In September 2023, scientists around the world detected a mysterious seismic signal that lasted for nine straight days. An international team of scientists, including seismologists Alice Gabriel and Carl Ebeling of UC San Diego's Scripps Institution of Oceanography came together to solve the mystery.
A new study published today in Science provides the stunning solution: In an East Greenland fjord, a mountaintop collapsed into the sea and triggered a mega-tsunami about 200 meters (650 feet) tall. The giant wave rocked back and forth inside the narrow fjord for nine days, generating the seismic waves that reverberated through Earth's crust, baffling scientists around the world. This rhythmic sloshing is a phenomenon known as a seiche. Fortunately, no people were hurt, but the waves destroyed some $200,000 in infrastructure at an unoccupied research station on Ella Island.
"When we set out on this scientific adventure, everybody was puzzled and no one had the faintest idea what caused this signal," said Kristian Svennevig, a geologist at the Geological Survey of Denmark and Greenland (GEUS) and the study's lead author. "All we knew was that it was somehow associated with the landslide. We only managed to solve this enigma through a huge interdisciplinary and international effort."
Climate change set the stage for the landslide by melting the glacier at the base of the mountain, destabilizing the more than 25 million cubic meters (33 million cubic yards) of rock and ice - enough to fill 10,000 Olympic-sized swimming pools - that ultimately crashed into the sea. As climate change continues to melt Earth's polar regions it could lead to an increase in large, destructive landslides such as this one.
"Climate change is shifting what is typical on Earth, and it can set unusual events into motion," said Gabriel, whose work on this study was supported by the European Research Council, Horizon Europe, the National Science Foundation (NSF) and NASA.
When seismic monitoring networks first detected this signal in September 2023, it was puzzling for two main reasons. First, the signal looked nothing like the busy squiggle that earthquakes produce on seismographs. Instead, it oscillated with a 92-second-interval between its peaks, too slow for humans to perceive. Second, the signal stayed strong for days on end, where more common seismic events weaken more rapidly.
The global community of Earth scientists started buzzing with online discussion of what could be causing the strange seismic waves. The discussion turned up reports of a huge landslide in a remote Greenland fjord that occurred on Sept. 16, around the time the seismic signal was first detected.
To figure out if and how these two phenomena might be connected, the team, led by Kristian Svennevig of the Geological Survey of Denmark and Greenland, combined seismic recordings from around the world, field measurements, satellite imagery and computer simulations to reconstruct the extraordinary events.
The team, comprised of 68 scientists from 41 research institutions, analyzed satellite and on-the-ground imagery to document the enormous volume of rock and ice in the landslide that triggered the tsunami. They also analyzed the seismic waves to model the dynamics and trajectory of the rock-ice avalanche as it moved down the glacial gully and into the fjord.
To understand the tsunami and resulting seiche, the researchers used supercomputers to create high-resolution simulations of the events.
"It was a big challenge to do an accurate computer simulation of such a long-lasting, sloshing tsunami," said Gabriel.
Ultimately, these simulations were able to closely match the real-world tsunami's height as well as the long-lasting seiche's slow oscillations.
By integrating these diverse data sources, the researchers determined that the nine-day seismic signal was caused by the massive landslide and resulting seiche within Greenland's Dickson Fjord.
"It was exciting to be working on such a puzzling problem with an interdisciplinary and international team of scientists," said Robert Anthony, a geophysicist with the United States Geological Survey's Earthquake Hazards program and co-author of the study. "Ultimately, it took a plethora of geophysical observations and numerical modeling from researchers across many countries to put the puzzle together and get a complete picture of what had occurred."
The study's findings demonstrate the complex, cascading hazards posed by climate change in polar regions. While no people were in the area when the landslide and mega-tsunami occurred, the fjord is close to a route commonly used by cruise ships, highlighting the need to monitor polar regions as climate change accelerates. For example, a landslide in western Greenland's Karrat Fjord in 2017 triggered a tsunami that flooded the village of Nuugaatsiaq, destroying 11 houses and killing four people.
Gabriel said the results could also inspire researchers to comb back through the seismic record to look for similar events now that scientists know what to look for. Finding more seiches could help more clearly define the conditions that give rise to the phenomenon.
"This shows there is stuff out there that we still don't understand and haven't seen before," said Ebeling, who co-authored the study with support from NSF and helped manage a network of seismic sensors that detected the seiche's vibrations. "The essence of science is trying to answer a question we don't know the answer to - that's why this was so exciting to work on."
