In September 2023, a strange, regular and global seismic signal, intrigued surveillance networks. Every 92 seconds, a very low frequency wave shakes the earth's crust for nine days. The phenomenon is repeated in October, less intense but just as confusing. For months, its origin remains uncertain. It will be necessary to wait for a study coordinated by the University of Oxford, in collaboration with several scientific institutes, to establish a link with two mega-tsunamis caused by landslides in an isolated fjord of Greenland.
A global seismic signal with unknown origins
It all starts on September 16, 2023, a strange very low frequency signal spread through global seismic networks. This regular rumble, perceived every 92 seconds, persists nine days without interruption. The sensors do not detect a classic epicenter or usual tectonic activity. A replica, shorter and less intense, manifests itself almost a month later, triggering the questioning of seismologists. The hypothesis of an arctic local phenomenon ends up emerging. Two massive landslides would have successively struck an isolated valley east of Greenland, the Dickson Fjord. They would have triggered tsunamis confined in this sealed sea. But these waves were not content to dissipate. They would have rebounded from one edge to another, trapped in the geometry of the fjord, forming a stationary wave – a cuttlefish – powerful enough to transmit a movement to the earthly coat.
Until then, no phenomenon of this type had never been correlated with a measurable seismic activity on a planetary scale. Above all, known cuttlefish is usually short -lived and geographically limited. This case defies the previous ones. On the ground, no one observes the scene. Neither the local populations, rare in this region, nor even a Danish military ship present shortly after the first event. The phenomenon escapes any direct detection for weeks, accentuating its elusive character. To get out of the shadows, we had to combine years of interdisciplinary research, complex models and, above all, a new generation of spatial observation tools.
Swot: a satellite to see the invisible even in Greenland
The SWOT (Surface Water and Ocean Topography) satellite), the fruit of a collaboration between the CNES, NASA and other international partners, was designed to fill a major gap in the measurement of continental and coastal waters. Put in orbit in December 2022, he embarked an unprecedented instrument, Karin (Ka-Band Radar Interferometer). The latter is capable of mapping in two dimensions the height of the surface of the water with an unequaled resolution. Where the traditional altimeters provide only occasional measurements under their trajectory, SWOT sweeps a strip 120 kilometers wide. It then offers a detailed image of liquid relief on a scale never reached.
In the case of events that occurred at the Dickson Fjord, this ability to scrutinize narrow and complex areas as arctic fjords played a central role. The satellite made it possible to observe variations in slope on the surface of the water. This type of measure was previously inaccessible, especially in polar regions poorly covered by sensor networks.
© © Thomas Monahan.Copernicus Sentinel-2 satellite image of Fjord Dickson, in eastern Greenland, with the measurements of the sea surface observed by the SWOT satellite during the seismic wave of October 11. © Thomas Monahan.
What Swot has recorded is only a moment in the dynamics of the fjord. However, these data were sufficient to invalidate the meteorological or tide origin of the phenomenon, and to corroborate the thesis of a stationary wave generated by a tsunami.
Immobile waves in Greenland that shake the earth
But what exactly? These are stationary waves, oscillating in a closed or semi-fermed body of water, such as a fjord or a lake. They occur when a brutal displacement of water – often caused by a landslide, a storm wave or an earthquake – has the mass of water oscillate between two walls. Usually these waves are not very powerful, localized and go out in less than an hour. What happened in the Dickson Fjord, in September 2023, radically exceeded this framework. The cutter observed persisted for nine days, before disappearing, then reappearing almost a month later with less intensity.
This exceptional behavior is linked to the initial violence of the phenomenon. Two massive landslides have generated mega-tsunamis confined to the fjord. Scientific simulations, especially those conducted with the high -resolution Hysea digital model, believe that the initial oscillation of the cuttlefish reached between 7.4 and 8.8 meters in height. To produce such a wave, it took a horizontal force between 160 and 500 gigawatts – an intensity rarely envisaged in this type of context.
© © Thomas Monahan.
Copernicus Sentinel-2 satellite image of Fjord Dickson, in eastern Greenland. Copernicus Sentinel-2 satellite image of Fjord Dickson, in eastern Greenland. © Thomas Monahan.
The SWOT satellite data, combined with the seismic signals recorded up to more than 1,300 kilometers, indicate a maximum slope of approximately 1.8 meters per kilometer in the fjord. The more moderate October phenomenon has a lower slope. These two episodes reveal persistent instability. The decline in the neighboring glacier, favored by global warming, could make such events more frequent in the years to come.
Climate, data and new scientific boundaries
The event observed at the Dickson Fjord is not only a geophysical curiosity. It marks a turning point in our ability to monitor the consequences of climate change in previously inaccessible regions. The Arctic, in particular, is one of the spaces most affected by the rise in temperatures. However, until recently, he widely escaped fine observation, for lack of tools adapted to his complex topography and isolation.
The Swot mission changes this situation. Its data makes it possible to better understand how local events, induced by the melting of ice, can have a global impact, by generating oscillations capable of spreading through the earth's crust.
But to fully exploit this technological revolution, researchers alert to several urgent needs. The installation of in situ sensors must be strengthened, improve SWOT data processing algorithms. Finally, it is necessary to democratize their access by open source tools. Today, only a handful of specialists can really use this data effectively.
As Thomas Adcock, professor at Oxford and co-author points out, this phenomenon forces us to review the established models. The interactions between the ocean, the cryosphere and the earthly crust are narrower than we thought. Understanding these links has become crucial to anticipate future impacts of climate change on the great geophysical balances of the planet.
Source: Monahan, T., Tang, T., Roberts, S. et al. “Observations of the Thook that Shook the World”. Common nat 164777 (2025).
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
