August 12, 2021 Tsunami: Southern Atlantic Ocean, UK Territories

Note: tsunami event was originally attributed to the magnitude 7.5 earthquake that occurred ~170s earlier, now attributed to magnitude 8.1 earthquake.

The August 12, 2021, Sandwich Islands tsunami was the result of a complex seismic sequence of strong earthquakes in the Scotia Subduction Zone in the South Atlantic. The tsunami apparently was generated by the Mw 8.1 earthquake (58.451°S 25.327°W), at 2021-08-12 18:35:20 (UTC) (according to the USGS). This strong earthquake occurred just 3 minutes after and about 90 km to the south of the smaller Mw 7.5 foreshock (USGS). In the 24 hours following the mainshock, 61 aftershocks of Mw 4.5 or larger were detected, including three aftershocks greater than Mw 6 (USGS). Our analysis suggests that the tsunami was generated by the strongest Mw 8.1 earthquake. The largest recorded coastal tsunami amplitude was ~75 cm and was detected at the King Edward Point tide station 3 hours after the earthquake, while the locations in Alaska recorded tsunami signal over 15 cm after more than 24 hours of propagation from the source. To our knowledge, this was the first tsunami since the 2004 Indian Ocean tsunami that was recorded in three different oceans. (reference #13064)

Waves of over 1m can be seen at the station King Edward Point (reference #13002)

The August 12, 2021, Mw 8.1 earthquake in the South Sandwich Islands demonstrated that the southern Atlantic seismic zone that encompasses the Scotia Arc Subduction Zone is a tsunami threat to PTWS [Pacific Tsunami Warning System] coasts. Tsunami waves from this earthquake reached PTWS coasts within 4-6 hours (e.g., southernmost Chile and Chilean bases on the Antarctic Peninsula) and tsunami waves of more than 0.2 m amplitude were recorded in the Pacific as far away as French Polynesia, Hawaii and Alaska. (reference #13088)

A careful and systematic analysis of coastal and deepwater sea‐level records highlights three points: (a) the tsunami propagated across four oceans following major submarine features; (b) despite its very low amplitude, it reached as far as the Canary Islands in the Atlantic Ocean, Hawaii and the US West coast as far as Alaska and the Aleutian Islands in the Pacific Ocean; (c) it was recorded twice on New Zealand DART system NZC, with one record of the tsunami from the East and one from the West. This event is an opportunity to highlight the lack of knowledge about the South Sandwich subduction region in terms of its tsunamigenic potential and the associated tsunami hazard in the Pacific ocean. It should lead to an improvement of national tsunami warning procedures, by including this region as a tsunami source zone, for neighboring regions but also for distant countries like New Zealand or French Polynesia. (reference #13863)