The 2021 South Sandwich Islands Earthquake and Regional Seismicity

The South Sandwich Islands region lies along the South Sandwich subduction zone in the southern Atlantic Ocean, where the South American Plate descends beneath the Scotia Plate. This convergent boundary drives frequent seismic activity, including megathrust events, and has shaped the volcanic arc that forms the islands themselves. The tectonic setting features a steeply dipping slab and relatively young oceanic lithosphere, contributing to a high rate of large earthquakes. On 12 August 2021 at 18:35 UTC, an M8.1 earthquake struck the South Sandwich Islands region at a depth of 22.7 km. The event occurred within the subduction interface and was followed hours later by an M7.5 earthquake approximately 90 km away. These shocks rank among the strongest recorded in the area since 2000. An earlier M7.1 event on 11 December 2018, located 67 km from the 2021 epicenter, further illustrates the persistent seismic productivity of the zone. The 2021 sequence released substantial strain accumulated along the plate boundary. Depths in the 20–30 km range are typical for interface ruptures in this segment, allowing efficient transmission of seismic energy. Historical records show that the South Sandwich subduction zone has generated multiple events exceeding M7.5 over the past century, consistent with its role as one of the more active margins in the Atlantic basin. Geologically, the islands represent the emergent portion of a volcanic arc built by magma generated from slab dehydration. Basaltic to andesitic volcanism has constructed the chain over millions of years, while ongoing subduction continues to recycle oceanic crust. The region remains remote, limiting direct observation, yet satellite and seismic networks provide continuous monitoring of both tectonic and volcanic processes. Seismic hazard assessments for the South Sandwich Islands emphasize the potential for future large events given the plate convergence rate of several centimeters per year. The 2021 earthquakes serve as a reminder of the dynamic nature of this margin and the value of sustained geophysical observation.

References

USGS Earthquake Catalog Global CMT Catalog