When a mountainside above Alaska’s Tracy Arm collapsed into the fjord on Aug. 10, 2025, the resulting tsunami ran 1,580 feet (481 meters) up the opposite wall, making it the second-highest tsunami in recorded history. A study in Science has now documented the warning signs that preceded the collapse, and the authors say those signs appeared at least a day before the slide. No cruise ships or tour boats were in the fjord when the rock fell.
Save more, waste less: The Cool Down’s newsletter sends smart, simple changes that help you and the planet in your everyday life.
We may earn a commission if you sign up.
The slide, the wave and the people who felt it
The collapse came early on a Sunday morning, around 5:30 a.m. local time, according to Michael West, the Alaska state seismologist, as reported by Alaska Public Media. It happened near the end of the fjord where South Sawyer Glacier reaches tidewater, south of Juneau. Estimated volume was tens to hundreds of millions of cubic meters of bedrock. West likened the event to the side of a mountain collapsing.
Three kayakers camped on Harbor Island in Holkham Bay were close enough to see the water. Kayaker Sasha Calvey described looking out of the tent and seeing “a massive wave coming, like, inches away,” and Nick Heilgeist called it “pure chaos out of nowhere.” Christine Smith, captain of the small cruise ship David B., anchored near Fords Terror, saw her tide rise and fall about 10 feet. The wave was also recorded by a tide gauge in Juneau, about 75 miles away.
Ships had been in the fjord shortly before. The authors write that passengers aboard the Hanse Explorer finished sightseeing in Tracy Arm on the evening of Aug. 9, and the landslide came about twelve hours later, just after 5 a.m. The water and debris stripped the fjord walls down to bare rock and continued down Tracy Arm. The 1,580-foot run-up is higher than the top floor of Taipei 101, the comparison the authors use, and Alaska Public Media’s reporting adds that wave heights near the source were still preliminary estimates, with West saying waves may have reached at least 100 feet in some areas.
Warning signs that preceded the collapse
West and Ezgi Karasözen of the Alaska Earthquake Center describe the precursors in their essay, first published by The Conversation and republished in ScienceDaily, which accompanies the paper at DOI 10.1126/science.aec3187. Three conditions stood out. The glacier below the slide area calved rapidly and retreated more than a third of a mile in the two months before the event. Heavy rain entered fractures in the mountain and raised water pressure, pushing the rock closer to failure. The authors add that August conditions, with warm ocean water and heavier precipitation, favor glacier retreat and slope failure. And thousands of small seismic tremors came from the slide area in the days before the collapse, growing exponentially in the hours before it and shifting to continuous tremor.
West told Alaska Public Media at the time that the landslide had a very clear precursory sequence, something seismologists have seldom observed. The seismic record is the part of the sequence the authors single out, because it arrived through instruments that were already operating for another purpose. The authors write that “most provocative are the thousands of small seismic tremors,” and that multiple major landslides have occurred at the terminus of a retreating glacier.
A graded alert system that does not yet exist
The authors argue the signals could have supported progressive alerts: a yellow alert from weather and glacier conditions, an orange alert from the seismic activity, and a red alert once a slide is detected, within minutes. The seismic data in this case came from regional earthquake monitoring as a byproduct, not a dedicated landslide program. In their words, “There is no landslide monitoring system operating yet at this scale in the U.S.”
The gap is one of scale and purpose. Seismometers across North America registered the Tracy Arm event within minutes, according to Alaska Public Media, yet nothing in place was designed to read those signals as a landslide forecast. Building one would take cooperation across state and federal agencies and stronger monitoring and communication networks. The authors say such a system would not be fail-proof, and a NOAA-style tsunami warning would offer little help to ships in the immediate area, though it could give roughly 10 minutes of warning to people farther away. For that wider audience, the official channel for warnings is tsunami.gov, the National Weather Service’s tsunami warning site, run from the National Tsunami Warning Center in Palmer, Alaska.
Some cruise lines began avoiding Tracy Arm after the event, according to the authors. They close the argument with a limit and a case for acting anyway: “Alert systems do not remove the risk entirely, but they are a better option than no warning at all.” Whether any agency takes up the proposal is not stated in the sources reviewed here.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
Save more, waste less: Subscribe to The Cool Down →
We may earn a commission if you sign up.
More from Morning Overview
- Seven of Earth’s nine planetary boundaries are breached, and all seven are worsening
- The NSA says three phone features should be off whenever you aren’t using them
- Hurricane Hunter radar shows four warning signs that a tropical cyclone is about to strengthen, a University of Miami study found
- Tropical Storm Rachel is dumping up to 12 inches on four Mexican states on its way to major hurricane strength
