Alaska experienced the second longest tsunami ever recorded in 2025, without any warning

ANCHORAGE, Alaska (KTUU) – A wave as tall as the Empire State Building swept through the Southeast Alaska Fjord last year.

A new study published on Wednesday says this is the second loudest tsunami ever recorded, and went largely undetected.

“The scale of what we are dealing with in Alaska is unprecedented,” Ezgi Karasuzen, a seismologist with the Alaska Earthquake Center, said in a news release from the University of Alaska Fairbanks. “We have stunning landscapes, but the risks that come with that are also very real.”

Karasozin and Michael West, director of the Alaska Earthquake Center, are among the 18 co-authors on the paper.

Tracy Arm landslide in 2025 (Google/Alaska News source)

A landslide about the size of 12 city blocks broke away from the fjord’s northern wall and collided with the South Sawyer Glacier — about 50 miles southeast of Juneau — just before 5:30 a.m. on Aug. 10, 2025.

The sheer force of the earth falling into a narrow, deep fjord pushed a huge wall of water far up the slope opposite the landslide, creating a large tsunami.

Alaska News Source interviewed Cyrus Reed, a geophysicist with the USGS, last March. He was one of the first scientists to fly over the landslide and see its effects for himself, just days after it happened.

“I think we were the first to provide an estimate of how high the water was splashing across the Gulf,” he said. “I think other people might have flown in perhaps earlier that day because it was the first good weather day and took pictures, but we haven’t seen them yet.

“So, we were among the first people to get there and look carefully. We were probably the first to look carefully.”

His flight was exclusively by air because the helicopter was unable to land.

“You can’t miss the big part of the mountain that fell, most of it fell on the glacier and partly into the water,” he said of the northern side of the fjord.

August 2025 Tracy Arm landslide (USGS/Cyrus Read)

He and the helicopter crew had an idea of ​​what they would see on the north side of the fjord, where the ground has separated from the rock underneath, but Reed said he had no idea what the south side of the fjord, where the tsunami was inundated, would look like.

“The wave was 1,500 feet high. I think that kind of caught us by surprise. We knew there was probably going to be a splash on the other side, but we didn’t realize how big it was,” he said. “I think this is somewhat unusual. It doesn’t happen very often.”

The study confirmed that this tsunami had the second highest slope on record at 1,578 feet above sea level, two miles from the landslide.

The world’s longest tsunami in recorded history occurred in 1958, when a landslide in Lituya Bay generated a tsunami that was about 1,730 feet high.

Could something like what happened in Lituya Bay in south-central Alaska happen?

Computer simulations using field data show that the tsunami wave was accompanied by a breaking wave about 330 feet high, and accelerated at more than 150 mph when it struck the opposite shore.

The research said that the landslide itself caused seismic waves similar to a 5.4-magnitude earthquake.

The furthest signs of flooding and erosion were found about 30 to 34 miles away, where waters had risen to about 50 feet above sea level, a few miles east of the Harbor Island kayak camp at the entrance to Tracy Arm.

There were kayakers camped on Harbor Island, an island near the mouth of Tracy Arm. The US Geological Survey reported that the wave swept their equipment away. At least one person was in nearby Endicott Arm Gorge when the incident occurred.

No injuries were reported, but the study authors noted that if a ship had been in the upper strait, there would have been deaths.

This chart from the USGS shows the before and after the landslide and tsunami (USGS)

Researchers state that the main cause of this tsunami was the retreat of the glacier, specifically over a period of four days in early August.

As the glacier retreated, it left the exposed slope above unstable, with nothing to hold it in place.

The study also reveals that the seismic activity occurred several days before the landslide. Signals equivalent to a 1-in-2 magnitude earthquake occurred about an hour apart, but were too small to be reliably detected or identified as earthquakes.

About two hours before slip, the signals became a continuous vibration that scientists interpreted as constant movement along the ultimate failure surface.

“These seismic signals were like the hills saying, ‘Hey, I’m going to fail,’” Karasuzen said.

August 2025: Landslide in Tracy Arm, tsunami aftermath (USGS)

Multiple cruise lines have decided not to travel through the region this summer. Usually, dozens of cruise ships and ships visit it every year.

West says the landslide has gone completely undetected, and that poses a danger to Alaska.

“Alaska has advanced earthquake monitoring and tracking,” West said. “We have procedures and observatories to monitor volcanoes. But this massive and dangerous landslide has not been detected, and this is a risk that Alaska must adapt to.”

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