Seismic Swarm Activity in the South Sandwich Islands Region: September 2021 Event

The South Sandwich Islands region lies along the southern Scotia Arc in the South Atlantic Ocean, where the South American Plate subducts westward beneath the Scotia Plate at a rate of approximately 7–9 cm per year. This convergent margin produces frequent intermediate-depth and shallow seismicity, accompanied by active volcanism on the island chain. The tectonic setting features a steeply dipping Wadati-Benioff zone that extends to depths exceeding 200 km, contributing to the area’s elevated seismic hazard.

Between 16:13 UTC on 4 September 2021 and 08:19 UTC on 6 September 2021, a swarm of six earthquakes occurred within the South Sandwich Islands region. The sequence lasted 40 hours and 6 minutes and comprised events with magnitudes ranging from 4.7 to 5.6. Focal depths varied between 10 km and 44 km, consistent with both upper-plate and interface seismicity in the subduction zone. The largest events reached magnitude 5.6 and were located at depths of 27 km and 28 km, respectively.

The temporal distribution showed two magnitude-5.1 events occurring within seconds of each other on 4 September, followed by three additional shocks on 5 September and a final magnitude-5.6 event on 6 September. Such clustered activity without a dominant mainshock is characteristic of swarm behavior in subduction environments, where fluid migration or aseismic slip can trigger multiple similarly sized events over short intervals.

Since 1 January 2000, ten seismic swarms have been documented in the region. Prior episodes occurred in 2015 (one swarm) and 2018 (one swarm), with the remaining eight recorded in 2021. This recent increase aligns with heightened regional strain following the magnitude-8.1 earthquake of 12 August 2021, whose epicenter lay 61 km from the September swarm centroid. An earlier magnitude-7.1 event on 11 December 2018 was located 79 km from the same area, indicating persistent stress accumulation along the plate interface.

The September 2021 swarm occurred at shallow to intermediate depths typical of the seismogenic zone in this arc. Depths around 10 km suggest possible involvement of the overriding Scotia Plate, while events near 27–44 km likely originated near the plate interface or within the subducting slab. No surface rupture or tsunami was reported, consistent with the moderate magnitudes and offshore locations.

Ongoing monitoring by global seismic networks continues to track aftershock decay and background seismicity in the wake of the 2021 great earthquake. The recurrence of swarms underscores the dynamic nature of the South Sandwich subduction zone and the value of high-resolution catalogs for understanding short-term clustering in tectonically active margins.

References

USGS Earthquake Catalog (events 2021-09) Global Centroid Moment Tensor Project International Seismological Centre Bulletin