M 7.3; South Sandwich Islands region; (15 Jul 2013) (57km from the swarm center)
Seismic Swarm PS20210823.1 in the South Sandwich Islands Region
A seismic swarm designated PS20210823.1 occurred in the South Sandwich Islands region from 19:20 UTC on 22 August 2021 to 14:28 UTC on 23 August 2021. Over this 19-hour period, eight earthquakes were recorded. The sequence began with a magnitude 5.0 event at 74 km depth, followed shortly by the largest event of magnitude 7.1 at a shallow depth of 6 km. Subsequent activity included five events between magnitudes 4.7 and 5.4, all at 10 km depth, and concluded with a magnitude 5.0 event.
This swarm featured a prominent shallow mainshock followed by aftershocks clustered at consistent moderate depths. The timing and distribution indicate typical aftershock behavior in a tectonically active subduction setting, with the initial deeper event possibly representing a foreshock or unrelated background seismicity.
The South Sandwich Islands lie along the South Sandwich Trench, where the South American Plate subducts beneath the Scotia Plate. This convergent margin produces frequent moderate to large earthquakes and volcanic activity along the island arc. The region experiences ongoing deformation due to the eastward motion of the Scotia Plate relative to the South American Plate.
Historical records since 2000 show four prior swarms in the area: one in 2015, one in 2019, and two in 2021. Strong earthquakes in the vicinity include a magnitude 7.3 event on 15 July 2013 located 57 km from the swarm center and the magnitude 7.1 event of 22 August 2021 situated 17 km away. These events underscore the persistent seismic hazard associated with the subduction zone.
Seismic swarms in this setting often reflect stress adjustments along the plate interface or within the overriding plate. The shallow depth of the magnitude 7.1 mainshock aligns with typical rupture depths observed in similar arc environments.
References
SeismoSight internal classification data for swarm PS20210823.1.
USGS Earthquake Catalog for regional historical seismicity.
Global tectonic summaries from the U.S. Geological Survey.