WATCH: Tracy Arm fjord tsunami in popular Alaskan cruise spot provides valuable lessons

When part of a mountain in southeast Alaska slid into the ocean last summer, it triggered the second-highest tsunami ever recorded.

The tsunami on Aug. 10, 2025, ran 481 metres — two-and-a-half times the height of the Calgary Tower — up the wall of the Tracy Arm Fjord more than a kilometre away and generated a seismic signal equivalent to a magnitude 5.4 earthquake.

Despite its massive size, no one was caught in the wave because it hit around 5:30 a.m. 

Hours later, some of the 20 or so cruise ships, boaters and kayakers that use the general area daily could have been in the narrow fjord, which is a popular sightseeing area for glaciers in the Tongass National Forest about 80 kilometres south of Juneau, Alaska.

The “near miss” has now been studied by a team of international researchers, including two from the University of Calgary, and newly published in the prestigious journal Science. The study has determined that the event provided valuable lessons for those who spend time in steep, mountainous terrain.

“We reconstructed the event from a suite of perspectives, including eyewitness accounts from a variety of ship passengers and kayakers,” says geomorphologist Dr. Dan Shugar, PhD, lead author of the study and an associate professor with the Department of Earth, Energy, and Environment in the Faculty of Science.

A group of kayakers reported waking around 5:45 a.m. 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 mount of the fjord saw currents and white water, but no obvious wave.

The team of researchers, which also includes postdoctoral scholar Dr. Aram Fathian, PhD, also studied satellite data before and after the event, as well as seismic data, and did numerical modelling to understand exactly what happened.

Study co-author Dr. Thomas Monahan, DPhil, of the Department of Engineering Science at the University of Oxford, says satellite observations of remote ocean dynamics are becoming essential as Arctic hazards intensify.

"Until now, we simply didn't have a way to observe these waves directly, but our study has demonstrated that using data from the new Surface Water Ocean Topography satellite can reveal the full sea-surface structure of these events, even if no one witnesses them directly," Monahan says.

Shugar says there wasn’t much warning before the landslide hit in Tracy Arm Fjord.

“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,” he explains. “In this case, that didn’t happen.”

Instead, Shugar says there was some minor seismic noise that was so slight, it wouldn’t have caught anyone’s attention.

“This one was truly a surprise,” he says, noting this presents some challenges when it comes to disaster reduction in high-risk areas.

The research concludes that the glacial retreat in the Alaska fjord led to the near disaster.

There are lessons to be learned from the event — particularly that warming in cold regions is increasing the risk of hazards and that improved monitoring could mitigate some of those impacts.

Shugar says it’s important to pay close attention, particularly in areas on the West Coast and in polar regions where glaciers are thinning due to a changing climate.

“Ultimately what we hope is that coastal municipalities, the cruise ship industry and other stakeholders take these threats seriously,” he says, adding similar events could result in future disasters.

At least six cruise lines have altered their itineraries in Alaska this year due to the hazards that still exist in the Tracy Arm Fjord.

The U.S. Geological Survey has warned on its website that steep, mountainous landslide areas are “inherently unstable and will continue to change for years following an initial landslide.”