South Sandwich Islands Earthquake Swarm of September 2021

The South Sandwich Islands region lies along the South Sandwich Trench in the southern Atlantic Ocean, where the South American Plate subducts beneath the Scotia Plate. This convergent boundary produces frequent seismic activity, including both isolated large events and episodic earthquake swarms. The trench reaches depths exceeding 8,000 meters, and the associated volcanic arc forms the South Sandwich Islands themselves. High seismicity reflects ongoing plate convergence at rates of approximately 65–78 mm per year. On 9 September 2021, a swarm of five earthquakes was recorded in the region between 03:47 and 15:04 UTC. Magnitudes ranged from 4.9 to 5.3, with focal depths between 10 and 43 km. The sequence began with a 5.3 event at 43 km depth, followed within minutes by a 5.0 quake at 35 km. Subsequent events included two magnitude 5.1 shocks at 10 km depth and a final 4.9 event at 39 km. All activity occurred within an 11-hour, 17-minute window. This swarm fits a pattern of elevated seismic clustering observed in the region since 2000. Twelve swarms have been documented in that period, with nine occurring in 2021 alone. Earlier swarms were recorded in 2015, 2018, and 2019. The September 2021 activity followed the magnitude 8.1 South Sandwich Islands earthquake of 12 August 2021, whose epicenter lay approximately 77 km from the swarm centroid. A magnitude 7.1 event on 11 December 2018 occurred about 75 km away, underscoring the persistent seismic productivity along this segment of the subduction zone. Earthquake swarms in subduction settings often reflect fluid migration or localized stress transfer along the plate interface and within the overriding plate. Depths spanning the upper 50 km are consistent with both intraslab and interface seismicity. The brief duration and moderate magnitudes of the September 2021 swarm align with typical swarm characteristics rather than a classic foreshock–mainshock–aftershock sequence. Ongoing monitoring by global seismic networks continues to track activity along the South Sandwich Trench. The combination of deep subduction and back-arc spreading contributes to the region’s status as one of the most active seismic zones in the South Atlantic. References
United States Geological Survey Earthquake Catalog
Global Centroid Moment Tensor Project
International Seismological Centre Bulletin