In September 2023, a seismic mystery began when an unusual seismic signal repeated every 90 seconds over the course of 9 days, and then again a month later, creating a perplexing riddle until now.
It took two 2024 scientific studies to begin unraveling the complex chain of events that started with a melting glacier and ended with a pair of mega tsunamis. Researchers proposed that the melting glacier triggered a landslide, which in turn caused tsunamis in an East Greenland fjord.
The waves became trapped in the narrow channel, bouncing back and forth to generate the repeating signal. However, the authors lacked one key piece of evidence.
The Hidden Waves
These standing waves are known as seiches, first identified in Lake Geneva in 1890. Yet such waves had never been directly observed in the fjord. Even a Danish military ship cruising through the area during the event failed to detect anything unusual.
A team at Oxford University turned to satellite altimetry data and advanced data analysis methods to investigate how these waves could exist. Altimetry measures the Earth’s surface elevation over both land and sea by calculating the return time of satellite signals. In earlier studies, altimetry data were ineffective for identifying wave activity because they recorded only one-dimensional information directly below the satellite, often with long gaps between measurements.
An Eye in Space
In December 2022, the Surface Water and Ocean Topography (SWOT) satellite was launched, specifically designed to measure water height. Its breakthrough feature is the Ka-band Radar Interferometer (KaRIn), a novel instrument that uses tandem antennas mounted on a 10-meter boom extending from each side of the satellite. This setup allows the system to triangulate signal returns over water with 2.5-meter accuracy across a 30-mile-wide swath.
Using SWOT, the team produced elevation maps of the Greenland fjord from multiple time periods following the tsunamis. These maps revealed up to two-meter height variations in the cross-channel slopes. Crucially, the slopes were in opposite directions—evidence of water moving simultaneously in two directions across the channel, consistent with seiche behavior.
The final piece of the puzzle involved linking these observations to subtle movements in the Earth’s crust recorded thousands of kilometers away. By establishing this connection, the team was able to infer wave characteristics even during periods when no direct measurements were available, using data extrapolated from crustal displacement. A reconstruction of local weather patterns later ruled out wind or tides as possible causes of the observed phenomenon.
Commenting on the Seismic Mystery
“Climate change is giving rise to new, unseen extremes. 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,” said lead author Thomas Monahon, a DPhil student in the Department of Engineering Science, University of Oxford.
“SWOT is a game changer for studying oceanic processes in regions such as fjords which previous satellites struggled to see into,” he added.
“This study is an example of how the next generation of satellite data can resolve phenomena that has remained a mystery in the past,” said co-author Professor Thomas Adcock of the Department of Engineering Science, University of Oxford. “We will be able to get new insights into ocean extremes such as tsunamis, storm surges, and freak waves. However, to get the most out of these data we will need to innovate and use both machine learning and our knowledge of ocean physics to interpret our new results.”
The paper “Observations of the Seiche that Shook the World” appeared on June 03, 2025, in Nature Communications.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.