A 200-Meter-Tall Event We Didn't See Sent Signals Through The Earth For Nine Whole Days

Back in September 2023, scientists monitoring seismic activity across the world were all met with a mysterious signal making its way throughout the whole of the planet.

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"We were baffled – the signal was unlike any previously recorded," Stephen Hicks, Research Fellow in Computational Seismology at UCL, and Kristian Svennevig, Senior Researcher at the Geological Survey of Denmark and Greenland, explain in a piece for The Conversation. "Instead of the frequency-rich rumble typical of earthquakes, this was a monotonous hum, containing only a single vibration frequency. Even more puzzling was that the signal kept going for nine days."

The signal was detected from the Arctic to Antarctica, and was particularly unusual, looking distinct from waves seen during earthquakes. Initially, it was labelled by the team as a "USO", or "unidentified seismic object".

“Even though we know seismometers can record a variety of sources happening on Earth’s surface, never before has such a long-lasting, globally travelling seismic wave, containing only a single frequency of oscillation, been recorded," Hicks added in a statement. "This inspired me to co-lead a large team of scientists to figure out the puzzle."

The international team traced the signal back to its cause, and unfortunately, it has a somewhat concerning origin.

"Our combined analyses, involving multiscale imagery, field data, tsunami simulations, and remote seismological data, demonstrate a complex, cascading chain of events in East Greenland," the team explains in their paper. 

According to the team, the vibrations throughout Earth's crust were caused by a massive landslide in Greenland's Dickson Fjord, which triggered a 200-meter-high (656-foot) mega-tsunami as enough rock to fill 10,000 Olympic swimming pools fell into the fjord.

"This sequence was originally preconditioned by climate change–induced glacial thinning, culminating, on 16 September 2023, in a large rockslide, which entered the fjord to generate a 200-m-high tsunami," the team explains in their paper.

That's a pretty big tsunami at around the height of 136 Danny DeVitos stacked on top of each other, or a little taller than Seattle's Space Needle. It could be the largest wave we have seen since 1980.

Hicks and Svennevig explain that the seismic waves seen were the result of "a phenomenon known as a seiche: a wave in the icy fjord that continued to slosh back and forth, some 10,000 times over nine days."

"Our study of this event amazingly highlights the intricate interconnections between climate change in the atmosphere, destabilisation of glacier ice in the cryosphere, movements of water bodies in the hydrosphere, and Earth’s solid crust in the lithosphere," Hicks added.

"This is the first time that watersloshing has been recorded as vibrations through the Earth’s crust, travelling the world over and lasting several days."

The team's models clearly suggested that this was the cause of the seismic signals, but another team has since found further evidence from the Surface Water Ocean Topography (SWOT) satellite. Looking at the area where the tsunami occurred, they found clear evidence of this seiche sloshing back and forth during the right timeframe.

"Climate change is giving rise to new, unseen extremes," lead author of that study, Thomas Monahan of the Department of Engineering Science, University of Oxford, said in a statement. "These extremes are changing the fastest in remote areas, such as the Arctic, where our ability to measure them using physical sensors is limited."

"This study shows how we can leverage the next generation of satellite earth observation technologies to study these processes. SWOT is a game changer for studying oceanic processes in regions such as fjords which previous satellites struggled to see into."

The first study is published in Science, the second in Nature Communications.