Melting glaciers trigger the world's largest tsunami

In October 2015, a powerful wave – as high as the 55-story building – crashed into an Alaska fjord, stripping the mountain of trees and dirt and leaving behind rocks and debris, flattening of huge trees and scattering rocks.

The 150-meter tsunami, triggered by a huge landslide in Taan Fjord, was three times higher than the largest tsunamis caused by the 2004 earthquake in Indonesia, Sri Lanka, India and Thailand. destruction in Japan in 2011, severely damaging the Fukushima nuclear power plant.

The slope crossing the fjord failed, resulting in a wave in the mountain, culminating at about 190 meters altitude, not far from where this picture was taken. (Ground Truth Trekking, CC BY-NC 3.0 license)

Scientists say huge tsunamis triggered by landslides are becoming more frequent – and could be a growing danger in places like Western Canada – as climate change melts glaciers that hold back the slopes mountainous.

"When we think about who will feel the greatest impact of climate change, we need to look closely at these places, such as the BC coast, "said Michele Koppes, associate professor at the University of British Columbia and a glaciologist. new study on the Taan Fjord tsunami.

"You can have a tsunami that can reach hundreds of kilometers in the fjord, and that can affect infrastructure and people."

This kind of tsunami killed four people and washed houses in the sea in a village in Greenland in 2017.

Scientists define a tsunami as a wave generated by a single force. And although we generally think that this force is applied by an earthquake, it can be applied by landslide. It's like when a stone is thrown into the water: you get a ring of waves that moves to the outside.

Huge and invisible

And if that rock is 180 million tonnes – like the landslide of Taan Fjord – these waves can be huge.

It was one of the largest tsunamis in the world in the last 50 years. But no one was there to see it.

The tsunami occurred at the edge of an American national park and the closest people were not on its way directly; they were safe in a cabin in the most protected part of Icy Bay, unconsciously ignorant of destruction.

A rock is nestled on top of trees razed by the tsunami, about 30 meters above the ocean. (Ground Truth Trekking, CC BY-NC 3.0 license)

"He went through them," said Bretwood Higman, lead author of the study on the causes and effects of the tsunami.

The study – published this week in Nature Scientific Reports – shows that of the 14 tsunamis of the last century that had a peak height greater than 50 meters, only one was due to an earthquake. (The 2004 Indian Ocean tsunami). The rest was caused by landslides.

The figure includes Canada's largest tsunami, 51 meters high, which was not on the coast, but rather a lake on Mount Colonel Foster, British Columbia, in 1946.

Most tsunamis have occurred in very inaccessible areas and their effects have never been documented.

"Great opportunity"

The Taan Fjord landslide was detected by seismic sensors designed to track earthquakes in Alaska.

Colin Stark, an associate professor of research at Columbia University studying landslides, saw in sensor readings that this had happened in an area where Koppes had been studying glaciers for decades. Looking at the satellite images, they could see that the destruction went well beyond the slope where the landslide had occurred.

"It was a great opportunity," said Higman, a tsunami sedimentologist and executive director of an Alaska-based environmental organization called Ground Truth Trekking. He heard about Stark's apparent tsunami at a conference and immediately interested in mounting an expedition.

This black hill was moved here by the massive landslide. (Ground Truth Trekking, CC BY-NC 3.0 license)

When the team arrived, Higman and Koppes said they were struck by what they had seen.

Koppes was there for the last time in 1999, when the slopes were covered with trees and alders. Not anymore. There was just bare rock up to 193 meters above the water. "The tsunami completely eliminated everything on the surface. "

He had climbed and descended along the fjord; up to 18 kilometers downstream, the team said there were still "cuts" on either side of the entrance.

Where the trees had not been completely washed away, they had been knocked down, Higman said. "Sometimes all in the same direction, sometimes in a whirlwind, where the whirlpools have gone through, but we are talking about trees." Giant trees. "

The debris was very different from those found after the earthquake tsunamis: they included a lot of gravelly glacial debris, topped with rocks – some up to five meters in diameter, and other similarly sized blocks clustered up to two meters . in diameter.

"It's a real headache," Higman said.

Predict future tsunamis

The researchers hope their findings will help others identify and collect information about the debris left by these tsunamis, and predict the risks of future events in different parts of the world.

This stretch of coast along the Taan Fjord, where the scientific expedition was encamped, is relatively flat. When a tsunami fell in 2015, he stripped the surface of the vegetation, leaving behind "beaches" that reach an altitude of 45 meters. (Ground Truth Trekking, CC BY-NC 3.0 license)

This is important, they say, because climate change makes landslides and tsunamis more frequent, destabilizing the mountainsides in three ways:

• The glaciers that filled the valleys, pushing against the walls of the valley, melt, leaving the rubble of these walls without support.

• The permafrost and the ice that sticks the rubble melt.

• When glaciers melt and recede, they leave deep bodies of water behind, which can contribute to tsunamis.

In this case, Koppes stated that the Tyndall Glacier had been retreating into the Taan Fiord for decades. By melting and slimming, the ice reached a height of about 500 meters in about 30 years, exposing the walls on each side that had been dug and dug by the ice. The walls were covered with loose sediments dug by the glacier, but they were no longer supported.

"That's what's gone down," Koppes said, adding that the glaciers of western and northern Canada are melting in the same way.

"What we have learned is that these dangers increase in frequency and, in some cases, their size increases. "

This layer of small rocks is the top layer of a deposit of two meters thick left by the tsunami. (Ground Truth Trekking, CC BY-NC 3.0 license)

In fact, tsunamis caused by landslides may have occurred without the presence of glaciers, even far from the coast, said Dan Shugar, another member of the University of Washington Tacoma research team. and from Canada.

Shugar noted that there are glacial lakes in BC, the Yukon, and in Canada's Far North, where such landslides have occurred or could occur, and where risks also increase with climate change.

"When you combine a steep topography with water," he said, "there is a big risk."