The short answer: a mega-tsunami created waves that sloshed back and forth in a fjord in Greenland, creating vibrations that travelled around the world.

A landslide, a tsunami and a seiche

The long answer begins in the atmosphere. As greenhouse gas concentrations increase due to climate change, those heat-trapping gases accelerate ice melt, particularly around Earth’s poles. On September 16 last year, that extra heat thinned a glacier in eastern Greenland over time so much that it could no longer support the mountain rock above it.

A 150 metre-thick piece of metamorphic rock, about half a kilometre wide and long, fell and triggered a massive landslide. Rock and ice, enough to fill 10,000 Olympic-size swimming pools, let loose as fast as 47m per second and ran for more than 1.6km. The avalanche plunged into the Dickson Fjord, triggering a 198m-high tsunami – one of the highest seen in recent history.

Farther away from the fjord, tsunami waves reaching 4m high damaged an unoccupied research station and destroyed cultural and archaeological heritage sites, including an old trapper hut that had never been affected by tsunamis during its century-old history. It destroyed about US$200,000 worth of infrastructure. Although the Dickson Fjord is commonly visited by tourist cruise ships, no ships were nearby. No fatalities were reported.

Meanwhile, in the fjord, the mega-tsunami wave travelled back and forth in the inlet and created a standing wave called a seiche. We often see small-scale seiches – this rhythmic oscillation in water – in a swimming pool or bathtub. This tsunami source was so energetic that the seiche radiated seismic waves globally, shaking the planet for nine days before it petered out.

Members of the Danish military sailed into the fjord only days after the event to collect drone imagery of the collapsed mountain face and glacier front and scars left by the tsunami.

Of course, Svennevig and many of his close colleagues didn’t fully know of the connection between the tsunamigenic landslide and the seiche as the events unfolded, which is detailed in the study.

At the time, they were scratching their heads about the data at the seismograph stations. The seiche appeared as a single slow wave, like a monotonous-sounding hum, as opposed to the frantic lines of a typical earthquake reading. The wave peaked every 92 seconds, which is slow compared with an earthquake.

Seiches are well known, but no one had seen something of this variety. Svennevig, the lead author of the study, said it was like seeing a rainbow but with an extra colour that no one had observed before.

“When we started doing this research, nobody had any ideas about what was the root cause,” said Carl Ebeling, a co-author of the study and a seismologist at the Scripps Institution of Oceanography at the University of California at San Diego. “Even for a large landslide under normal circumstances, it would be hard to see that on a global scale, so something special is going on here.”

Not quite bathtub science

As some scientists investigated the peculiar seismic data, another group of authorities and researchers had heard of a large tsunami in a remote fjord in eastern Greenland. The two teams, among others, joined forces, quickly growing into a 24/7 international collaboration via a messaging system. The group brought a variety of local field data and remote, global-scale observations.

“We knew there was a landslide and a tsunami. You could pull the seismic signal of those,” Svennevig said. “But then there was this other seismic signal that continued for nine days, and they were taken from roughly the same area, so they must be associated somehow.”

The seiche signal was so befuddling that one team member tried to recreate the long-lasting seiche wave in his bathtub using a small Styrofoam float and a tape measure. It didn’t work – to no surprise to tsunami modellers.

Instead, the experts turned to advanced mathematical models to simulate how to create the wave’s height, slow movement and duration. Even then, they initially failed.

It would not be surprising if there are more destabilisation events in east Greenland and elsewhere.

The breakthrough came when they received new bathymetry data of the fjord, similar to a topographic map, from the Danish military, which allowed them to better map the seabed in the computer models. Once incorporated, the team used an unprecedentedly high-resolution model to show how the landslide direction, along with the uniquely narrow and bendy fjord channel, led to the nine-day seiche.

Svennevig and his colleagues aren’t the only ones who have been studying the global event. In August, a team of six German researchers studied the event primarily through satellite imagery. Their published findings revealed a similar story that a massive landslide caused the tsunami and long-lasting seiche, although the new study details the entire sliding process.

“This event with its fascinating, more-than-a-week-long oscillations triggered the interest of many working groups around the globe,” said Gesa Petersen, who was an author of the August study but was not involved in the new one. “The methods chosen by the teams are different, but the results agree well.”

While the landslide is the obvious instigator of events, “the root cause of this lies in climate change”, said Alice Gabriel, a co-author of the new study and a seismologist at the Scripps Institution of Oceanography. She said this is the first landslide of this size to occur in eastern Greenland. Large landslides are more well-known in western Greenland, like a massive event in 2017 that caused four fatalities and left two villages abandoned permanently.

Many aspects of climate change are already destabilising mountain slopes worldwide, whether from increased precipitation, increased air temperatures, or snow or ice loss, said Leigh Stearns, a glaciologist at the University of Pennsylvania who was not involved in either study. She said this landslide and tsunami event highlights the domino effect that can happen with the loss of even a small glacier, but it probably won’t be the last one as temperatures rise.

“It would not be surprising if there are more destabilisation events in east Greenland and elsewhere,” she said.