Climate change has increased the risk of a major landslide in an Alaskan fjord that could cause a catastrophic tsunami, scientists said on Thursday.
The warming prompted the withdrawal of a glacier that helps sustain a steep mile-long slope along a fjord flank in Prince William Sound, about 100 kilometers east of Anchorage. With only a third of the slope now supported by ice, the scientists said, a landslide could be caused by an earthquake, prolonged heavy rain or even a heat wave that could cause extensive snow to melt on the surface.
While the slope has been moving for decades, researchers estimated that a sudden and huge collapse was possible within a year and probably within two decades. "This can happen at any time, but the risk only increases as the glacier recedes," said Anna Liljedahl, an Alaskan hydrologist with the Woods Hole Research Center in Massachusetts, who was part of the team.
The Alaska Department of Natural Resources, after being informed of the findings, released a statement Thursday afternoon warning that "an increasingly likely landslide could generate a wave with devastating effects for fishermen and recreational workers".
Computer modeling showed that a collapse of the entire slope – estimated at about 500 million cubic meters of rock and dirt, or several hundred times the volume of the Hoover dam – could cause a tsunami that would start several hundred meters high. .
About 20 minutes later, when it reached Whittier, a town at the head of another narrow fjord 50 kilometers away, the water wall could still be 30 feet high and cause extensive destruction.
"As a risk, it's really worrying," said another researcher, Hig Higman, who studies geological risks and runs an organization called Ground Truth, based in Seldovia, Alaska.
Liljedahl said that while his findings have not yet been analyzed by peers, "we realized that we needed to inform people". She said the researchers hope the money will be made available for near real-time monitoring of the slope, which could provide a warning if a landslide and tsunami occur.
The fjord, Barry Arm and other nearby waters are frequently visited by tourist and fishing boats, and the surrounding area is popular with hunters. In good weather, potentially hundreds of people could be in the area. Although it has a year-round population of only several hundred, Whittier is typically a landing point for thousands of cruise ship passengers heading inland to Anchorage and pointing north.
Tsunami-causing landslides 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 a nearby earthquake caused 40 million cubic yards of rock to slide 2,000 feet into the narrow bay.
Clearing vegetation on a slope opposite the slide showed that the tsunami reached a maximum height of 1,720 feet – essentially a giant splash that is the largest tsunami ever documented. The water then rolled across the bay in a wave that was still about 12 meters high when it reached the mouth, flooding several fishing boats and killing two people.
A slip in Barry Arm would potentially be much greater in terms of the energy involved. "It's a totally different class than the one we studied after the fact, let alone before it happens," said Higman.
The researchers, from 14 organizations and institutions – including Ohio State University, the University of Southern California and the University of Alaska's Anchorage and Fairbanks campuses – only started studying the Barry Arm area about a month ago, as part of of a NASA-funded project. project to study the movement of land masses in 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 warned him of the potential risk in Barry Arm. Aware of her brother's work, she was on a tourist boat on her arm when she saw the slope, which looked to her as if she had fractures, suggesting that she was sliding slowly. She took some pictures and sent them to her brother.
Dr. Higman studied satellite images and determined that the slope had slipped over time. Further analysis showed that the rate of movement had sometimes been high: between 2009 and 2015, the landslide moved downhill about 600 feet, leaving a large scar.
Ohio State researcher Chunli Dai showed a connection between the landslide and the movement of the nearby Barry glacier, which, like glaciers worldwide, is melting and receding as the climate warms up.
The researchers say that permafrost, or permanently frozen soil, may exist in the area and may be helping to maintain a steady slope. Climate change has also caused permafrost to thaw in many parts of the world; therefore, if the thaw in Barry Arm could contribute to the risk of landslides. Higman, however, said he doubted that permafrost had an important role there.
The violent quake of an earthquake can cause a slope to decline, and Alaska is among the most earthquake-prone areas on the planet. Whittier, in fact, was heavily damaged by a tsunami during the 1964 Alaska earthquake, the second most powerful ever recorded.
But gravity can also cause a slope to fail, especially if it becomes saturated by water during periods of heavy rain or if a heat wave melts snow off the surface. In these cases, water can act as a lubricant, making it more likely that the earth will be pulled downhill by gravity.