Alaska landslide generated 481-meter tsunami in near-miss tourist disaster

At a glance:

• A 63.5 million cubic meter rockslide into Alaska's Tracy Arm fjord on August 10, 2025 produced a 481-meter runup tsunami — the second highest ever recorded on Earth.
• Glacial retreat driven by 1.1 °C of industrial-era warming left the slope unsupported, and the slide's seismic signature registered globally as a magnitude 5.4 event.
• The event missed tragedy only because it struck at 5:26 am; cruise-ship traffic in Alaska has risen from 1 million to 1.6 million passengers since 2016, and researchers warn similar events could strike in Canada, New Zealand, Greenland, Norway and elsewhere.

The slide and the wave

At 5:26 am local time on August 10, 2025, a massive wedge of rock detached from a mountain above Alaska's Tracy Arm fjord and plunged into the deep water at the terminus of the South Sawyer Glacier. The volume of material that fell was at least 63.5 million cubic meters. The impact generated an initial breaking wave 100 meters high that tore across the narrow fjord at speeds exceeding 70 meters per second. When the wave struck the opposite shoreline it surged up the steep rock face to a runup height of 481 meters above sea level. Aram Fathian, a researcher at the University of Calgary and co-author of a recent Science study reconstructing the event, called it "the second highest tsunami ever recorded on Earth." The only higher runup on record is the 1958 Lituya Bay landslide tsunami, which reached 530 meters. Since 1925, scientists have documented 27 landslide-generated tsunamis with runups exceeding 50 meters, and the Tracy Arm event now sits among the most extreme of that small but terrifying dataset. Because the landslide happened in the pre-dawn hours, there were no injuries or fatalities. But the handful of people in the vicinity were shaken. About 55 kilometers away on Harbor Island, a group of kayakers saw water flowing past their tents 20 minutes after the slide, and the surging tide carried away some gear and one kayak. In No Name Bay, observers on a motor vessel reported a 2 to 2.5-meter cresting wave followed by a secondary 1-meter wave. Eighty-five kilometers from the source, the crew of a small cruise boat anchored in Ford's Terror watched water pour over a nearby sandbar and physically lift their vessel three meters despite a falling tide.

Why the glacier mattered

The root cause of the Tracy Arm collapse, according to the study, was the retreat of the South Sawyer Glacier — a massive wall of ice that for centuries had braced the steep rock wedge on the northern side of the fjord. The glacier's headscarp sat roughly 1,025 meters above sea level, and its presence kept the slopes stable. But South Sawyer, like many glaciers in the Stikine Icefield, has been retreating as the climate warms. Researchers calculating regional temperature trends found a 1.1 °C increase in summertime temperatures since around 1875, which drove snowline elevations up by roughly 169 meters. Between 2013 and 2022 alone, the glacier ice at the failure site thinned by 100 to 130 meters. By July 2025, just weeks before the landslide, glacial retreat had exposed the very base of the slope. The "icy straitjacket" that had kept the rock from collapsing was gone.

Seismic warning signs missed in real time

Retrospective analysis of optical and radar satellite imagery from the weeks before the slide showed no visible tension cracks or major deformational scarring on the slope's surface — from the outside it looked perfectly sound. But deep within the rock, surfaces were already grinding. Regional seismometers registered localized repeating earthquakes beginning as early as August 5. By August 9, these micro-earthquakes were occurring once every hour. In the six hours leading up to the main failure, the gaps between seismic signals shrank to between 30 and 60 seconds. About an hour before the landslide, the signals merged into a continuous grinding slip — and then the rock fell. The impact of 63.5 million cubic meters of rock hitting the fjord released forces large enough to be registered globally. The seismic waves cascaded across the planet and were equivalent in energy to a magnitude 5.4 earthquake. Within the fjord, the sloshing water established a 66-second long-period seiche — a standing wave — that reverberated back and forth for 36 hours.

The cruise-ship exposure problem

Tracy Arm is a highly frequented tourist destination. During the summer, more than 20 boats navigate the Tracy and Endicott arms every day, including up to six large cruise ships. At the mouth of the fjord, a National Geographic Venture cruise ship carrying around 150 people was anchored in dense fog when the wave arrived. The captain noted currents, white water, and a significant amount of ice and debris near the fjord edges. Because the jagged, shallow seabed near the mouth acted like a speed bump that sapped the wave's energy, everyone onboard came through unscathed. Fathian put it bluntly: "It could easily turn into a catastrophic disaster." The number of cruise-ship passengers visiting Alaska has risen from roughly 1 million in 2016 to 1.6 million in 2025. "Some of these cruise ships carry up to 6,000 passengers. This is literally a floating city," he said. "Imagine one of these ships getting hit by a mega tsunami wave."

A global warning

The conditions that produced the Tracy Arm event are not unique to Alaska. Fathian notes that similar settings exist in Canada, New Zealand, Greenland, Norway, and many other places where tidewater glaciers are retreating and permafrost is thawing. "And a similar event could happen in these areas," he says. The researchers' goal now is to develop early-warning tools based on the precursory signals they identified — particularly the accelerating pattern of micro-earthquakes in the days before the slide. "Tracy Arm was not on the radar — it was not on anyone's hazard or risk map," Fathian explains. "These signals could be promising for developing early warning systems in similar conditions or areas. Hopefully this kind of data ends up on desks of policymakers and regulators to come up with practical and appropriate measures." Science, 2026. DOI: http://dx.doi.org/10.1126/science.aec3187