The USO-like tsunami of September 16, 1923, from a massive fjord in Greenland, Canada, when climate change was triggered

On September 16, 1923, seismologists around the globe registered a strange signal coming from eastern Greenland. Missing were the variations in frequency that typically accompany events such as earthquakes: the signal was ‘monochromatic’, resembling the ringing of a bell, and lasted nine days. It was registered as a USO: an unidentified seismic object.

You could compare the vibrations from an earthquake to smashing your hands into a piano full force and removing them again and it would set off a flurry of sounds and tones that decays fairly quickly.

The signal was traced back to a massive fjord in eastern Greenland, triggered by climate change, according to research published in the journal Science.

Svennevig and his colleagues began documenting the landslide and the resulting tsunami. They calculated the total amount of material that was carried after the collapse of the top of the mountain. The earthen material smashed into a local glacier at the bottom of a gully, creating a rock–ice avalanche that cascaded sidelong into the fjord.

“Only when we had access to declassified army bathymetrical mapping and had tweaked our tsunami models could we prove it through modelling: that part of the tsunami got caught between the parallel sides of the fjord and resonated for nine days,” he says.

Their work toward an answer was slow, Svennevig says. They had to get more time and data to prove that they were correct.

A seismological event that set the stage for the 2023 landslide in a lake or fjord: Evidence of global warming?

To the researchers, the signal was like an alarm bell — one that “rang” the Earth for nine days. They say that the effects of climate change are one of the most striking yet. And they warn that we could see similar cascading events in the future.

It is likely that there will be similar signals if a landslide occurs in a lake or fjord. As a result of global warming, we expect the incidence of landslides in the top of the world to increase.

With the mystery of the global seismic signal solved, Svennevig says there is more work to be done to understand how the planet is adapting to climate change.

Precedents for such seismological signals existed in the scientific literature going back more than a decade. Landslides in closed water basins had produced a back-and-forth sloshing motion, known as a seiche, yielding a monochromatic seismic signature similar to the 2023 one. These events lasted less than an hour and were only registered in the local area.

It’s a nice study that explains an “extremely weird and unusual” seismological event, says Göran Ekström, a geophysicist at the Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York. He chalks it up to teamwork and the sharing of data. “The speed at which the team was able to document, describe and explain the sequence of events shows how science can work these days.”

In the end, Svennevig and his team suggests that the actual culprit was global warming, which thinned the glacier underpinning the mountain and ultimately set the stage for the landslide. “We will probably see more of these funky events in the future,” he says.