The South Sandwich Tsunami of 12 August 2021: An Underestimated Widespread Tsunami Hazard Around the World

On 12 August 2021 a large Mw 8.1 earthquake, detected by global seismic networks, occurred on the South Sandwich subduction zone in the southern Atlantic Ocean. Approximately 1.5 hr later, a tsunami was clearly recorded on King Edward Point coastal tide gauge (South Georgia Island), approximately 800 km north‐west of the earthquake location. Subsequently it was recorded on other coastal stations both in the Atlantic Ocean, and also in the Indian and Pacific Oceans. 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. Plain Language Summary A Mw 8.1 earthquake occurring on the South Sandwich subduction zone on 12 August 2021 triggered a small but widespread tsunami. Showing less than 1 m maximum amplitude on the nearest coastal gauges, it was recorded in many locations of the Southern, Atlantic, Indian and Pacific Oceans, as far as Alaska and the Aleutian Islands. This event highlights the urgent need to improve the knowledge of the tectonics of this subduction zone, and increase the tsunami monitoring capacity in this remote region, which has the potential to affect not only neighboring countries, but most of the World's coastlines. Key Points The South Sandwich Islands Subduction Zone triggered a widespread transoceanic tsunami on 12 August 2021 propagating in four oceanic basins Tsunami simulation of a potential maximal tsunami source scenario showing its worldwide potential impact on the coasts The Scotia subduction zone has to be considered in the tsunami hazard assessment for the Pacific, Atlantic, and Indian Ocean as well