Second largest megatsunami triggered long-lived waves in Alaska fjord

A ‘near-miss’ tsunami that occurred in Tracy Arm fjord, Alaska last year reached an incredible height of 481 meters. Remarkably, despite the fact that this fjord is heavily visited by cruise ships in summer, no boats were caught when it struck at 5:30 a.m. on 10 August 2025.

The event happened when more than 60 million cubic meters of rock collapsed into the fjord, triggering the wave which ran 481 meters up the wall of the fjord. Eyewitnesses in the area reported chaotic conditions: kayakers tens of kilometres away were awakened by surging water that swept away equipment, while observers elsewhere described waves and strong currents moving through the fjord system. The event has already led major cruise companies to cancel trips into Tracy Arm for the 2026 season.

However, nobody observed the wave directly. In the new study, an international team of researchers led by UC Calgary used a combination of satellite data, seismic recordings, and numerical modelling to understand exactly what had happened.

Engineering researcher Dr Thomas Monahan was part of the team that analysed seismic data to identify the signal of the wave. This revealed the surprising discovery of a series of long-lived oscillations that continued to reverberate through the fjord long after the initial impact.

The signals indicated that rather than dissipating, the energy from the tsunami became trapped within the steep-sided fjord, causing water to slosh back and forth for more than a day. This produced a standing wave (seiche), only the second such event ever recorded. Dr Monahan helped document the first such wave with occurred in the Dickson Fjord in East Greenland a year ago.

Unlike the Greenland event, where the water moved in a single, simple rhythm, the Tracy Arm fjord produced a much more complex motion pattern. Instead of one steady pulse, the water oscillated in several overlapping rhythms at once, similar to how a bell can produce multiple tones when struck. This demonstrates that these resonant oscillations can act as a kind of unique ‘calling card’ for each basin.

"This study shows that enclosed basins like fjords can effectively act as giant tuning forks, with the resonance determined by their shape and geometry", says Dr Monahan. "This gives each fjord a unique “signature” when they are affected by energetic events such as megatsunamis."

Because these landslide-induced seiches generate subtle seismic signals that can travel around the globe, this opens up new possibilities for detecting and monitoring hazardous events, even in remote regions with little direct observation.

The findings also suggest that such oscillations may leave lasting traces in fjord sediments, offering a potential way to identify similar events in the past. As climate change increases the likelihood of large landslides, understanding how these hidden waves behave could become increasingly important for assessing risks and predicting how landscapes will evolve.

Seeing the waves directly

In order to capture the wave directly in satellite images, the research team applied novel analysis techniques to interpret satellite altimetry data. This measures the height of the Earth’s surface (including the ocean) by recording how long it takes for a radar pulse to travel from a satellite to the surface and back again. Conventional satellite altimeters are not able to capture differences in water height needed to spot tsunamis due to long gaps between observations, and the fact that they sample data directly beneath the spacecraft, producing 1D profiles along the sea surface.

This study used data captured by the new Surface Water Ocean Topography (SWOT) satellite, which uses two antennas to triangulate returning radar pulses, enabling them to measure surface water levels over a large area with metre-level horizontal accuracy.

"Until now, we simply didn’t have a way to observe these waves directly," adds Dr Monahan, who helped develop the technique to use and interpret SWOT data. "Traditional satellite altimeters only sample a narrow line across the ocean surface, so they miss the cross-fjord structure entirely. SWOT changes that by giving us a two-dimensional view of the water surface, allowing us to actually see these standing waves for the first time."

The satellite data revealed clear patterns of water moving back and forth within the fjord, like waves sloshing in a basin. These patterns demonstrate that several different wave motions were happening at once, rather than a single simple oscillation.

Strikingly, the SWOT satellite observations indicated that these waves were larger and more powerful than even the most advanced computer simulations had predicted.

"Accurately simulating these kinds of waves is extremely challenging,’ says Dr Monahan. ‘Even with very high-resolution models, it is difficult to fully capture how energy is trapped, reflected, and amplified within a complex fjord geometry. The SWOT observations suggest the real system may be more energetic than our best simulations predict."

This gap highlights a major challenge in modelling and predicting extreme natural events. In complex environments like fjords, the shape of the landscape and the way waves interact can trap and amplify energy in unexpected ways - making these events harder to model and forecast accurately.

“Climate change is happening particularly quickly in arctic regions, and that’s accelerating the processes that lead to events like this. That makes it even more important that we develop robust ways to monitor them. Combining satellite remote sensing with seismic observations will be critical for detecting and understanding these hazards, especially in remote areas.”

Dr Thomas Monahan, Study co-author

A growing hazard in a warming world

The Tracy Arm event was triggered by rapid glacier retreat, which destabilized the slope and allowed the landslide to occur- part of a broader pattern of cascading hazards emerging in glaciated regions. At the same time, human activity in these environments is increasing. Cruise tourism and recreation are expanding into fjords that are becoming more dynamic as glaciers retreat.

Co-author Dr Stephen Hicks (UCL Earth Sciences) says: "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."

As climate change continues to reshape glaciated landscapes, such tools will be essential for understanding and mitigating the cascading hazards that follow.

The study ‘A 481 m-high landslide-tsunami in a cruise ship-frequented Alaska fjord’ has been published in Science.