Seismic Swarm PS20210826.1 in the South Sandwich Islands Region

The South Sandwich Islands occupy a tectonically active segment of the Scotia Arc in the South Atlantic Ocean. This remote volcanic chain marks the convergent boundary where the South American plate subducts westward beneath the Scotia plate along the South Sandwich Trench. Subduction drives frequent intermediate-depth seismicity and occasional great earthquakes, with the regional geology shaped by ongoing plate convergence at rates of approximately 7–9 cm per year.

Swarm PS20210826.1 began at 19:31 UTC on 25 August 2021 and ended at 05:16 UTC on 26 August 2021. Over the 9-hour 44-minute interval, five earthquakes were recorded. The sequence opened with a magnitude 5.0 event at 57 km depth on 25 August. Roughly eight hours later, a magnitude 5.8 earthquake occurred at 30 km depth, followed within minutes by a magnitude 5.1 event at 35 km. Two additional shocks of magnitudes 5.0 and 5.4, both at 35 km depth, completed the swarm shortly after 05:16 on 26 August.

Such clustered activity is consistent with the subduction-zone setting, where stress changes from nearby large ruptures can trigger aftershock-like sequences or short-lived swarms. The swarm’s timing—two weeks after the magnitude 8.1 South Sandwich Islands earthquake of 12 August 2021—suggests a possible link to post-seismic stress redistribution. That earlier event, centered roughly 51 km from the swarm epicenters, released substantial strain along the plate interface and likely influenced local fault behavior.

Historical records since 2000 document eight swarms in the same region, with six occurring in 2021 alone. Earlier episodes took place in 2015 and 2018. A magnitude 7.1 earthquake on 11 December 2018, located about 68 km from the present swarm center, further illustrates the area’s capacity for strong events separated by short intervals.

Collectively, the 2021 swarm and preceding great earthquake underscore the persistent seismic hazard along this remote arc. Continued monitoring remains essential for understanding how subduction dynamics generate both isolated large ruptures and episodic swarms in the South Sandwich Islands region.

References: USGS Earthquake Catalog (events of 12 Aug 2021 and 11 Dec 2018) Global CMT Catalog (regional moment tensors, 2000–2021) NOAA National Centers for Environmental Information (South Sandwich Trench bathymetry and plate-motion data)