2021 South Sandwich Islands earthquakes

The **2021 South Sandwich Islands earthquakes** were a pair of powerful Intraplate earthquakes followed by dozens of strong aftershocks which struck along the South Sandwich Trench in August 2021. The major quakes measured 7.5 and 8.1 on the moment magnitude scale, according to the United States Geological Survey. The mainshock was the largest earthquake in the region since 1929, and was one of the largest earthquakes of 2021.

## Tectonic setting

The mainshock occurred as the result of reverse faulting at approximately 48 km depth in the Scotia subduction zone. The focal mechanism solution indicates slip occurred on either a steeply dipping fault dipping to the northwest, or on a shallowly dipping plane dipping to the southeast. Note that in this location, the subduction zone interface dips to the west. The earthquake occurred about 3 minutes after a 7.5 Mw foreshock that was located at a depth of 63 km and 90 km to the north. At the location of the M8.1 earthquake, the South America plate subducts westward beneath the Scotia plate and South Sandwich microplate (a component of the broader Scotia plate). The South American plate subducts at a rate of 71 mm/yr relative to the South Sandwich microplate. The depth and the mechanism of this event are consistent with the earthquake rupturing within the subducted South American plate (This would make this an intraplate earthquake), rather than on the interface between the two plates. Given the temporal proximity of these two large earthquakes, however, a faulting mechanism for the M8.1 mainshock is difficult to accurately constrain due to the overlapping seismic waves. Over the previous century, eight other M7+ earthquakes, including the M7.5 foreshock, occurred within 250 km of the August 2021 M8.1 earthquake. The largest of these previous earthquakes was an M7.8 in May 1964. M7+ earthquakes in the South Sandwich Island region have historically occurred at intermediate-depths (broadly defined as 70-300 km depth). However, the largest earthquake in the region was a shallow (10 km deep) M8.1 earthquake on June 27, 1929 that was located ~450 km northwest of the of August 2021 M8.1 earthquake along the northern boundary of the Scotia plate with the South American 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.

## Earthquakes

The first earthquake was initially reported as a 7.6 magnitude then later downgraded to a 7.5 magnitude, which struck at 16:32 PM local-time but did not cause any damage or casualties, the second event was much stronger at magnitude 8.1, and occurred 3 minutes after the foreshock, a tsunami warning was issued by the Pacific Tsunami Warning Center but was later cancelled for both events. The two quakes are considered an Intraplate event.

### Other events

The South Sandwich Islands mainshock came just 2 weeks after a magnitude 8.2 earthquake struck the Alaskan Peninsula, which was the 7th largest earthquake in the US and triggered a small tsunami. On March 4 of that year an 8.1 earthquake earthquake struck the Kermadec Islands, a sparsely populated territory owned by New Zealand. This clustering-in-time is unusual since earthquakes of magnitude eight or higher occur only about once per year on average. There is no evidence of a causal relationship between these three events, and 2021 has had the most M8+ earthquakes in one year since 2007.

## Tsunami

A tsunami with a maximum run-up height of 1 meter was measured at South Georgia Island.