Seismic Swarm PS20210812.3 in the South Sandwich Islands Region

The South Sandwich Islands region lies along the South Sandwich Trench, where the South American plate subducts beneath the Scotia plate. This convergent boundary produces frequent seismic activity, volcanic arcs, and occasional large-magnitude earthquakes. The tectonic setting features intermediate-depth seismicity driven by slab dehydration and stress accumulation within the subducting lithosphere.

SeismoSight internal classification recorded swarm PS20210812.3 from 18:32 on 12 August 2021 to 03:34 on 14 August 2021. Over 33 hours and one minute, 27 earthquakes were detected. The sequence began with a magnitude 7.5 event at 47 km depth, followed by events ranging from magnitude 4.8 to 5.9 at depths between 6 km and 54 km. Notable subsequent shocks included a magnitude 5.9 at 24 km, another magnitude 5.9 at 25 km, and a magnitude 5.7 at 19 km. Depths clustered predominantly in the 10–35 km range, consistent with upper-plate and intraslab fracturing in this subduction zone.

The swarm concluded with a magnitude 5.3 event at 35 km depth. All events remained within a compact spatial footprint centered near the initial mainshock. Historical records since 1 January 2000 document six prior swarms in the same region, occurring in 2010, 2014, 2015, 2017, and twice in 2018. These episodes illustrate recurring clustered seismicity without a single dominant mainshock, a pattern typical of fluid migration or stress triggering along the trench.

The magnitude 7.5 earthquake of 12 August 2021 stands as the strongest event recorded in the region since 2000. Its proximity—approximately 29 km from the swarm centroid—highlights the capacity for large subduction-related ruptures to initiate prolonged aftershock sequences. Regional monitoring indicates that such activity aligns with the long-term seismic hazard of the South Sandwich arc, where convergence rates exceed 70 mm per year.

This swarm provides further evidence of dynamic stress transfer within the subduction interface and overlying crust. Continued observation of similar clusters will refine understanding of rupture mechanics and volcanic-seismic interactions in this remote yet tectonically active setting.