Alaskan Mountain Slope Has Become Unstable and May Collapse Producing a Tsunami Hundreds of Feet High, Scientists Warn

An unstable mountain slope in Alaska has the potential to suddenly collapse at some point in the next two decades, resulting in a rapid landslide and huge tsunami, a group of scientists has warned.

The mountain slope is located above a fiord known as Barry Arm on the southern edge of the state that lies 60 miles east of the state's most populous city Anchorage. Barry Arm is adjacent to two other bodies of water known as Harriman Fiord and Port Wells that are in turn connected to the wider Prince William Sound—an inlet of the Gulf of Alaska.

Currently, the newly discovered landslide on the slope is moving slowly. But while it is difficult to predict exactly when the slope could fail, the scientists—who have expertise in climate change, landslides and tsunami hazards—say it has the potential to happen within the next year, and is "likely" within 20 years.

Such a tsunami could have a significant impact on areas frequented by potentially hundreds of people at any one time—including tourists, fishermen, hunters and local residents—the scientists said in an open letter published on the website of the Alaska Department of Natural Resources, Division of Geological and Geophysical Survey (DGGS.)

Over the past decade, landslides have been responsible for generating giant waves elsewhere in Alaska and Greenland. For example, on October 17, 2015, 180 million tons of rock slid into Taan Fjord, Alaska, produced a tsunami wave that reached a height of 633 feet above the water—the fourth-highest ever recorded—according to the National Park Service.

While this giant wave had almost no impact on humans due to the remoteness of the area, the wave stripped trees across eight square miles of forest.

"Alaska parks are prone to events like this. Land-slides that generate large tsunamis most often happen in landscapes with retreating glaciers. In the last century, 10 of the 14 highest tsunamis in the world were in glaciated mountains and four were in Alaska parks, which include vast tracts of glaciated terrain," another team of scientists wrote in an article published in the journal Alaska Park Science.

"Though landslides like this can happen at any time, these events are becoming more frequent—driven by climate change-induced glacial retreat and permafrost thaw. The Taan Fiord tsunami can help us understand subaerial landslide tsunami hazards and prepare for potential impacts," they wrote.

Another landslide that occurred at Karrat Fiord, Greenland, in June 2017, also produced a large tsunami that led to the deaths of four people, and destroyed a large chunk of the town of Nuugaatsiaq, located around 20 miles away.

But according to the authors of the DGGS letter, the unstable slope in Barry Arm is much bigger than the ones responsible for the landslides in Taan Fiord and Karrat Fiord, meaning that any resulting tsunami could also be larger, with impacts potentially felt across Prince William Sound.

In their research, the scientists analyzed photographs and measurements of elevation that were collected previously by satellites and aircraft. This revealed that portions of the slope had moved 600 feet downhill between 2009 and 2015. Furthermore, preliminary analysis of radar satellite data from 2019 indicated that some sections are still moving, albeit at a slower rate than during the aforementioned period.

The scientists then estimated how much rock the slow-moving landslide contains, and simulated any potential tsunamis. If the majority of the material in the landslide fell suddenly, all at once, the researchers say the resulting wave could reach hundreds of feet in elevation along the shores of Barry Arm and Harriman Fjord, before dwindling to lower heights as it made its way into Port Wells and, subsequently, the main body of Prince William Sound.

"A complete failure could be destructive throughout Barry Arm, Harriman Fiord, and parts of Port Wells. The tsunami would propagate throughout Prince William Sound, including into bays and fiords far from the source. The results suggest that there could be a destructive tsunami in Whittier about 20 minutes after the landslide, reaching over 30 feet above the tide," the authors wrote in the letter.

"Valdez, Tatitlek, and Cordova could see noticeable waves of a few feet that are unlikely to impact anyone onshore, but could produce dangerous currents at docks and in harbors. Chenega Bay appears largely insulated from the tsunami, and no significant wave is expected outside Prince William Sound," they said.

The researchers say that the landslide could result in only a partial or gradual collapse, in which case the tsunami would be less severe, with the effects mainly felt within Barry Arm and Harriman Fiord.

The researchers note that their tsunami models could be refined with the help of further field measurements and analysis, but they say they are confident in their findings.

"Considerable effort is needed to maximize the accuracy of the tsunami model results, however based on testing of the model on other tsunamis we expect that these preliminary simulations accurately reflect what might happen far outside of Barry Arm and Harriman Fiord, assuming there is a complete or nearly complete failure of the landslide mass," the authors wrote.

"In other words, we believe these initial results are sufficiently detailed to support initial assessment of the hazard faced in Prince William Sound."

According to the researchers, the gradual retreat of Barry Glacier—which partially supports the slope—over past years is making the mountainside more unstable. In this context, there are several factors that could cause the unstable slope to change from a slow-moving landslide into a rapidly-moving one capable of causing a tsunami.

"Often, heavy or prolonged rain is a factor. Earthquakes commonly trigger failures. Hot weather that drives thawing of permafrost, snow, or glacier ice can also be a trigger. Commonly, large landslides are preceded by rockfalls and other signs of increasing instability," they wrote.

"It is plausible failure could happen any time. If the slope doesn't fail immediately, failure is still likely as time passes. However, it is also possible that the slope will eventually stabilize for centuries or millennia," they said.

It is important to note that the latest research is only preliminary and is yet to go through the standard peer-review process, which can take several months. While one of the team is currently producing a manuscript that will be submitted for peer review in around a month, the researchers said they released the information to the public early due to the imminent nature of the threat, Anchorage Daily News reported.

Despite the results being preliminary, Alaska's Department of Natural Resources appeared to agree with the findings after being briefed on the research, saying in a statement that "an increasingly likely landslide could generate a wave with devastating effects on fishermen and recreationalists."

In addition, the department "strongly recommend people avoid all the identified danger zones until the hazards can be adequately understood and characterized."