“Near-miss” Tsunami in Alaskan Cruise Area Offers Lessons for Steep Landscapes Near Glaciers

Adapted from a story written by Colette Derworiz for the University of Calgary

When part of a mountain in southeast Alaska slid into Tracy Arm fjord last summer, it generated a seismic signal equivalent to a magnitude 5.4 earthquake and triggered a tsunami. The wave crossed the fjord and ran more than 1,500 feet (481 meters) up the opposite wall—a height greater than that of the Empire State building—nearly a mile away.

Despite its massive size, no one was caught in the wave because it hit around 5:30 a.m.
Had it occurred a few hours later, some of the 20 or so cruise ships, boaters and kayakers that travel through the general area each day could have been in the narrow fjord—a popular sightseeing area for glaciers in the Tongass National Forest about 50 miles south of Juneau, Alaska.

The “near miss” has now been studied by an international team of researchers and published today in Science. The study concludes that the event offers important lessons for people who live, work or travel in steep coastal landscapes near retreating glaciers.

“We reconstructed the event from a suite of perspectives, including eyewitness accounts from a variety of ship passengers and kayakers,” says University of Calgary’s Dan Shugar, lead author of the study.

A group of kayakers reported waking around 5:45 a.m. that morning to see water flowing past their tent, carrying away one kayak and much of their gear. Another observer described a 6- to 8-foot wave coming along the beach, while those on a cruise ship anchored near the mouth of the fjord saw currents and white water, but no obvious wave.

The researchers analyzed seismic data, numerical models and satellite images taken before and after the event to better understand what happened.

Shugar says there wasn’t much warning before the landslide hit. “Often these gigantic rock avalanches give some sort of warning signs in the weeks, months or years beforehand, when the slope is slowly moving down the mountain,” he says. “In this case, that didn’t happen.”

Instead, he says, there was some minor seismic noise so slight it likely would have not drawn attention. “This one was truly a surprise,” Shugar adds, noting that such events pose challenges for disaster reduction in high-risk areas.

The research team also concludes that glacier retreat in the Alaska fjord helped set the stage for the near disaster. It also points to broader lessons: as glaciers retreat in warming regions, the risk of related hazards can increase, and improved monitoring may help reduce some of those dangers.

“This event shows how glacier retreat can set off a chain of hazards in steep coastal landscapes,” says coauthor Göran Ekström, a seismologist at the Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and a professor of Earth and Environmental Sciences. “The seismological detection of cracking in the rock many hours before the landslide occurred gives us new insights into the landslide process and provides opportunities to develop practical monitoring and warning systems.”

The researchers say it’s important to pay close attention in places along the West Coast and in polar regions where glaciers are thinning as the climate warms.

“Ultimately, we hope that coastal municipalities, the cruise ship industry and other stakeholders take these threats seriously,” says Shugar, noting that similar events could result in future disasters. At least six cruise lines have altered their itineraries in Alaska this year because of hazards that remain in Tracy Arm fjord.

The U.S. Geological Survey has warned on its website that steep, mountainous landslide areas are “inherently unstable and will continue to change for years following an initial landslide.”