In September 2023, 100 seismic stations around the world took turns recording a signal. The oscillation detected was puzzling. Seismometers measured a single-frequency wave, comparable to a monotone hum, very different from the signals associated with earthquakes, which are usually multi-frequency. The phenomenon was recorded over an unusually long period of nine days.

After several months of investigation, a group of scientists have revealed the causes of this mysterious tremor, in a study published in the journal Science on September 12. Using satellite data and aerial photographs, geophysicists had reconstructed the sequence of events.

On September 16, 2023, in eastern Greenland's Dickson Fjord, a mountain peak collapsed, taking a glacier with it. Some 25 million cubic meters of rock and ice – the equivalent of 10,000 Olympic-sized swimming pools – tumbled down the slope at high speed and plunged into the fjord. The surge generated a colossal impulse, triggering a 200-meter-high mega-tsunami in this desert region of the Arctic Circle. No casualties were reported, only damage to scientific equipment at the Ella Ø station, 70 kilometers from the fjord.

"The closed topography of the fjord allowed the wave to ricochet from one side to the other," explained Antoine Lucas, a researcher at the Institut de Physique du Globe in Paris and co-author of the study, using the image of a basin full of water being carried at arm's length. "The energy generated was such that the wave maintained this pendulum movement and created seismic waves that propagated under the fjord to the various station networks." It was at this point that the seismometers went into a frenzy, recording a frequency that had never before been observed for over a week.

To reach these conclusions, more than 68 scientists from 40 institutions in 15 countries combined data from seismometers, infrasound, field measurements and ground and satellite images. It was a lengthy process. "The location of the signal source was quickly identified, and satellite data confirmed the landslide," explained the French researcher. "A helicopter mission then enabled photographic surveys to be carried out to reconstruct the course of events in high resolution."

Mathematical models were then used to simulate the magnitude of the phenomenon. Geophysicist Anne Mangeney, from the Université Paris-Cité, who was also involved in the study, recalled how challenging this was. "At first, the numerical models seemed to indicate that it was impossible for this pendulum movement to last for nine days. It was astonishing. We had to readapt the numerical methods and resort to finer resolution," she explained. "This has opened up new perspectives for tsunami modeling."

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