Seismic Swarm Activity in the South Sandwich Islands Region, August 2021

The South Sandwich Islands region lies along the South Sandwich subduction zone, where the South American Plate descends beneath the Scotia Plate. This tectonic setting produces frequent earthquakes and occasional seismic swarms due to ongoing plate convergence and associated faulting. The islands themselves are volcanic, formed by this subduction process, with a history of both tectonic and volcanic seismicity documented over decades.

On 20 August 2021, a seismic swarm was recorded in the South Sandwich Islands region. The sequence began at 03:21 UTC and concluded at 22:25 UTC, lasting 19 hours and 4 minutes. During this period, six earthquakes were registered, with magnitudes ranging from 4.6 to 5.2 and focal depths between 10 km and 55 km. The events occurred in rapid succession, characteristic of swarm behavior where multiple earthquakes of similar size take place without a single dominant mainshock.

The individual events included a magnitude 5.2 earthquake at 03:21:33 UTC (35 km depth), followed later by another magnitude 5.2 at 15:07:40 UTC (10 km depth) and a magnitude 5.1 at 15:07:46 UTC (55 km depth). Subsequent activity comprised a magnitude 5.0 at 18:59:13 UTC (20 km depth), a magnitude 4.6 at 19:36:27 UTC (51 km depth), and a final magnitude 5.0 at 22:25:55 UTC (35 km depth). Depths varied across the sequence, indicating activity across different segments of the subducting slab and overlying crust.

This swarm represents the second such event recorded in 2021. Historical data since 2000 show a total of eight swarms in the region, occurring in 2010, 2014, 2015, 2017, 2018 (two instances), and 2021 (two instances). These swarms highlight recurring episodic seismic release along the subduction interface.

The timing of the August 2021 swarm followed closely after two major earthquakes earlier in the month. A magnitude 8.1 event struck on 12 August 2021, centered approximately 65 km from the swarm location, and a magnitude 7.5 earthquake occurred the same day about 33 km away. Such large events can alter local stress conditions, potentially triggering subsequent swarm activity through afterslip or fluid migration along faults.

Seismic monitoring in this remote region relies on global networks, given the limited local instrumentation. The South Sandwich subduction zone remains one of the most active in the South Atlantic, with a long-term record of both moderate swarms and infrequent great earthquakes. Continued observation helps refine understanding of subduction dynamics in this isolated setting.

References

SeismoSight internal swarm classification PS20210820.1
USGS Earthquake Catalog (historical events and regional tectonics)