Scientists Identify Cause Behind Mysterious ‘Trapped Wave’ That Shook The Planet For 9 Days

OXFORD, England — In September 2023, something unprecedented happened: a mysterious seismic signal echoed around the globe for nine straight days. Seismometers from Greenland to Antarctica detected a repeating vibration every 92 seconds, puzzling scientists worldwide. Even more baffling, an “identical signal” with half the magnitude and duration occurred. Now, for the first time, researchers have captured direct visual evidence of what caused this global phenomenon using cutting-edge satellite technology.

A study by British researchers presents the first direct satellite observations of a massive seiche — essentially a standing wave sloshing back and forth in a remote Greenland fjord with such force that it caused detectable vibrations in the Earth’s crust. The research, published in Nature Communications, confirms what scientists had theorized but never directly observed: that a tsunami triggered by a catastrophic landslide became trapped in Dickson Fjord and continued oscillating for over a week.

The Mystery That Captivated Scientists Worldwide

When the seismic signal first appeared on September 16, 2023, scientists were baffled. Unlike typical earthquakes that last minutes, this signal persisted with metronomic regularity for over a week. Some researchers initially thought their instruments were broken when they detected the strange vibrations.

Stephen Hicks, a seismologist at University College London who co-authored earlier research on the event, told CNN the event didn’t roll and rumble like most earthquakes do. Rather, this activity was like a “monotonous hum” stirring the underground.

Previous studies had theorized that the signal originated from a seiche in Dickson Fjord following a massive landslide-triggered tsunami. The landslide itself was extraordinary: 25 million cubic meters of rock and ice crashed into the fjord after glacial thinning destabilized a mountain slope. But direct observational evidence of the seiche remained elusive until now.

A Satellite Revolution Captures the Invisible

Enter the Surface Water and Ocean Topography (SWOT) mission, a joint NASA-CNES satellite launched in December 2022. Traditional satellites provide only narrow snapshots of ocean surfaces. SWOT captures ultra-high resolution measurements across a 50-kilometer swath. Its Ka-band Radar Interferometer (KaRIn) enables it to see into narrow coastal environments, including fjords.

Lead researcher Thomas Monahan from Oxford University and his team analyzed SWOT’s observations of Dickson Fjord following the September landslide. One pass occurred about 12 hours after the event.

“SWOT is a game changer for studying oceanic processes in regions such as fjords which previous satellites struggled to see into,” Monahan said in a statement.

Catching a Seiche in Action

The satellite data revealed extraordinary evidence: clear cross-channel slopes in the fjord’s water surface. It was exactly what you’d expect from a massive standing wave. Using KaRIn data, the team created elevation maps of the fjord that showed height differences of up to two meters, indicating water was sloshing from one side to the other.

To connect this visual evidence to seismic data, researchers compared it with filtered signals from a seismic station in Alert, Canada, about 1,300 km (808 miles) away. The alignment was striking: the timing and direction of water movement matched the seismic displacements exactly as predicted by seiche theory.

This correlation allowed the team to reconstruct the wave’s behavior even during periods not captured by the satellite.

Ruling Out Other Explanations

Before concluding they had captured a seiche, the team ruled out other potential causes. Local wind data and tidal modeling didn’t match the observed cross-channel slopes.

“The KaRIn radar’s resolution was fine enough to make observations between the relatively narrow walls of the fjord,” said Lee-Lueng Fu, SWOT project scientist, in a NASA interview. “The footprint of the conventional altimeters used to measure ocean height is too large to resolve such a small body of water.”

A Wave That Shook the World

The research confirms that the trapped wave in the fjord initially reached astonishing heights. Earlier modeling and remote sensing suggest tsunami runups as high as 200 meters near the point of impact before the energy settled into a rhythmic seiche that lasted nine days.

“SWOT happened to fly over at a time when the water had piled up pretty high against the north wall of the fjord,” said Josh Willis, a sea level researcher at NASA’s Jet Propulsion Laboratory. “Seeing the shape of the wave — that’s something we could never do before SWOT.”

Even a Danish military vessel that visited the fjord three days after the event did not see the wave, demonstrating how elusive it was despite its global seismic impact.

A Climate Change Warning

Beyond solving a scientific mystery, the research reveals how climate change can trigger unexpected and extreme events in remote places. The landslide that sparked the tsunami was caused by glacial thinning—a process accelerating across the Arctic.

“Climate change is giving rise to new, unseen extremes,” said Monahan, who works in Oxford’s Department of Engineering Science.

The fact that a localized event in a remote Greenland fjord could generate seismic signals detectable across the globe highlights the planet’s interconnectedness. As Arctic temperatures rise faster than anywhere else on Earth, scientists expect to see more of these rare but powerful events.

“This study is an example of how the next generation of satellite data can resolve phenomena that has remained a mystery in the past,” added co-author Professor Thomas Adcock of Oxford. “We will be able to get new insights into ocean extremes such as tsunamis, storm surges, and freak waves.”

A New Era of Earth Observation

The study marks a turning point for ocean science, showing that modern satellite altimetry, which once tracked only slow, broad ocean trends, can now capture fast, localized, extreme events in places like fjords.

As climate change continues to reshape Earth’s most remote regions, tools like SWOT will be essential for detecting and understanding the extreme events to come. The seiche that shook the world may have been unprecedented, but it likely won’t be the last.

---