At 5:26 a.m. on Aug. 10, 2025, more than 64 million cubic meters of rock and debris collapsed near South Sawyer Glacier and plunged into Alaska’s Tracy Arm fjord. The impact sent water 481 meters, about 1,580 feet, up the opposite mountainside. That measurement is the runup, the highest elevation the water reached on land, rather than the height of a wave moving through open water.
Researchers writing in Science reconstructed an initial breaking wave about 100 meters high traveling faster than 70 meters per second. NOAA’s historical database places the 481-meter runup second only to the 1958 Lituya Bay event among documented tsunamis.
A collapse with warning signs
The study found that glacier retreat driven by climate change preconditioned the landslide. Several days of small seismic events preceded the collapse, increasing in rate and magnitude until roughly an hour before the slope failed. Even with those signals, NOAA says no operational warning system currently exists for landslide-generated tsunamis.
The collapse shook the Earth with long-period seismic waves comparable in size to a magnitude 5.4 earthquake. It also trapped a standing wave, or seiche, inside the fjord. The resulting roughly 66-second seismic signal persisted for as long as 36 hours, only the second time scientists had observed a days-long seiche this way.
A near miss with a long reach
No deaths, injuries or major infrastructure damage were reported. NOAA said the early hour and near-low-tide timing likely limited the consequences in a fjord used by cruise ships and smaller boats. More than 30 miles away, campers on Harbor Island woke to water flowing past their tent; the surge swept away a kayak and much of their gear. A tide gauge in Juneau recorded a 36-centimeter wave about an hour after the landslide.
The danger did not end when the water settled. USGS warns that the exposed scar can continue shedding rock for years, potentially generating smaller local tsunamis. The researchers say climate change is making similar events more likely as more fjords attract cruise ships and other traffic.
Comments