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Thursday, July 9, 2020

“It could happen anytime”: Scientists warn of the threat of the Alaska tsunami

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Climate change has increased the risk of a huge landslide in an Alaskan fjord that could cause a catastrophic tsunami, scientists said on Thursday.

Warming temperatures have caused the retreat of a glacier that helps support a steep, mile-long slope along a flank of a fjord in Prince William Sound, about 60 miles east of Anchorage. With only a third of the slope now supported by ice, scientists say, a landslide could be triggered by an earthquake, prolonged heavy rain or even a heat wave that could cause significant snow melt. area.

Although the slope has been shifting for decades, the researchers estimated that a sudden and huge collapse was possible in a year and probably in two decades. “It could happen at any time, but the risk increases as this glacier recedes,” said Anna Liljedahl, an Alaska-based hydrologist at Woods Hole Research Center in Massachusetts, who was part of the team.

The Alaska Department of Natural Resources, after being informed of the results, issued a statement Thursday afternoon warning that “an increasingly likely landslide could generate a wave with devastating effects on fishermen and leisure enthusiasts. “

Computer modeling has shown that a collapse of the entire slope – estimated to be about 500 million cubic meters of rock and earth, several hundred times the volume of the Hoover Dam – could cause a tsunami that would start several hundred feet height.

About 20 minutes later, when it reached Whittier, a town at the head of another narrow fjord 30 miles away, the wall of water could still reach 30 feet high and cause significant destruction.

“As a danger, it’s really worrying,” said another researcher, Hig Higman, who studies geological hazards and heads an organization called Ground Truth based in Seldovia, Alaska.

Dr. Liljedahl said that while their results have not yet been peer reviewed, “we realized we had to let people know.” She said the researchers were hoping that money would be made available for near-real-time slope monitoring that could provide a warning in the event of a landslide and tsunami.

The fjord, Barry Arm and other nearby waters are frequently visited by tourist and fishing boats, and the surrounding land is popular with hunters. In good weather, hundreds of people could be in the area. Although it has a population of several hundred people throughout the year, Whittier is generally a landing point for thousands of cruise ship passengers inland to Anchorage and north.

Landslides causing a tsunami are rare, but have occurred in Alaska and elsewhere. Perhaps the most famous occurred on July 9, 1958, in Lituya Bay, on the southeast coast of Alaska, when an nearby earthquake caused 40 million cubic meters of rock to slide. at 2000 feet in the narrow bay.

Scouring the vegetation on a hill in front of the slide showed that the tsunami reached a maximum height of 1,720 feet – essentially a giant splash which is the highest tsunami ever documented. The water then rolled into the bay in a wave that was still about 75 feet high when it reached the mouth, submerging several fishing boats and killing two people.

A slide at Barry Arm would potentially be much larger in terms of the energy involved. “It’s in a class completely different from the one we never studied after the fact, much less before that happened,” said Dr. Higman.

Researchers from 14 organizations and institutions – including Ohio State University, University of Southern California, and the University of Alaska Anchorage and Fairbanks campuses – have only begun to study the Barry region. Arm only about a month ago, as part of a NASA-funded project. project to study land mass movements across the North American Arctic.

Dr. Higman was aware of several areas in Alaska that were at risk of landslides. But it was his sister, Valisa Higman, an artist, who alerted him to the potential danger to Barry Arm. Aware of her brother’s work, she was on a tourist boat in her arm when she saw the slope, which seemed to be fractured, suggesting that it was sliding slowly. She took photos and sent them to her brother.

Dr. Higman studied satellite imagery and determined that the slope had indeed slipped over time. Additional analysis showed that the movement rate was sometimes high: between 2009 and 2015, the landslide moved downhill by approximately 600 feet, leaving a large scar.

Ohio State researcher Chunli Dai has shown a link between the slide and movement of the nearby Barry Glacier, which, like glaciers all over the world, melts and recedes as the climate warms.

Researchers say that permafrost, or permanently frozen ground, can exist in the area and can help keep the slope stable. Climate change has also caused the thawing of permafrost in many parts of the world; if it melted at Barry Arm, it could contribute to the risk of landslide. But Dr. Higman said he doubted that permafrost plays a big role in it.

The violent shaking of an earthquake can cause a slope to collapse, and Alaska is among the most earthquake-prone regions on the planet. Whittier, in fact, was severely damaged by a tsunami during the 1964 earthquake in Alaska, the second most powerful ever recorded.

But gravity can also cause a slope to fail, especially if it becomes saturated with water during periods of heavy rain or if a heat wave melts surface snow. In these cases, water can act as a lubricant, making it more likely that the earth will be carried away by gravity.

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