Scientists have finally solved the mystery behind two strange events that shook the entire planet for nine days straight. Their findings close the book on a years-long effort to trace the seismic signals back to their source.

In September 2023, global seismometers detected something strange. The Earth was experiencing minor tremors every 90 seconds—and the shaking went on for nine days. One month later, it happened again. Scientists were baffled, as natural tectonic processes couldn’t explain the anomalies. 

After roughly a year of scientific sleuthing, two studies published in 2024 independently hypothesized that the shaking resulted from two huge landslides, setting off two “mega-tsunamis” in the Dickson Fjord in East Greenland. These enormous standing waves—or seiches—sloshed back and forth inside the fjord and triggered small movements within the Earth’s crust, they posited. 

It was an intriguing possibility, and both studies presented compelling evidence to support their claims. But, “there were some big uncertainties that made it difficult to fully corroborate that this was actually the root cause,” Thomas Monahan, a Schmidt AI in Science fellow at the University of Oxford, told Gizmodo.

So, Monahan and his colleagues set out to confirm whether this hypothesis was correct. In a study published today in the journal Nature Communications, they unveiled the first direct satellite observations of the seiches and definitively linked them to the seismic anomalies. 

The researchers used data captured by NASA’s Surface Water Ocean Topography (SWOT) satellite, which launched in December 2022 to map the height of water across 90% of Earth’s surface. This type of data collection—known as satellite altimetry—works by transmitting radar pulses from a satellite to Earth’s surface, and then measuring the time it takes for the signals to bounce off the surface and return to the satellite.

Conventional altimeters failed to capture evidence of the seiches due to long gaps between observations, according to an Oxford statement. As such, they generally struggle to gather data in fjord regions due to the complexity of the terrain, Monahan said. But SWOT is equipped with a cutting-edge altimeter instrument that uses two antennas to triangulate the return signals. This allows the satellite to take very high-resolution measurements of surface water levels.

“What we’re able to get is essentially an incredibly high-resolution snapshot of what the elevation of the sea surface is doing in these complex regions,” Monahan said. These snapshots provided a more accurate picture of how the fjord’s sea surface height changed during the 2023 seismic events, allowing his research team to calculate the slopes of the massive waves that had formed. 

“We were able to essentially unearth the fact that there was a significant anomaly in the fjord exactly when we would expect this wave to occur,” Monahan said. 

But determining that these seiches formed at the same time as the strange seismic signals still wasn’t enough to prove the two events were linked. The researchers needed direct evidence to prove that these huge waves were capable of generating global tremors. 

To that end, they linked the SWOT snapshots to small movements in the Earth’s crust detected at seismic stations located thousands of miles away from the fjord. Coupling this continuous seismic data with the intermittent satellite observations allowed them to reconstruct the characteristics of the wave, even for periods that SWOT did not observe. The researchers also ruled out the possibility that the seismicity stemmed from weather or tidal conditions and ultimately determined that the seiches were the sources of the tremors.

These waves formed when a warming glacier collapsed in on itself, Monahan said. “This created very large landslides, which—when they struck the fjord—produced massive mega-tsunamis on the order of 200 meters or 600 feet tall,” he explained.

“This was the first time that a mega-tsunami of that nature had occurred in Eastern Greenland,” Monahan said, adding that this type of event has been documented on the territory’s west coast. To see this phenomenon spreading to other parts of the ice sheet “is concerning,” he said, because it shows that climate change is accelerating in this region.

“I think what this study really emphasizes is that—well, it sounds silly to say—but climate change is a global phenomenon,” Monahan said. 

“Some of the biggest and fastest changes are occurring in the Arctic and in remote regions where we may not see it every single day. But it’s important to understand and quantify those changes as they will eventually come to impact us where we live,” he said. Indeed, the waves that shook the world offer a stark reminder of the sweeping impacts of rising global temperatures.