Seismic Swarm in the South Sandwich Islands Region: August 2021

The South Sandwich Islands region lies in the southern Atlantic Ocean along a tectonically active boundary where the South American Plate subducts beneath the Scotia Plate. This convergence produces the South Sandwich Trench and fuels frequent seismic and volcanic events. The islands themselves form part of an intra-oceanic arc built on the South Sandwich Plate, with earthquakes commonly occurring at depths ranging from shallow crustal levels to intermediate depths associated with the subducting slab.

Between 18:36 UTC on 12 August 2021 and 02:01 UTC on 15 August 2021, a swarm comprising 41 earthquakes was recorded in this region. The sequence lasted 55 hours and 24 minutes. The largest event reached magnitude 6.7 at a depth of 35 km, followed shortly by a magnitude 6.1 event at the same depth. Subsequent activity included multiple events in the magnitude 5.0–6.0 range, with focal depths varying between 10 km and 73 km. Notable shocks occurred at 19:43 UTC on 12 August (magnitude 6.0, 35 km depth) and 19:44 UTC (magnitude 5.9, 40 km depth). The swarm exhibited a typical pattern of clustered moderate-magnitude events without a single dominant mainshock-aftershock sequence.

Seismic swarms in the South Sandwich Islands region remain uncommon. Since 1 January 2000, only three prior swarms have been documented: one each in 2015, 2018, and 2019. These episodes underscore the episodic nature of clustered seismicity along this plate boundary, distinct from the more regular occurrence of isolated large earthquakes.

On 22 August 2021, a magnitude 7.1 earthquake struck approximately 97 km from the swarm centroid, highlighting continued elevated seismic hazard in the days following the swarm. Depths and magnitudes observed during the August swarm align with typical intermediate-depth activity along the subducting slab in this arc system.

The South Sandwich region experiences ongoing tectonic strain accumulation due to rapid plate convergence rates exceeding 7 cm per year in places. Historical records indicate that large thrust earthquakes and associated aftershock sequences can occur, yet swarm-type activity appears limited to specific stress conditions along fault segments or within the downgoing slab.

References
USGS Earthquake Catalog
Global CMT Project
SeismoSight internal swarm classification PS20210812.2