Seismic Swarm Activity in the South Sandwich Islands Region

A seismic swarm designated PS20210826.2 occurred in the South Sandwich Islands region from 21:26 on 25 August 2021 to 20:18 on 26 August 2021. Over 22 hours and 51 minutes, six earthquakes were recorded. The sequence began with a magnitude 5.3 event at a depth of 9 km, followed by a magnitude 5.0 at 35 km depth early on 26 August. Two magnitude 5.1 events occurred near midday, at depths of 10 km and 70 km respectively. A magnitude 4.4 shock at 10 km depth was recorded in the afternoon, and the swarm concluded with another magnitude 5.3 at 35 km depth.

This activity aligns with the region's established pattern of clustered seismicity. Historical records since 2000 indicate ten prior swarms in the area, occurring in 2010, 2014, 2015, 2017, 2018 (two events), and 2021 (four events). The swarm took place shortly after a magnitude 7.5 earthquake on 12 August 2021, centered approximately 55 km away, highlighting the potential for aftershock sequences and triggered events in this tectonically dynamic zone.

The South Sandwich Islands region forms part of the Scotia Sea plate boundary system in the South Atlantic Ocean. It features the South Sandwich Trench, where the South American Plate subducts westward beneath the Sandwich Plate at rates exceeding 70 mm per year. This subduction drives intense seismic activity, with frequent intermediate-depth and shallow earthquakes. The islands themselves represent a volcanic arc resulting from this process, with active volcanism contributing to the overall geological instability.

Seismicity in the region has been documented extensively since instrumental recording began, reflecting both the subduction interface and associated crustal faulting. The August 2021 swarm exemplifies typical swarm behavior, characterized by multiple events of moderate magnitude without a dominant mainshock. Depths ranging from 9 km to 70 km indicate activity across both crustal and upper mantle levels consistent with subduction zone dynamics.

Such events underscore the importance of continuous monitoring in remote subduction settings, where clusters can signal evolving stress conditions along the plate interface.

References:
USGS Earthquake Hazards Program – South Sandwich Islands regional seismicity reports
Global CMT Catalog – subduction zone parameters and event data
SeismoSight internal swarm classification records (PS20210826.2 and historical statistics)