On September 16, 2023, seismic monitoring stations worldwide detected an unusual signal that gradually diminished but remained detectable for an entire nine days.
“We were surprised; this signal continued to come in, which is completely different from typical earthquake activity,” said a researcher from University College London. This phenomenon was labeled as an unidentified seismic object or USO.
Subsequent investigations revealed that the signal originated from water sloshing back and forth across the 2.7-kilometre-wide Dickson Fjord in eastern Greenland, triggered by a massive landslide that generated a 110-metre-high tsunami.
Unlike typical earthquake signals, which last only a few minutes and feature a mix of various frequencies, the USO maintained a single frequency of approximately 11 millihertz, occurring every 90 seconds. After establishing that the signal coincided with the Greenland landslide, researchers recognized a probable connection.
Research indicates that objects, including bells, resonate at specific frequencies when struck. This principle also applies to bodies of water, from swimming pools to oceans. Seismic disturbances like earthquakes and wind can initiate rocking movements, creating standing waves known as seiches.
Based on its dimensions, researchers calculated the resonant frequency of Dickson Fjord to match the signal at 11 millihertz. However, they took considerably longer to understand why this seiche persisted so long.
Immediately following the tsunami, the seiche reached heights of 7 metres on either side of the fjord. Within days, the movement reduced to just a few centimetres—small enough that a Danish naval vessel passing through three days post-landslide couldn’t detect it.
However, the seiche continued, likely enduring well beyond the nine days during which it was detectable by seismic stations. “No previous reports have documented seiches persisting for such extended durations or dissipating energy so slowly,” the researcher noted.
Crucial to understanding this phenomenon was the fjord’s morphology. Computer simulations demonstrated that the landslide site, located 200 kilometres inland, had a glacier obstructing one end of the fjord, alongside a sharp bend at the opposite end. The fjord’s rounded bottom functioned akin to a rocking chair, facilitating minimal resistance to the water’s movement.
These features contributed to an exceptional degree of energy retention, preventing the wave from dissipating quickly, commented the researcher.
This landslide resulted directly from climate change. A steep glacier was pivotal in supporting the mountainside; its thinning led to the collapse of an estimated 25 million cubic metres of rock and ice into the fjord, marking the first recorded landslide in eastern Greenland.
While no one was present in the region during this event, the fjord is frequented by cruise ships. The tsunami damaged monitoring equipment and two abandoned hunting huts in the vicinity.
As global temperatures continue to rise, the likelihood of similar landslides increases. This incident underscores how climate change impacts not just the atmosphere and oceans but also geological activities beneath our feet. Researchers emphasize that this discovery reveals some of the far-reaching consequences of climate change.
