Landslide triggered a mega-tsunami in Alaska, new study shows

Before-and-after satellite imagery showing locations and extent of the August 10, 2025 landslide. South Sawyer Glacier terminus positions since 1979 and the locations of three precursory slope failures since 2017 are also shown. In panel B, the white line indicates the main landslide, while the dashed yellow delineates the additional landslide-affected area. The pink line indicates the area of landslide deposition on top of the glacier. Coloured dots (which are close enough together that they look like a line) represent the tsunami runup. (Planet Labs.) Click on the figure to open it in a bigger format.

If any of this rings a bell, it may be because you have heard of a similar event that took place in Dickson Fjord in East Greenland in 2023. There, too, a major landslide occurred, leading to a 200-metre-high tsunami and subsequently a seiche that generated a seismic signal detectable for nine days – across the globe. At the time, Kristian Svennevig led an international team of researchers who documented the event. It was the first time that researchers had documented a seiche causing the globe to ‘ring’ for several days in a row.

“This is only the second time we have been able to document a seiche caused by a landslide being recorded on seismometers worldwide,” explains Kristian Svennevig.

The study of the event in Dickson Fjord has paved the way for the work to locate and understand the similar signal following the seiche in Tracy Arm Fjord to be much easier, simply because we now know what such a signal from a seiche can look like. This was simply not known before the study of the events in Dickson Fjord, where it took over six months to reach the conclusion. Kristian Svennevig highlights that there are several recognisable elements, and he also mentions a previous landslide in Karrat Fjord in West Greenland in 2017. He expects that massive landslides, tsunamis and seiches are something we will experience more frequently in the future.

“We know that climate change means we’re going to see more landslides like these in the Arctic. In some places, this is because the permafrost there – which acts as a kind of glue holding the rock faces together – is drying out and becoming unstable. In other places, such as here at Tracy Arm, it is because the glaciers are retreating, so the mountainsides lose their support,” explains Kristian Svennevig.

In Karrrat Fjord in Greenland in 2017, the subsequent tsunami had tragic consequences when it struck the village of Nuugaatsiaq and four people lost their lives. Fortunately, there were no loss of life in Tracy Arm Fjord in 2025, as the tsunami occurred at a time when the fjord was empty of cruise ships. In summer, there are at least three cruise ships in the fjord every day, so it could easily have turned into a disaster, the researchers write in the article. As far as we know, the only loss associated with the incident was that some people camping near the mouth of the fjord lost a kayak with some equipment. Observations from them and from crews on board ships who saw waves in parts of the fjord systems further away are included in the study.

“The study is the result of an international effort involving collaboration across national borders and institutions. It has been a pleasure to lead this important collaboration,” says Dan H. Shugar, Professor at the University of Calgary in Canada and lead author of the article.

Infographic showing the relative runup heights of several of the largest historic tsunamis with some of the world’s tallest buildings. (Steve Hicks/UCL.) Click to open the infopraphic in a bigger format.

Closeup oblique aerial photo of the highest runup resulting from the August 10, 2025 landslide-triggered tsunami in Tracy Arm during a U.S. Geological Survey field reconnaissance overflight on August 13, 2025. View direction is approximately south. (Cyrus Read/U.S. Geological Survey.)