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On September 16, 2023, scientists recorded a seismic event so strange that they named it a USO (unidentified seismic object).
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Seismic data collected around the world (along with satellite imaging) confirmed that the event—which propagated around the world for a staggering nine days—was caused by a rockslide that triggered a megatsunami and an undulating seiche (a type of sloshing wave).
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This event was likely caused by the thinning of a glacier due to climate change, and researchers expect more events like it to occur as the world warms.
The world’s largest waves aren’t found off the coasts of surfing hotspots like California or Portugal. Instead, the real record-breakers take place in much colder climates, within the fjords carved out by glaciers in the far North. These megatsunamis, as they’re called, form from rock slides that send tons of material down these narrow waterways that trigger seiche—a kind of back-and-forth sloshing of water.
Although megatsunamis can impact local communities (as dramatized in the 2015 Norwegian disaster film The Wave), they typically occur in remote locations so that no one actually sees them occur in real time. But, luckily for scientists, they’re usually far less hidden from seismic sensors.
One September 16, 2023, monitoring stations designed to detect seismic activity picked up a strange signal that reverberated around the entire world for nine days. Scientists knew it wasn’t an earthquake, so they labeled the event a USO (unidentified seismic object) and began searching for a cause. The investigation (involving 68 scientists, 40 institutions, and 18 countries) eventually revealed that the likely culprit was a rockslide in Dickson Fjord, located on the central east coast of Greenland, 124 miles inland from the Greenland Sea.
In a new study, published in the journal Science, the researchers estimate that the rockslide delivered enough material to the fjord below to fill 10,000 Olympic swimming pools, spawning the creation of a megatsunami that reached a staggering 200 meters (656 feet) high. That would make this Dickson fjord tsunami the tallest wave recorded on Earth since 1980.
“The signal looked nothing like an earthquake,” Stephen Hicks, a co-author of the study from University College London, said in a video explaining the paper’s results. “If we were to hear the vibrations from earthquakes, they would sound like a rich orchestra of rumbles and pings. Instead, the symbol from Greenland was a completely monotonous hum … it lasted for nine days.”
Although megatsunamis aren’t a new phenomenon, the era of climate change will only increase their likelihood, as melting permafrost destabilizes slopes above these fjords. This rockslide, for example, was likely caused by the thinning of the glacier below the nearby mountaintop due to warming weather. The researchers estimate that the glacier had thinned by some 100 feet in the last few decades, and that it simply could no longer support the weight of the mountaintop above it.
The last lingering mystery was why the event lasted nine days, when waves created by tsunamis typically dissipate within hours. The researchers compared seismic surface waves generated by the tsunami’s monotonous signal and determined that the Dickson Fjord’s unique features—particularly, the fact that it dead ends on its western end and contains a sharp bend toward the east—created seiche that could easily escape. Because of this, it slowly dissipated over nine days and sent vibrations throughout the entire world.
“This event underscores a deeper and more unsettling truth: climate change is reshaping our planet and our scientific methods in ways we are only beginning to understand,” Hicks—along with fellow co-author Kristian Svennevig with the Geological Survey of Denmark and Greenland—wrote in an article for The Conversation. “Just a year ago, the idea that a seiche could persist for nine days would have been dismissed as absurd … yet, these once-unthinkable events are now becoming our new reality.”
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