The Aug. 10, 2025 event, detailed in a new study published Wednesday in Science, began when more than 64 million cubic meters of rock collapsed into the fjord near South Sawyer Glacier, about 80 miles south-southeast of Juneau. The landslide generated an initial 100-meter breaking wave traveling more than 70 meters per second, followed by a megatsunami that stripped vegetation from the steep fjord walls.
No one was killed or injured. Scientists say that was largely a matter of timing, with the landslide striking in the early morning hours when no cruise ships or large numbers of visitors were in the immediate area.
“The fact that the landslide occurred this early in the morning was unbelievably lucky. Next time — and there will be a next time — we may not be so lucky,” said Dan Shugar, a University of Calgary geomorphologist and lead author of the study.
Tracy Arm is a popular destination for cruise ships, tour boats and kayakers drawn by tidewater glaciers, steep cliffs and floating ice. The study warns that those same features can amplify landslide-generated tsunamis, creating extreme local hazards with little warning.
The U.S. Geological Survey said the landslide occurred at about 5:26 a.m. local time and was detected by seismic instruments. Kayakers camped at Harbor Island near the mouth of Tracy Arm later reported that their gear had been swept away by waves. A NOAA tide gauge in Juneau, roughly 80 miles away, recorded tsunami waves up to 36 centimeters above tide level that continued for hours.
Researchers found the landslide was preceded by several days of increasing microseismic activity, raising the possibility that future collapses in glacier fjords could be detected before failure. The event also produced seismic waves equivalent to a magnitude 5.4 earthquake and generated a long-period seiche—a sloshing motion trapped inside the fjord—that persisted for up to 36 hours.
The study links the failure to glacial retreat driven by climate change. As glaciers pull back, they can remove support from steep mountain slopes, leaving unstable rock faces exposed above narrow fjords and coastal waters.
Alaska has a long history of extreme landslide tsunamis, including the 1958 Lituya Bay event, which produced a record runup of about 530 meters. But the Tracy Arm event is notable because it occurred in a heavily visited cruise region, turning a remote geologic hazard into a direct maritime safety concern.
For the cruise industry, the warning is blunt: glacier fjords are becoming more unstable just as more vessels and passengers are entering them.
The Tracy Arm tsunami was not a mass-casualty disaster. But scientists say it easily could have been.
