A landslide in Alaska threw water 1,580 feet up a mountainside hours after tourists had left

A mountainside collapse in Alaska’s Tracy Arm fjord on Aug. 10, 2025, shoved water and debris 1,580 feet (481 meters) up the opposite wall, the second-highest tsunami run-up ever recorded. The rock let go just after 5 a.m., roughly 12 hours after the cruise ship Hanse Explorer had passed through the fjord, and no ships were nearby when it came down. A peer-reviewed reconstruction in the journal Science, led by Dan Shugar of the University of Calgary, has since pieced together what happened and what the slope was doing beforehand.

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The numbers describe a rare event. More than 64 million cubic meters of rock, roughly a cube a quarter mile on each side, plunged into the fjord. The initial breaking wave stood about 100 meters (330 feet) high and moved faster than 70 meters per second, and the disturbance registered on seismometers as the equivalent of a magnitude 5.4 earthquake. Water then raced down the fjord, stripping the walls to bare rock, and a trapped oscillation with a period of about 66 seconds kept sloshing for up to 36 hours.

Second only to Lituya Bay: where 1,580 feet ranks

ScienceDaily’s repost of the Tracy Arm study puts the run-up at 1,580 feet, a height it compares to the top floor of Taipei 101. A summary from the University of Alaska Fairbanks Geophysical Institute gives the same figure, about 481 meters above sea level and about 2 miles from the slide, and ranks it behind only the 1958 Lituya Bay, Alaska, event at 1,730 feet. Simulations suggest the breaking wave stood about 330 feet tall and exceeded 150 mph when it reached the opposite shore.

The research appeared in Science, volume 392, issue 6803, pages 1142 to 1153, published as “A 481 m-high landslide-tsunami in a cruise ship-frequented Alaska fjord,” according to the USGS publication record, which lists 20 authors and a May 5, 2026, publication date. The paper’s DOI is 10.1126/science.aec3187. The ScienceDaily page carrying the story is dated Oct. 19, 2026, months after the paper itself appeared, and it draws on an article by two of the authors, so the findings below are the study’s rather than a fresh event. Its abstract notes that climate-driven glacial retreat had preconditioned the slope and that cruise traffic in such fjords, together with climate change, makes similar events more likely.

South Sawyer Glacier let go of the slope

The Geophysical Institute summary identifies the glacier that had been buttressing the failed hillside as South Sawyer Glacier, which retreated rapidly over about four days, Aug. 2 through 5, leaving a steepened slope without support. ScienceDaily’s account adds that the glacier below the slide area pulled back more than a third of a mile in the two months before the collapse.

Rain added to the load in the same weeks. The authors cite heavy rainfall as a precursor because water pressure in rock fractures rises when a wet spell soaks a slope, which loosens its grip. They list three warning signs in all: rapid glacier retreat and calving, heavy rain, and a long run of small earthquakes.

Thousands of tiny tremors, then continuous shaking

The seismic record is the most striking part of the study. Days before the failure, signals equivalent to magnitude 1 to 2 earthquakes arrived about an hour apart. In the six hours before the collapse they came every 30 to 60 seconds, and about two hours before it they merged into continuous vibration that scientists read as steady movement along the failure surface. The study calls that behavior unusual for landslides, and ScienceDaily describes the run-up as thousands of small tremors in the days before the collapse.

Ezgi Karasözen, a research seismologist at the Alaska Earthquake Center, described those signals as the hillside effectively announcing that it was going to fail, and she called the scale of what researchers face in Alaska unprecedented.

The authors estimate that a monitoring system could have issued alerts at least a day before the slope gave way, and that a tsunami warning system could have given about 10 minutes of notice to people farther down the fjord. Nothing of the kind was running at the time, and the authors say no comparable landslide monitoring operates at this scale in the United States. According to the Geophysical Institute summary, the slide was found only after boaters reported surging water in Endicott Arm, and the Alaska Earthquake Center then located it using its landslide-detection tools.

A three-color alert scale for fjords nobody monitors

In an article for The Conversation, Karasözen and Michael West, who directs the Alaska Earthquake Center, proposed a three-level scale. Yellow would mean weather and glacier conditions have temporarily raised the hazard over a general area. Orange would mean seismic signals point to immediate concern for a specific region, such as keeping vessels out of a fjord. Red would mean seismic data picked up within a couple of minutes of a landslide starting show an event in progress.

West put the gap bluntly in the Geophysical Institute summary: “But this tremendous and dangerous landslide went undetected,” he said, adding that it is a hazard Alaska has to come to terms with. The authors note that major landslides have also struck the ends of retreating glaciers in Alaska, Greenland and Norway, which places Tracy Arm among several such events rather than alone. A related study on detecting landslides within minutes points toward the faster, red-level end of that scale.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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