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

Published 08-05-2026

On 10 August 2025, a 481-metre-high tsunami surged through a fjord in Alaska following a massive mountain landslide. A new article in Science describes the sequence of events.

På en fjeldside i Alaska ses hvor et fjeldskred og efterfølgende tsunami har ændret fjeldsiden. I fjorden under ses sedimenter fra fjeldskreddet.
Oblique aerial photo of the August 10, 2025 landslide, terminus of South Sawyer Glacier, and Tracy Arm taken from across the fjord during a U.S. Geological Survey field reconnaissance overflight on August 13, 2025. Note the trimline along the far side of the fjord, caused by the tsunami stripping the walls of vegetations. View direction is approximately north. (Cyrus Read/U.S. Geological Survey.)

On 10 August 2025, a massive landslide occurred in Tracy Arm Fjord in Alaska, triggering a mega-tsunami. Scientists estimate that at least 63.5 million cubic metres – that is 63,500,000 m³ – of material slid into the fjord during the landslide. That is enough material to fill approximately 25,000 Olympic-sized swimming pools – or 26 times the volume of the Great Pyramid of Giza. The landslide generated a very high tsunami. Traces of the tsunami can be seen 481 metres above sea level on a cliff face 1.6 km away from where the landslide struck the fjord. The tsunami thus ranks second among the highest tsunamis ever observed.

The material originated from an area measuring 1,000 x 600 metres, 1,000 metres above sea level.

“This was a massive landslide from a great height, and combined with the shape of the fjord, it created the conditions for the massive tsunami and a subsequent seiche,” explains Kristian Svennevig, a senior researcher at the Geological Survey of Denmark and Greenland (GEUS) and Denmark’s leading expert on landslides. A seiche is a phenomenon where the water sloshes back and forth in the fjord, creating a seismic signal that can be measured by seismometers around the world.

Kristian Svennevig is among the researchers who have just published a research article on the event in Science. Dan H. Shugar from the University of Calgary in Canada is the lead author of the article.

Landslides occur regularly in Alaska’s fjords, which are characterised by steep mountains and glaciers. Kristian Svennevig explains that there are several factors that make this landslide particularly interesting:

“Apart from the massive volume of material, the record-breaking tsunami is, of course, exceptional. Furthermore, the fjord is a popular destination for cruise tourism, which underlines the need for us to understand these phenomena, even though they occur in remote Arctic fjords. And then, of course, this is only the second time we have recorded a seiche triggered by a landslide,” says Kristian Svennevig.

 

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.

Common features may point to a future with more landslides

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.

tsunami-heights-V2

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.

How the researchers do it

How can we tell how high the water rose during the tsunami?

The tsunami swept vegetation away from the slopes of the fjord. This means that the traces left by the wave can be mapped quite accurately across the entire fjord using satellite images. In addition, the researchers also use advanced computer models to understand how the tsunami wave propagated through the fjord and developed into a seiche.

How do we know how much material has slid down?

The researchers estimate the volume of the landslide by creating elevation models before and after the event and subtracting one from the other, arriving at a figure of at least 63.5 million cubic metres. This figure is used as input in the computer model of the tsunami.

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.)

Seismometers

Seismometers are sensitive scientific instruments that detect vibrations travelling through the earth – known as seismic waves. Traditionally, seismology focuses on measuring seismic vibrations caused by earthquakes. However, seismic records can also contain information about the movement of large masses on the Earth’s surface, such as landslides and waves of water.

The research article

A 481 m-high landslide-tsunami in a cruise ship-frequented Alaska Fjord.
By Dan H Shugar, Katherine R Barnhart, Mira Berdahl, Jacqueline Caplan-Auerbach, Göran Ekström, Aram Fathian, Marten Geertsema, Stephen P Hicks, Bretwood Higman, Erin K Jensen, Ezgi Karasözen, Patrick Lynett, John Lyons, Thomas Monahan, Gerard Roe, Kristian Svennevig, Liam Toney, Maximillian Van Wyk de Vries, Michael E. West.
DOI: 10.1126/science.aec3187
Published in Science, 6 May 2026

Media

Images, illustrations and videos including captions and credits to be used by media in relation to mentions of the study are available here: Media

Portræt af en person på et kontor. I baggrunden er der en bogreal med diverse rammer og tidsskrifter. Kristian Svennevig. (Jacob Lind Bendsten, GEUS)

Kristian Svennevig
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