The side of a mountain slid into an Alaskan fjord last August, producing a tsunami 481 metres high, the second highest ever recorded, according to a new study involving a UCL researcher.
The tsunami occurred in the Tracy Arm fjord in southeastern Alaska, a popular spot for cruise ships, but no ships were caught in the wave and no one was hurt as it hit early in the morning, at around 5.30am.
The landslide was ultimately a result of climate change, as the glacier holding up the mountain had retreated, leaving the rock unsupported.
It is the second such event occurring over two years - the first, in a fjord in Greenland in 2023, resulted in a smaller wave, a mere 200 metres high.
For the new study, published in the journal Science, researchers reconstructed what happened using eyewitness accounts, numerical modelling, and data from satellites and seismometers measuring Earth's vibrations.
The wave scraped off trees and plants along the walls of the fjord, leaving a sheer rock face. This enabled researchers to infer that the wave reached 481 metres high (taller than the Empire State Building) over the first kilometre, before dissipating as it flowed along the fjord.
A group of kayakers camped out further down the fjord reported waking around 5.45am to see water flowing past their tent, carrying away one of their kayaks and much of their gear. Another observer described a two-metre wave coming along the beach, while those on a cruise ship anchored near the mouth of the fjord saw currents and white water, but no obvious wave.
The landslide and tsunami were also recorded by seismometers around the world, with ground vibrations generated by the water in the fjord moving back and forth lasting for 36 hours. (The movement of water trapped in the Dickson fjord following the Greenland tsunami in 2023 generated similar vibrations around the world, but for nine days.)
The amount of rock that crashed into the water was equivalent to 24 Great Pyramids of Giza, the researchers estimated. Prior to the collapse, the glacier underneath had retreated rapidly, by 500 metres in a matter of weeks.
Lead author Dr Dan Shugar, based at the University of Calgary, said: "Normally with these gigantic rock avalanches, they often give some sort of warning signs in the weeks, months, years prior when the slope is slowly moving down the mountain. It's sagging and then it catastrophically gives way in a rock avalanche. In this case, that didn't happen."
Co-author Dr Stephen Hicks (UCL Earth Sciences) said: "Fjords with retreating glaciers have seen a sharp increase in tourism. More people are visiting these beautiful places in the polar and sub-polar regions but they are at the forefront of climate change impacts and so can be dangerous.
"This event took everyone by surprise. The area had not been identified as hazardous. We need to reduce the risk of these expeditions by better identifying the riskiest areas and getting investment into warning systems that might give us a few hours' or days' notice of a potentially catastrophic event.
"With hindsight, there were some warning signs. Tiny earthquakes occurred at an increasing rate in the days to hours before the landslide, signalling that this mass of rock was starting to crack. Many seismic monitoring stations provide data in real-time, so this gives us some optimism that we can turn what we have learned into a warning system."
At least six cruise lines have changed their itineraries in Alaska this year to avoid the Tracy Arm fjord where the tsunami occurred.
Dr Shugar said: "Ultimately what we hope is that coastal municipalities, the cruise ship industry and other stakeholders take these threats seriously."
