How a remote landslide triggered a global seismic mystery
In a twist on the old "if a tree falls in the forest" thought experiment, a global team of scientists worked together to connect mysterious seismic signals that were detected around the planet last year to a remote-but-powerful landslide and tsunami in an eastern Greenland fjord.
The event in Greenland's Dickson Fjord registered on sensitive seismographs from the Arctic to Antarctica for nine days following a massive landslide that sent more than 32 million cubic yards of rock and ice crashing into the fjord and triggering a tsunami with a wave height that reached 650 feet. As the tsunami rocked back and forth in the fjord, the impacts were strong enough to ripple through the Earth's crust and register as a single frequency hum that seismologists initially categorized as a USO, or unidentified seismic object.
The study's findings, first published in Science, were the results of collaboration of 68 scientists representing 41 research facilities in 15 countries who pieced together the events using satellite imagery, site photographs and digital models to study the avalanche of rock and ice and its impact in the water.
How big was the rockslide?
The Sept. 16, 2023, landslide was caused by the collapse of a mountain top that until then had stood nearly 4,000 feet above the fjord. A glacier at the base of the mountain that had been thinning for decades from climate change had worn down enough to release 32.7 million cubic yards — enough to fill 10,000 Olympic-sized swimming pools — of rock and ice into the fjord.
Imagery of the glacier and missing mountain top was captured by the Danish military, who sailed to the fjord days after the event to document the impact. The September 2023 photo shows the scarring left by the tsunami on the face of the glacier.
The fjord is roughly the width of Manhattan, and over the span of the nine-day period following the collapse, the water in the fjord sloshed back and forth, oscillating every 90 seconds. This matched the patterns picked up by seismologists around the planet.
“When I first saw the seismic signal, I was completely baffled,” said Stephen Hicks of the University College of London and co-author of the report. “Never before anywhere on Earth have we directly recorded water sloshing over days caused by a single event lasting just minutes — in this case a landslide.”
A model prepared by Hicks shows how seismic monitoring stations picked up the ripples from the Greenland event within an hour of the landslide.
“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 of the Geological Survey of Denmark and Greenland 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.”
While no one was injured in this event, the fjords are on a route used by cruise ships touring the fjords. If ships had been present, the study noted, the consequences "could have been devastating."
The study concluded that rapidly accelerating climate change creates instability in other regions thought to be stable and highlighted the need for monitoring in those areas.
SOURCES Science, University College of London, UC San Diego Scripps Institute of Oceanography