A landslide linked to climate change shook the Earth for nine days, researchers say: NPR

Images from Dicksonfjord show before (August 2023) and after (September 2023) photos of the mountain peak and glacier where a large landslide triggered a tsunami.

Soren Rysgaard

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Soren Rysgaard

The global seismic signal was extremely unusual and reverberated for more than a week, so it's no surprise that it caught the attention of researchers around the world. In the end, it took 68 experts from 15 countries to figure out what caused the “USO” (Unidentified Seismic Object) last September.

The USO designation is “an informal term that some seismologists use when they encounter a seismic signal that they cannot explain,” Kristian Svennevig, lead author of a new report on the unusual signal, tells NPR.

When scientists examined the signal, one thing was certain from the start: it was not caused by an earthquake.

“You can compare the vibrations of an earthquake to hitting a piano with full force and then pulling it away again,” says Svennevig. This creates a cacophony of tones and noises that dies down fairly quickly.

“The signal we are describing is more like hitting a single piano key to produce a very pure tone that lasts for nine days.”

The signal was traced to a massive avalanche along Dickson Fjord in eastern Greenland, triggered by melting glaciers due to climate change, according to a study published in the journal Science by Svennevig and nearly 70 co-authors.

About 1.2 kilometers above the remote fjord, a mountain peak collapsed and washed more than 25 million cubic meters of rock and ice into the water. The volume of material is large enough to fill 10,000 Olympic-sized swimming pools, say the researchers.

The tsunami was massive: 200 meters high, according to the researchers. Because the energy of the wave was trapped in a rocky fjord, the water sloshed back and forth, a phenomenon known as a seiche. The scientists were able to trace the seismic signal recorded by sensors from the Arctic to the Antarctic to this pattern.

Satellite images show a mountain peak and a glacier 30 minutes before and 7 minutes after a landslide.

Satellite image from Planet Labs

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Satellite image from Planet Labs

Their work on an answer was slow, Svennevig says. Even when they understood what had probably happened, they needed more time and data to prove it.

“It was only when we had access to declassified army bathymetric maps and adjusted our tsunami models that we were able to prove it using model calculations: This part of the tsunami got between the parallel banks of the fjord and reverberated for nine days,” he explains.

The seiche was seven meters high and had a long duration, “with a frequency (11.45 millihertz) and a slow amplitude decay that were almost identical to the seismic signal,” which had a frequency of 10.88 millihertz as it oscillated approximately every 90 seconds, the article said.

For the researchers, the signal was like an alarm bell – one that “rang” across the Earth for nine days. Of the many wake-up calls humanity has received about the impacts of climate change, this is one of the most striking yet, they say. And they warn that we could see similar cascading events in the future.

“We can expect similar signals to occur when a landslide occurs in a fjord or lake of similar shape,” says Svennevig. “We definitely expect the frequency of landslides and tsunamis in the Arctic to increase as a result of global warming.”

Now that the mystery of the global seismic signal has been solved, Svennevig says more work needs to be done to understand how the planet is adapting to climate change.

“There is a lot to learn from this event: climate change is causing new natural phenomena that we could not even dream of a year ago,” he says. “There are still many unsolved mysteries waiting to be solved by the next generation of geoscientists.”