In October 2015, a powerful wave — as tall as a 55-storey building — crashed through a fjord in Alaska, stripping the mountainsides of trees and dirt, and leaving behind bare rock and debris, flattening huge trees and scattering boulders as big as SUVs.

The 150-metre tsunami, unleashed by a massive landslide into the Taan Fjord, was triple the height of the tallest earthquake-triggered tsunamis that devastated Indonesia, Sri Lanka, India and Thailand in 2004 and nearly four times taller than the one that caused widespread destruction in Japan in 2011, badly damaging the Fukushima nuclear plant.

The slope shown across the fjord here failed, sending a wave raging up the mountainside, peaking at about 190 metres elevation, not far from where this photo was taken. (Ground Truth Trekking, licensed under CC BY-NC 3.0)

Scientists say huge, landslide-triggered tsunamis are becoming more frequent — and could pose an increasing hazard in places like Western Canada — as climate change melts the glaciers that hold mountainsides in place, both on the coast and far inland.

"When we think about who's going to feel the repercussions of climate change the most, we have to look closely at these places like coastal B.C.," said Michele Koppes, an associate professor at the University of British Columbia and a glaciologist who co-authored a new study on the Taan Fjord tsunami.

"You can have a tsunami that can go hundreds of kilometres down fjord, and it can affect infrastructure and people."

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

Scientists define a tsunami as a wave generated by a single force. And while we typically think of that force as being applied by an earthquake, it can be applied by landslide. It's similar to when a rock is thrown into the water: you get a ring of waves travelling outward.

Huge and unseen

And if that rock is 180 million tonnes — as in the Taan Fjord landslide — those waves can be huge.

It was one of the biggest tsunamis in the world in the past 50 years. But no one was there to witness it.

The tsunami occurred on the edge of a U.S. national park and the nearest people weren't in its direct path; they were safe in a cabin in the most protected part of Alaska's Icy Bay, blissfully unaware of the destruction as it happened.

A boulder is nestled atop tsunami-flattened trees, about 30 metres above the ocean. (Ground Truth Trekking, licensed under CC BY-NC 3.0)

"It snuck right past them," said Bretwood Higman, lead author of the study documenting the causes and effects of the tsunami.

The study — published this week in Nature Scientific Reports — shows that of the 14 tsunamis in the past century that had a peak height greater than 50 metres, only one was caused by an earthquake. (The 2004 Indian Ocean tsunami.) Ten were caused by landslides into fjords or lakes in glaciated mountains.

The figure includes Canada's tallest tsunami, at 51 metres tall, which wasn't on the coast at all, but rather at a lake at British Columbia's Mount Colonel Foster in 1946.

Most of the tsunamis occurred in very inaccessible areas and their effects were never documented.

'Big opportunity'

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

Colin Stark, an associate research professor at Columbia University who studies landslides, saw from the sensor readings that it had happened in an area where Koppes had studied glaciers for decades. When they looked at the satellite images, they could see that the destruction went far beyond the slope where the landslide occurred.

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

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

When the team arrived, both Higman and Koppes said they were struck by what they saw.

Koppes had last been there in 1999, when the slopes were covered with trees and alder bushes. Not anymore. There was just bare rock up to 193 metres above the water. "The tsunami completely wiped out everything on the surface."

It had moved up and down the length of the fjord; as far as 18 kilometres downstream, the team says there were still "buzz cuts" on either side of the inlet.

Where the trees hadn't been completely washed away, they had been knocked down, Higman said. "Sometimes all in the same direction, sometimes in a swirling pattern, where the eddies have come through. But we're talking trees. Giant trees."

The debris was quite different from that found after earthquake-triggered tsunamis: it included a lot of gravelly glacial debris, topped with boulders — some as large as five metres in diameter, and other, similar-sized boulders grouped together, up to two metres in diameter.

"It's a real mind bender," Higman said.

Predicting future tsunamis

The researchers hope their findings will help others identify and gain information from the debris left by such tsunamis, as well as help predict the risk of future events in different parts of the world.

This section of coast along the Taan Fjord, where the science expedition camped, is relatively flat. When a tsunami swept across in 2015, it stripped the surface of vegetation, leaving behind 'beaches' that extend up to an elevation of 45 metres. (Ground Truth Trekking, licensed under CC BY-NC 3.0)

That's important, they say, because climate change is making landslides and tsunamis more frequent, destabilizing mountainsides in three ways:

Glaciers that filled up valleys, pushing up against the valley walls, are melting, leaving the rubble of those walls unsupported.

Permafrost and ice that glues the rubble together is melting.

When glaciers melt and retreat, they leave behind deep bodies of water, a condition that can help generate tsunamis.

In this case, Koppes said, the Tyndall Glacier has been retreating down the Taan Fjord for decades. As it melted and thinned, the height of the ice dropped about 500 metres in about 30 years, exposing walls on either side that had been carved and steepened by the ice. The walls were covered in loose sediment dug up by the glacier, but no longer supported by it.

"That's what came unravelling," Koppes said, adding that glaciers across western and northern Canada are melting in a similar fashion.

"What we've learned is these hazards are increasing in their frequency, and in some cases, they're increasing in their size."

This sheet of small boulders is the upper layer of a two-metre-thick deposit left by the tsunami. (Ground Truth Trekking, licensed under CC BY-NC 3.0)

In fact, it's possible to have landslide-triggered tsunamis without the presence of glaciers at all, even far from the coast, said Dan Shugar, another member of the research team, who is a University of Washington Tacoma assistant professor and originally from Canada.

Shugar noted there are glacial lakes in B.C., the Yukon and Canada's Far North, where such landslides have or could occur, and where the risk is also increasing with climate change.

"When you combine steep topography with water," he said, "there's a pretty big hazard."