Alaska Megatsunami reached 481 metres in Tracy Arm fjord
A megatsunami reached 481 metres in Tracy Arm fjord in southeast Alaska on 10 August 2025 after a massive rockslide collapsed 1km vertically onto the South Sawyer glacier. The wave became the world's second-tallest tsunami ever recorded, and it hit a route visited by about three cruise ships daily.
South Sawyer glacier and Tracy Arm
Dan Shugar, a geomorphologist at the University of Calgary, led the new research published in Science. Researchers said the wave measured 1,578 feet and began at 5.26am local time, after the landslide dropped into the narrow 48km fjord.
The same research linked the rockslide and tsunami to the climate crisis. Researchers said that without rapid glacier retreat, the landslide would likely not have produced such a wave.
Cruise traffic near the fjord
The timing mattered for people on the water. A sightseeing vessel from Juneau and a National Geographic tour boat, each capable of carrying more than 100 passengers, were due to enter the fjord just hours after the landslide. Two cruise ships carrying thousands of passengers had already visited the area the day before, and another cruise ship was scheduled to arrive the following day.
Researchers also said landslide-generated tsunamis can produce substantially higher runups than earthquake tsunamis in confined water bodies like fjords. That leaves cruise routes through places such as Tracy Arm exposed to a different kind of hazard than the one most passengers expect when they enter glacier-lined waters.
Dennis Staley on the event
Dennis Staley of the US Geological Survey called the tsunami a historic event. After seeing the scale of the wave and the 36-hour seiche it triggered, he said, “I feel like we dodged a bullet.”
Long-period seismic waves from the landslide were equivalent to those of a 5.4 magnitude earthquake. A kayaker camp on Harbor Island, about 55km away, reported water surging past a tent and sweeping away one of the group’s kayaks along with other gear. An observer aboard a motor vessel in No Name Bay, roughly 50km from the landslide, described a 2 to 2.5 metre wave cresting along the shoreline from the direction of Tracy Arm.
The Alaska event came within 530 metres of the world record tsunami recorded in Lituya Bay, Alaska, in 1958. For cruise operators and passengers moving through Tracy Arm, the practical lesson is immediate: a fjord can turn fast when a slope fails above retreating ice.
