5.0 event at 11:45:17 at 10 km depth; a magnitude 5.4 shock at 16:26:05 at 10 km depth; a magnitude 5.5 earthquake at 16:26:11 at 40 km depth; and a magnitude 4.7 event at 17:20:46 at 10 km depth. Depths remained predominantly shallow, consistent with upper-crustal deformation along the megathrust interface. The tight temporal clustering and moderate magnitudes indicate a swarm rather than a classic foreshock-mainshock-aftershock sequence, likely driven by fluid migration or stress transfer within the subduction interface.

Since 1 January 2000, eleven swarms have been recorded in the vicinity. Earlier episodes include two in 2002, two in 2004, five in 2005, one in 2008, and one in 2010. These recurrent swarms underscore persistent strain accumulation and release patterns in the region.

Notable larger earthquakes since 2000 further illustrate the area's seismic potential. A magnitude 7.8 event struck 75 km east of Sinabang on 6 April 2010, located 88 km from the swarm centroid. Additional magnitude 7.4 earthquakes occurred 56 km northwest of Sinabang on 20 February 2008 (52 km from the swarm center) and 50 km northwest on 2 November 2002 (51 km from the swarm center). Such events reflect the capacity for significant rupture along adjacent segments of the Sunda megathrust.

Geologically, Sinabang lies on Simeulue Island, positioned directly above the Sumatran portion of the Sunda Trench. The island's geology comprises accreted sedimentary and volcanic rocks deformed by ongoing convergence, with historical great earthquakes, including the 2004 Indian Ocean event, demonstrating the zone's ability to generate tsunamigenic ruptures exceeding magnitude 9. Updated assessments from global monitoring networks confirm continued moderate seismicity in the decade following 2011, reinforcing the long-term hazard profile without altering the internal classification of the 2011 swarm itself.

• USGS Earthquake Catalog
• Global CMT Project
• Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports