The landslide that triggered a powerful tsunami in Alaska’s Inside Passage early on 10 August was a close call, say scientists, tour operators and agency officials, with the risk of such events apparently increasing as glaciers retreat because of climate change.
“It’s a historic event,” said scientist Dennis Staley from the US Geological Survey of the slide, which occurred in the Tracy Arm fjord 50 miles (80km) south of Juneau.
“I feel like we dodged a bullet,” he continued, of reports that no one had been hurt, at a destination that can see more than 500,000 annual visitors.
Staley studies landslide hazards in Alaska, including in popular tourism areas. Tracy Arm is a narrow 30-mile (48km) fjord, with sheer walls looming thousands of feet above the water. It ends at the face of the South Sawyer glacier. The dramatic scenery, part of the Tongass national forest, attracts kayakers, boaters and a steady stream of packed cruise ships. But the channel was apparently empty at 5.30am on Sunday, 10 August, when without warning a colossal piece of the fjord wall collapsed.
Staley says it’s too early to know the precise size of the slide. But by analyzing its seismic signal, he and others estimate up to 100m cubic meters of rock, or more, fell near the face of the South Sawyer glacier. It threw a wave of ocean water an incredible 1,400ft (425 meters) up the opposite side of the fjord, which will likely make it one of the largest such waves ever recorded. Staley can see its impact on trees and brush in satellite imagery and aerial photographs.
Seconds later, a tsunami estimated at up to 100ft (30 meters) tall barreled down the inlet, stripping forest and brush from either shore. Thirty miles away, the wave was still 15ft high (5 meters) as it swelled up against the remote Harbor Island, coming within inches of a tent where Sasha Calvey and two other kayakers were camped.
‘Tracy Arm is a narrow 30-mile (48km) fjord, with sheer walls looming thousands of feet above the water.’ Photograph: Landslide Hazards Program/US Geological Survey
The kayakers scrambled awake to find half of their gear had been washed away. One kayak hung from a bluff, another was wedged against a tree and a third bobbed in the ocean a quarter mile away. The water remained chaotic for hours, says Calvey.
Although they had to be rescued, ending a 78-day expedition from Washington state, Calvey knows they were lucky. If the tsunami had occurred only three hours earlier, when the tide level was 10ft higher, they may not have survived.
Jeffrey and Christine Smith had planned to anchor their 65ft tour boat, the David B, for an overnight stay in Tracy Arm. But rainy weather led them to anchor about 45 miles away instead. The morning of the slide, they saw water rise 10ft along the shore near their anchorage.
“We’ve been feeling a little mortal,” Christine Smith said after learning how narrowly they missed the slide.
The landslide occurred just hours before a sightseeing vessel from Juneau and a National Geographic tour boat, each with capacity for more than 100 passengers, were scheduled to visit Tracy Arm. Two cruise ships carrying thousands of passengers each had visited the area less than 24 hours earlier. Another cruise ship was scheduled for the next day.
Staley and others marvel at the luck, where poor weather, early morning timing and a falling tide apparently conspired to prevent casualties. In another bit of luck, a portion of the slide landed on the South Sawyer glacier. If its full weight had hit the water, says Staley, the tsunami could have been even larger.
But some geologists worry that luck may be running out. Landslides near retreating glaciers appear to be increasing, in Alaska and elsewhere. One possible cause is that glacial ice, which can be hundreds of feet thick, may help support mountain walls above them. Scientists debate whether rapid glacial retreat, like is happening at Tracy Arm and across the globe, may “de-buttress” mountainsides by removing supportive ice that has been in place for centuries.
Climate change is also thawing high-elevation permafrost around the world. When permafrost thaws, explains Staley, changes in pressure, temperature and hydrology can cause instability deep within steep slopes . Scientists believe thawing permafrost contributed to a massive landslide and glacier collapse that wiped out the Swiss village of Blatten earlier this summer.
Staley emphasizes that it’s too early to say what led to the Tracy Arm slide.
“We have to be careful,” he says. “We can’t say with any real certainty what triggered this event.”
Michael West, the state seismologist who directs the Alaska Earthquake Center in Fairbanks, agrees it’s too early to identify a trigger for the slide, which he calls “shockingly huge”. But he says it’s notable that this and other tsunami-causing landslides have occurred near retreating glaciers in recent years. He says it’s reasonable to explore the ideas of de-buttressing and thawing permafrost in Tracy Arm, along with other factors like recent rain in the area.
In 2015, a landslide near a receding glacier in Taan fiord in south-east Alaska sent water 630ft up a nearby slope. A 2024 landslide near the Surprise glacier in Prince William Sound, more than 500 miles north-west of Tracy Arm, caused a small tsunami.
And last summer, a large landslide alongside the receding Pedersen glacier in Kenai Fjords national park near Seward, Alaska, triggered a wave estimated at 60-180ft.
Scientists also have a wary eye on a recently discovered weak slope above the receding Portage glacier, an hour south-east of Anchorage. If it fails, researchers say, it could trigger a tsunami in Portage Lake that could slam into a nearby Chugach national forest visitor center that sees up to 1,000 people daily in summer.
They’re also monitoring a 3,000ft unstable slope in the Barry Arm inlet in Prince William Sound. If it fails, modeling shows it can trigger a tsunami comparable to the one in Tracy Arm. Barry Arm and surrounding waters are also popular among boaters, kayakers, cruise ships and others.
Since the Barry Arm instability was discovered in 2020, scientists have installed high tech equipment to track precipitation, slope movement and other conditions on the mountain. They hope the work will improve understanding of the ties among landslides, glaciers and climate change.
Although no such instrumentation was present at Tracy Arm, West says that work at Barry Arm helped geologists quickly pinpoint the event by analyzing real-time data from more than 250 seismic stations around Alaska.
“That’s a new capability,” says West, explaining that scientists took days to locate the 2015 Taan fiord slide. But using the new method, they identified the Tracy Arm location shortly after calls came in from boats in the area.
“But this is where it gets wild,” continues West.
He says that review of seismic monitoring sites shows a seismic “popping” was occurring at the Tracy Arm site for roughly 24 hours before the slide, with hundreds of small signals detectable 100 miles away. They suddenly ended with the slide.
“Not every landslide does this,” says West, who hopes to research this phenomenon to better understand possible warning signals.
