On September 16, 2023, earthquake monitoring stations around the world picked up a strange signal that weakened over time but remained detectable for nine days.

“We thought, ‘Oh, this signal is still coming. This is definitely not an earthquake.'” Stephen Hicks Researchers at University College London have dubbed the object an “Unidentified Seismic Object,” or USO.

Hicks and his colleagues found that the signal was caused by water moving across the 1.7-mile-wide Dikson Fjord in eastern Greenland. The wave was triggered by a massive landslide, which produced a 360-foot-high tsunami.

Earthquake signals typically last only a few minutes and are a mix of different frequencies, Hicks said. USO’s frequency is about 11 millihertz and repeats every 90 seconds. When it became clear that the signal started at the same time as the Greenland landslide, Hicks and his colleagues thought there was probably a connection.

Many objects, such as bells, vibrate at a particular resonant frequency when struck. The same is true for bodies of water, from swimming pools to oceans. Disturbances such as earthquakes or wind can cause a body of water to shake, setting off a kind of standing wave called a seiche.

Based on its width and depth, the researchers calculated that Dikson Fjord has a resonant frequency of 11 millihertz, which matches the signal. What took them longer to figure out was why the fjord continued to oscillate for so long.

Immediately after the tsunami, the seiche rose seven metres on both sides of the fjord. Within a few days it receded to a few centimetres, but this was so small that it went unnoticed by a Danish navy ship sailing up the fjord three days after the landslide.

But the seiche didn’t stop, likely continuing long after the nine days had passed and becoming undetectable by distant seismic stations, Hicks said. “No seiche has ever been reported before that lasted that long or that the energy dissipated that slowly.”

The team’s computer modelling suggests that the shape of the fjord was a crucial factor: The landslide occurred 200 kilometres inland, and the fjord is blocked by a glacier at one end and curves sharply at the other. The fjord’s rounded bottom acts like a rocking chair, allowing the water to flow through with little resistance.

All these factors caused the wave to have a high degree of energy trapped inside, rather than dissipating quickly as it normally would, Hicks said.

The slide itself was a direct result of climate change. A steep glacier supported the mountainside. As the glacier thinned, it collapsed, sending an estimated 25 million cubic meters of rock and ice into the fjord. It was the first landslide ever recorded in eastern Greenland.

No one was in the area at the time, but cruise ships were traveling up the fjord. The tsunami destroyed equipment used to monitor the area and two abandoned hunting lodges.

As the planet continues to warm, we’ll likely see more of these kinds of landslides, Hicks said, noting that the findings show that climate change is affecting not just the atmosphere and oceans, but also the ground beneath our feet. “For the first time, we’re looking down at our feet and seeing some of the devastating effects of climate change,” he said.