In September 2023, a megatsunami in remote eastern Greenland sent seismic waves around the world, piquing the interest of the global research community.
The event created a week-long oscillating wave in Dickson Fjord, according to a new report in The Seismic Record.
Angela Carrillo-Ponce of GFZ German Research Centre for Geoscience and her colleagues identified two distinct signals in the seismic data from the event: one high-energy signal caused by the massive rockslide that generated the tsunami, and one very long-period (VLP) signal that lasted over a week.
Their analysis of the VLP signal—which was detected as far as 5000 kilometers away—suggests that the landslide and resulting tsunami created a seiche, or a standing wave that oscillates in a body of water. In this case, the seiche was churning for days between the shores of Dickson Fjord.
"The fact that the signal of a rockslide-triggered sloshing wave in a remote area of Greenland can be observed worldwide and for over a week is exciting, and as seismologists this signal was what mostly caught our attention," said Carrillo-Ponce.
"The analysis of the seismic signal can give us some answers regarding the processes involved and may even lead to improved monitoring of similar events in the future. If we had not studied this event seismically, then we would not have known about the seiche produced in the fjord system," she added.
The findings will help researchers as they study the impacts of landslides in Greenland and similar regions around the world where global warming and the loss of permafrost are making rocky slopes and glaciers increasingly unstable.
In western Greenland, recent tsunamis have had devastating consequences, such as the 2017 Karrat Fjord event where an avalanche caused a tsunami that flooded the village of Nuugaatsiaq and killed four people. Megatsunamis over 100 meters high off the east coast of Greenland have also reached Europe.
The 16 September 2023 megatsunami took place in Dickson Fjord in a remote part of East Greenland, and was first noted in social media posts and in a report of waves hitting a military installation on Ella Island.
Carrillo-Ponce and colleagues studied both seismic signals and satellite imagery from the area to precisely locate and reconstruct the series of events.
Their analysis of an initial high-energy seismic signal, combined with satellite images of a missing rock patch along a cliff along Dickson Fjord, allowed them to trace the direction of the landslide as it picked up glacier ice and became a mixed rock-ice avalanche before it reached the water. The resulting megatsunami run-up was more than 200 meters near the water entry point and an average of 60 meters along a 10-kilometer stretch of the fjord.
"While we were able to obtain information on the direction and magnitude of the force exerted by the landslide, we do not have data to investigate the original cause of the landslide," Carrillo-Ponce said.
The strength, radiation pattern and duration of the later seismic VLP signal best fit a scenario where the tsunami created a long-lasting seiche in the fjord, the researchers found.
VLP signals have been observed previously in Greenland, but they are usually associated with iceberg collapse due to glacial earthquakes. "In our case we observed a VLP signal too, but the main difference is the long duration," Carrillo-Ponce explained. "It is quite impressive to see that we could use good-quality data from stations located as far as Germany, Alaska and North America, and that those records were strong enough for at least one week."
The researchers say their approach might prove useful in studying similar past events, and their possible link to climate and environmental change.
"We have compared our results with remote sensing data to validate our solutions, and our study shows that the force produced by the signals is well resolved," Carrillo-Ponce said. "Therefore it becomes a useful analysis as seismic signals contain information on the type of source generating the signal and how the energy is radiated."
