M 7.5 - South Sandwich Islands region

Seismic observations suggest this earthquake is part of a complex seismic sequence. Our current interpretation is that this earthquake is a foreshock to a M8.1 that occurred ~170s later. Research is being conducted on this sequence to better understand the faulting geometry and details of rupture. This analysis will take time and our understanding will likely evolve. This page will be updated as we learn more about these interesting events.

Tectonic Summary

The August 12, 2021 M 7.5 earthquake in the South Sandwich Islands occurred as the result of reverse faulting at approximately 63 km depth in the Scotia subduction zone. This earthquake was a foreshock to an M 8.1 reverse faulting earthquake that occurred ~3 minutes later and ~90 km to the south. The focal mechanism solution indicates slip occurred on either a shallowly dipping fault striking east-southeast and dipping to the southwest, or on a steeply dipping fault striking to the northwest and dipping to the northeast. At the location of this earthquake, the South America plate subducts westward beneath the Scotia plate and South Sandwich microplate. The South America plate subducts at a rate of ~71 mm/yr relative to the South Sandwich microplate. The depth and focal mechanism solution of this event indicate it occurred as the result of intraplate faulting within the lithosphere of the subducting South America plate, rather than on the shallower thrust faulting plate boundary between the South America and South Sandwich plates. The top of the subducting plate in the vicinity of this earthquake is located at a depth of ~25km. Given the estimated depths of the earthquake and of the subducted slab, the August 2021 earthquake likely initiated in the lower half of the subducted South America plate as a result of compressional forces caused by downward bending of the subducted plate.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Events of the size of the August 12, 2021 earthquake are typically about 100 km x 35 km in size (length x width).

Over the previous century, seven other M7+ earthquakes occurred within 250 km of the August 2021 earthquake. The largest of these earthquakes was an M7.8 in May 1964. M7+ earthquakes in the South Sandwich Island region historically have occur at intermediate-depths (broadly defined as 70-300 km depth). The largest earthquake in the region was an M8.1 earthquake on June 27, 1929 and was located ~360 km northwest of the of August 2021 earthquake along the northern boundary of the Scotia plate with the South America plate. None of these earthquakes are known to have caused fatalities, likely because of their remote location far from population centers that might be vulnerable to earthquake shaking, as well as their intermediate depths which are not normally conducive to the generation of tsunamis.