Seismic Swarm PS20210812.4: Analysis of the South Sandwich Islands Event

The seismic swarm designated PS20210812.4 occurred in the South Sandwich Islands region of the South Atlantic Ocean. It began at 18:35 UTC on 12 August 2021 and concluded at 03:10 UTC on 17 August 2021, spanning 104 hours and 35 minutes. During this period, 53 earthquakes were recorded, with magnitudes ranging from 5.0 to a peak of 8.1. The swarm initiated with the magnitude 8.1 event at a depth of 22 km, followed by numerous aftershocks clustered at depths between 8 km and 53 km.

This sequence reflects typical aftershock patterns following a major mainshock in a subduction setting. The events were distributed across a compact area, with the magnitude 8.1 and a subsequent magnitude 7.5 shock located 71 km and 66 km from the swarm center, respectively. Depths remained predominantly shallow to intermediate, consistent with activity along the plate interface and overlying crust.

The South Sandwich Islands lie along the South Sandwich subduction zone, where the South American Plate converges with and subducts beneath the Scotia Plate at rates of approximately 6–7 cm per year. This tectonic interaction forms the South Sandwich Trench, one of the deepest oceanic trenches in the Atlantic, and drives frequent seismicity. The region features active volcanism on islands such as Montagu and Saunders, resulting from the release of volatiles during subduction. The underlying geology includes oceanic crust of varying ages, with the overriding Scotia Plate incorporating both continental fragments and back-arc basin structures.

Historical records indicate eight prior swarms in the region since 2000, occurring in 2008, 2010, 2014, 2015, 2017, 2018 (two events), and 2021. These episodes underscore the persistent seismic productivity of the subduction zone. The 2021 swarm aligns with this pattern, contributing to the area's documented history of clustered earthquake activity.

Notable strong events since 2000 include the magnitude 8.1 South Sandwich Islands earthquake of 12 August 2021 and the magnitude 7.5 event in the same region on the same day. Both occurred in close proximity to the swarm center, highlighting the potential for large-magnitude ruptures along the megathrust.

Insights from the swarm data reveal a rapid onset of activity dominated by the initial magnitude 8.1 shock, followed by a decay in frequency over subsequent days. Multiple magnitude 5.0–5.8 events persisted through 16 August, with a notable magnitude 6.9 aftershock on 16 August at 16 km depth. Depths showed variability, suggesting involvement of both the subduction interface and shallower crustal faults.

Such swarms provide valuable data for understanding stress transfer and rupture dynamics in subduction environments. Monitoring efforts by global seismic networks continue to track activity in this remote yet tectonically active region.

References
USGS Earthquake Catalog
Global CMT Project
NOAA National Centers for Environmental Information