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.
Oxford University researcher Dr Thomas Monahan (Department of Engineering Science) 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,’ said 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.
