Seismic Swarm PS20160902.1 in the South Georgia Island Region

The South Georgia Island region lies within a tectonically complex zone of the South Atlantic Ocean, where the Scotia Plate interacts with the South American Plate along transform and subduction boundaries. This setting produces frequent seismic activity, primarily driven by the nearby South Sandwich subduction zone and associated strike-slip faulting. The island itself forms part of the Scotia Arc, a chain of islands marking the northern edge of the Scotia Plate, with crustal deformation influenced by both plate convergence and lateral motion.

On 1 September 2016 at 22:56 UTC, a seismic swarm designated PS20160902.1 began in this region. The sequence lasted 17 hours and 59 minutes, concluding at 16:55 on 2 September 2016. During this period, five earthquakes were recorded, with magnitudes ranging from 4.8 to 5.5 and focal depths between 8 and 10 km. The events occurred in close temporal succession, characteristic of swarm behavior where multiple shocks of similar size occur without a single dominant mainshock.

The sequence initiated with a magnitude 5.1 event at 22:56 on 1 September. Subsequent shocks included a magnitude 5.5 at 01:57 on 2 September, followed by a magnitude 5.3 at 06:48, a magnitude 5.0 at 16:38, and a final magnitude 4.8 at 16:55, all on 2 September. Depths remained shallow throughout, consistent with activity along upper crustal faults in the area.

This swarm represents one of only four documented swarms in the South Georgia Island region since 2000. Earlier episodes occurred in 2004 and 2014, with two additional swarms recorded in 2016. Such infrequent clustering suggests episodic stress accumulation along local fault systems rather than continuous background seismicity.

Notably, a magnitude 7.4 earthquake struck the same region on 19 August 2016, approximately 47 km from the swarm center. This larger event may have altered local stress conditions, potentially contributing to the subsequent swarm activity observed two weeks later.

Seismic swarms in subduction-influenced margins like the Scotia Arc often reflect fluid migration, afterslip, or triggered fault slip following major ruptures. The shallow depths and moderate magnitudes observed here align with typical upper-plate deformation in this tectonic environment. Ongoing monitoring remains essential given the region's proximity to plate boundaries capable of generating larger events.

References

USGS Earthquake Catalog
Global CMT Catalog
SeismoSight internal swarm classification data