In September 2023, seismometers all around the globe detected a unique seismic signal that was measurable for up to nine days in some places. It was caused by a massive rockslide in the Dickson Fjord in Greenland. What triggered this signal, and why did it last that long? Researchers at Karlsruhe Institute of Technology (KIT) investigated the phenomenon together with other scientists in an international collaboration. They discovered the source of the vibrations: the continuous sloshing of water back and forth in the narrow fjord. The researchers also pointed out that the rockslide was linked to the thinning of a glacier - a phenomenon they traced back to climate change. They have published their findings in the journal Science. DOI: 10.1126/science.adm9247
Sensitive scientific instruments, so-called seismometers, can be used to record vibrations that ring the Earth. The instruments usually measure the seismic waves, which are generated during earthquakes, but they can also record information about huge masses of water or earth moving on the planets's surface. "In September 2023, we discovered a signal that could be detected all around the globe," says Dr. Thomas Forbriger from KIT's Geophysical Institute. "The signal was totally different from those we know from earthquakes. It was an oscillation with a single dominant frequency, like a monotonous hum that decayed very slowly."
In order to find the underlying cause of the phenomenon, 68 researchers from 40 institutions in 15 countries and various disciplines joined forces. They combined seismometer and infrasound data, field measurements, ground and satellite images, simulations of tsunami waves, and photos provided by the Danish Army. "Thanks to this cooperation across disciplines and the unique combination of local field observations and global data, we were able to retrace the extraordinary sequence of events," explains Forbriger. "High-quality data as those recorded by KIT and the University of Stuttgart at the geoscientific Black Forest Observatory essentially contributed to this achievement."
Reconstruction of the Event Using Mathematical Models
Going beyond the results published on this topic before and using on-site observations and measurement data acquired directly at the remote Dickson Fjord in Greenland, the researchers were able to create a realistic high-resolution numerical model to reconstruct the event and unequivocally establish the origin of the signal. The model showed that a "seiche" (standing wave of water in a basin) caused vibrations that could be measured all across the globe. This phenomenon is similar to water sloshing back and forth in a bathtub when you step in on one side and thereby displace the water.
The seiche was caused by a massive rockslide in the fjord, triggered by the collapse of a mountain peak that had previously risen 1,200 meters above the fjord. " The volume of the falling material was enormous - more than 25 million cubic meters. That is enough to fill 10,000 Olympic swimming pools," states Kristian Svennevig from the Geological Survey of Denmark and Greenland (GEUS), who coordinated the research. The falling masses displaced a huge amount of water, which was pushed out of the fjord as a mega-tsunami with an initial height of 200 meters. The waves generated by the event, which lasted only a few minutes, then sloshed back and forth in the narrow fjord for days.
Seismic Waves Due to the Movement of Water Masses
The researchers' calculations revealed that the water had sloshed back and forth across the fjord with a period of around 90 seconds, which matches the oscillation period of the observed seismic waves. "It seems to be a special property of the Dickson Fjord that it is capable of such an oscillation. Seiche oscillations of such a frequency, which decay so slowly, have never been reported in scientific literature before. "Our findings are unique in this respect as well," emphasizes Rudolf Widmer-Schnidrig from the University of Stuttgart.
The movements of these huge masses of water generated seismic waves that could still be measured nine days later at the nearest measuring stations. The waves circled the Earth and they could even be detected in Antarctica, which is almost 20,000 kilometers away. According to the study, the tsunami was one of the highest in recent history. Outside the fjord, waves as high as four meters damaged a research base on Ella Island, 70 kilometers away, and destroyed cultural and archaeological heritage sites across the fjord system.
Interrelations between Climate Change and Geological Processes
The researchers also investigated how this massive rockslide came about. Satellite images showed that the glacier at the foot of the mountain had thinned out considerably in recent decades. What is more, the rockslide and the tsunami were the first ones to be observed in the northeastern part of Greenland. The researchers trace the event back to climate change. It showed that its impact is already strong, even in this region. The investigations illustrate how climate change and processes in the atmosphere, the destabilization of the glacier ice (cryosphere), the movements of water masses (hydrosphere), and of the solid Earth's crust (lithosphere) are interrelated.
The researchers are planning to install seismic instruments in Dickson Fjord to get an even better understanding of the area. "We were lucky that no one was injured in this event. But our study shows that, given the rapidly accelerating climate change, it will be more important than ever to characterize and monitor regions that were previously considered stable," says Svennevig. "This is the only way we can warn in good time about such massive landslides and tsunamis in the future."
Original Publication
Kristian Svennevig et al.: A rockslide-generated tsunami in a Greenland fjord rang Earth for 9 days. Science, 2024. DOI: 10.1126/science.adm9247.
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