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On September 16, 2023, scientists detected an intriguing seismic signal. Every 90 seconds, a vibration resonated through the Earth’s surface. The rhythm was far too regular to be the cacophonous rumble of an earthquake. So what was causing this “unidentified seismic object”?
Nearly a year later, an international team of 68 researchers comes up with the solution to this geophysical enigma in the journal Science.
Their analysis reveals that the quake originated from a tsunami bouncing off the walls of a remote Greenland fjord. What amazes the scientists is that this stationary wave persisted for more than a week in the narrow inlet.
“No one has ever seen anything like this,” one of the study’s authors, geologist Kristian Svennevig, told an American media outlet in an interview.
The sequence of events goes as follows:
- In recent years, a glacier upstream of Dickson Fjord on the east coast of Greenland has thinned due to warm weather.
- Deprived of the glacier supporting it, one side of the mountain collapsed. The landslide took away as much rock and ice as 10 pyramids of Giza…
- As it plunged into the fjord, this material displaced a huge amount of water, causing a 200-metre wave, as high as Place Ville-Marie.
- This wave transformed into a standing wave 7 meters high whose frequency – 90 seconds – corresponded to that of the seismic signal.
- Lasting for nine days, this stationary wave, which oceanographers call a “seiche,” is the longest ever recorded.
A growing risk
Coming to these conclusions was no piece of cake. First, it was necessary to find the exact origin of the signal. Shortly after the seismic vibration began, a cruise ship noticed that the small island of Ella, not far from Dickson Fjord, had been razed by a flood.
When scientists visited the fjord in question, they found that debris had been deposited high on the walls of the glacial valley. Satellite images also showed the extent of the mountain’s amputation.
But to truly understand the cause of the “unidentified seismic object,” the researchers had to model the wave’s behavior in the narrow fjord. Using a bathymetric map declassified by the military, they were able to refine their numerical model.
The result spat out by their computers: a 7-meter seiche oscillating every 90 seconds. And this, for several days. The correspondence with the seismic signal was perfect and the mystery, solved.
“This is a cascade of events that has never been observed before,” American seismologist Alice Gabriel was quoted by NBC News as saying. “The Earth is a very dynamic system. We are currently in a phase where this very delicate balance is being violently disrupted by climate change.”
Due to global warming, experts expect tsunamis and landslides to become more frequent in the Arctic. These phenomena obviously pose dangers for local communities.
In 2017, four people died after a tsunami caused by a landslide hit the village of Nuugaatsiaq in West Greenland. The wave was nearly 10 metres high when it hit the hamlet. Since then, the villages of Nuugaatsiaq and Illorsuit have been abandoned, fearing a similar disaster could happen again.