A mysterious, globally observed and unprecedented 9-day-long seismic signal in September 2023 was caused by a massive landslide in Greenland, scientists have revealed.
Twenty five million m3 of rock and ice fell into the remote Dickson Fjord and, in turn, caused a 200 metre-high mega-tsunami that continued sloshing back and forth - a phenomenon called a seiche - in the narrow fjord for 9 days.
That is the conclusion of research published in the journal Science by a group of almost 70 scientists from 40 institution, including the University of Aberdeen.
This movement of a large mass of water generated vibrations through the Earth, shaking the planet and radiating globally observed seismic waves.
Never before have scientists observed such an unusual mechanism causing a global seismic signal.
When it was first noticed in September 2023, the signal looked completely different to frequency-rich earthquake recordings - it contained only a single vibration frequency, like a monotonous-sounding hum. At the same time, news of a large tsunami in a remote North East Greenland fjord reached authorities and researchers working in the area. The two teams joined forces in a multidisciplinary group involving a unique collaboration of 68 scientists from 40 institutions in 15 countries, combining seismometer and infrasound data, unique field measurements, on-the-ground and satellite imagery, and simulations of tsunami waves.
The team also used imagery captured by the Danish military who sailed into the fjord just days after the event to capture the collapsed mountain-face and glacier front along with the dramatic scars left by the tsunami. It was this unique harmony of local field data and remote, global-scale observations that allowed the team to solve the puzzle and reconstruct the extraordinary cascading sequence of events in September 2023. The results are now published in Science.
Seismometers are sensitive scientific instruments that record vibrations traveling through the ground - called seismic waves. Traditionally, seismology focuses on measuring seismic vibrations arising from earthquakes in the ground. However, seismic records can also contain information about movements of large masses on Earth’s surface, such as landslides and water waves. The study found that the landslide was from the collapse of a mountaintop that previously towered 1.2 km above the fjord. The volume of material that collapsed was massive - more than 25 million cubic metres - enough to fill 10,000 Olympic-sized swimming pools. This collapse was caused by glacial thinning at the base of the mountain over recent decades, ultimately caused by climate change.
Numerical simulations, data from local oceanographic sensor network, satellite and on-the-ground imagery confirm that the resulting mega-tsunami is one of the highest seen in recent history. Further out of the fjord, 4 m high tsunami waves damaged a research base at Ella Ø (island) 70 km away and destroyed cultural and archaeological heritage sites across the fjord system. The fjord is on a route commonly used by tourist cruise ships visiting the Greenland fjords. Fortunately, no cruise ships were close to Dickson Fjord on the day of the landslide and tsunami, but if they had been, the consequences of a tsunami wave of that magnitude could have been devastating.
Dr David Cornwell, a seismologist at the University of Aberdeen, said: “It was very strange. There was this mystery signal which was being detected across the entire Earth and it was the type of signal that nobody has ever seen before.
“With an earthquake you normally have a big signal at the start and then the signal has lots of different frequencies in it and then gradually over time it gets smaller and smaller. But this signal was different because it just had a single frequency and then it went on for nine days.”
">The signal was so puzzling, that one of the scientific team members tried to recreate the long-lived sloshing effect in their bathtub at home. They failed to simulate the same effect, so it was left to detailed mathematical models to show that the landslide direction, together with the uniquely narrow and bendy fjord channel, was the last missing piece of the puzzle of how climate change rang the Earth for 9 days.
The predictions showed that water sloshed back and forth every 90 seconds, the same oscillation period observed in the seismic waves. This perfect match shows how the force of the moving water body, generating a distinct oscillation due to the width and depth of the fjord, creates seismic energy in the crust.
Among the early theories bandied around were unusual glacial dynamics, unreported military activity – even aliens!
Dr Will Harcourt a glaciologist at the University of Aberdeen, said: “Once we started looking at the satellite images, we found this really big change on one of the mountains in one of the smaller fjords in East Greenland. A whole mountain seemed to just disappear out of nowhere.”
Dr Cornwell added: “We think this is related to climate change because the ice is melting in that region which caused the instability. Then this massive volume of rock fell into the water causing a 200m tsunami which then passed through this quite narrow fjord and instead of that water disappearing it bounced off the other end of the fjord and then sloshed back and forth over nine days.
“We've had a look and there are other similar type of rockfalls that actually generate these large waves but nothing like the same size and nothing that's been going on for this period of time. We think these events might become more frequent in the future. as the climate gets warmer, as more ice is melting, as more similar rock faces become unstable.
Dr Harcourt adds: “Particularly in the Arctic, in places like Greenland, a lot of the ice is decreasing in size and that's exposing a lot of the mountainsides, which destabilise them. And then we're seeing more events like this where we get big rockfalls falling onto glaciers, then going into the fjord and causing these massive waves. If we do see ice decreasing in size as much as we do, we may see more of these events. And that has particular significance for local communities because we may see fatalities because of this. We see rockfalls across the world, not just in Greenland but in the Alps, the Rockies in America. We need to be prepared to monitor these and be able to warn people in advance.”
Professor Matteo Spagnolo, glaciologist at the University of Aberdeen added: “We had a fascinating mystery and many, many scientists that kept joining our group – up to 70 scientists across 15 countries and 40 institutions and from many different subjects so it was truly interdisciplinary. All of them trying to work out what had happened and why. The power of this is unimaginable, really. As a small team of 3 or 4, as we usually work it would have probably taken years figuring out what happened here. But with the combined power of 70 or so scientists working together it took us a matter of weeks. Whilst what happened is worrying in a way and needs further investigation. The way we came together to solve this problem is quite inspiring.”
The study concludes that with rapidly accelerating climate change, it will become more important than ever to characterise and monitor regions previously considered stable and provide early warning of these massive landslide and tsunami events.