It didn’t sound like an earthquake. More like a dull thud. A “thud” every 90 seconds. Recorded by seismographs worldwide – from the Arctic ice to the Antarctic. For nine days, the Earth vibrated. No one knew why – only that it wasn’t normal. A seismic pulse is a wave of energy that travels through the Earth’s layers, typically caused by sudden geological events such as earthquakes, volcanic eruptions, or landslides. These pulses are detected by seismometers and appear as distinct signals on seismic recordings. Unlike the complex shaking of a typical earthquake, a seismic pulse can sometimes take the form of a single, powerful wave or a repeating signal with a consistent frequency. Seismic pulses can carry vital information about the source event and the materials they pass through, helping scientists understand Earth’s internal structure and the dynamics of natural phenomena—like the unexpected “donk” event triggered by a glacier collapse in Greenland.
“A mysterious seismic pulse rippled through the Earth, echoing the hidden force of a collapsing glacier in remote Greenland.”
What seismologists first identified as a “USO” – an unknown seismic object – was later revealed to be a dramatic consequence of climate change. In a remote fjord in eastern Greenland, a glacier had melted to the point that it could no longer support the weight of the towering 1.2-kilometer-high mountain peak above it. When the peak collapsed into Dickson Fjord, it launched a megatsunami, sending water soaring up to 200 meters into the air. Model simulations estimated the initial wave height at around 110 meters. Within the narrow confines of the fjord, this triggered a seiche – a standing wave that rebounded back and forth for several days. The resulting seismic vibrations created a single-frequency signal that traveled around the globe through Earth’s crust. Interestingly, the peculiar name “donk” for this global seismic ripple was coined by the science team at Popular Science magazine in New York and featured in their podcast The Weirdest Thing I Learned This Week.
The Donk was triggered by a glacier weakened by climate change , which could no longer support the weight of the mountain above it. The ice field at the base of the mountain had retreated so much due to global warming that the unstable rock face above it collapsed. The water masses in the fjord were trapped by the narrow, winding valley, leaving little room for energy to escape. As a computer simulation showed, the wave moved back and forth exactly every 90 seconds – in time with the globally recorded seismic vibrations. The vibration was so uniform and long-lasting that it differed significantly from conventional earthquakes. Instead of the usual multitude of frequencies, there was only one—a monotonous humming that was recorded by seismographs worldwide.
Only an international team of 68 researchers from 15 countries was able to solve the mystery. They combined measurement data from seismometers and infrasound sensors with satellite images, model calculations, and recordings from the Danish military, which sailed through the fjord shortly after the landslide. Dr. Kristian Svennevig of the Geological Survey of Denmark and Greenland (GEUS) in Copenhagen explained: “When we started, no one had any idea what caused this signal. The fact that it was triggered by climate change makes it all the more remarkable.” Dr. Stephen Hicks from University College London (UCL) in the United Kingdom was also part of the team: “Never before has a standing wave been recorded as a seismic signal that travels around the world and persists for days.”
As global warming accelerates, such events are likely to occur more frequently. The study , published by the international team in the journal Science , emphasizes the importance of monitoring and developing early warning systems in the future, even in regions considered stable. Thomas Forerider of the Karlsruhe Institute of Technology (KIT) says: “Without the worldwide networks of highly sensitive seismographs, we would never have discovered this event. They are our only window to such unique signals.” Perhaps it was precisely this steady thud that gave the global tremors a clear frequency for the first time – as an echo of climate change that has long since triggered more than just glaciers.
