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For over a week in 2023, the planet got the shakes and nobody knew why. Now, geologists believe a fjord in Greenland is to blame for the odd seismological event.
You likely didn’t notice the quavering, which began last year on September 16. The initial cause was easy enough to identify, as that was the day a massive rockslide rocked an eastern Greenland inlet, but what followed was weird. Seismic equipment picked up a 92-second long oscillation for nine days after the landslide, an atypically long period of time for a seismic event. The shaking was far too slow and subtle for humans to perceive, but the signal was there. Experts were flummoxed as they detected the odd, subtle tremor reverberating around the world.
An impressively large team of 60 scientists from various specialties came together to solve the mystery. In the resulting paper, published in Science, they reveal what they believe is the answer: The rockslide, which was just under 10 square miles (25 square kilometers) in size, set off a localized tsunami. That, in turn, triggered 23 foot (seven meter) waves, which sloshed back and forth at regular intervals within the fjord, until finally dying down, a phenomenon known as a seiche. The waves were powerful enough to reverberate through the Earth’s crust and trigger the seismic instruments.
Nobody was hurt in the initial rockslide or from the seiche, although the waves did destroy around $200,000 of gear at an unoccupied research station on nearby Ella Island.
“When we set out on this scientific adventure, everybody was puzzled and no one had the faintest idea what caused this signal,” said Kristian Svennevig, a geologist at the Geological Survey of Denmark and Greenland, who led the study, in a press release. “All we knew was that it was somehow associated with the landslide. We only managed to solve this enigma through a huge interdisciplinary and international effort.”
The scientists figured out the seiche was to blame by analyzing satellite images and photos of the landslide area. They then used supercomputers to simulate the dirt and avalanche’s local effects, and determined the oscillations of the seiche matched those of the nine-day signal.
“This shows there is stuff out there that we still don’t understand and haven’t seen before,” said Carl Ebeling, a development engineer at UC San Diego’s Scripps Institution of Oceanography, who also contributed to the study. “The essence of science is trying to answer a question we don’t know the answer to—that’s why this was so exciting to work on.”
The geologists said they hope other researchers will go through historical records to determine if similar events have happened in the past. They also warned there could be a repeat in the future, thanks to the increasing frequency of landslides—a development they said is at least partially attributable to the effects of climate change on polar regions. A 2019 study found Arctic landslides have increased 60-fold between 1984 and 2015, driven by the melting of ground ice.
