Greenland megatsunami led to week-long oscillating fjord wave
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.”
Journal
The Seismic Record
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
The 16 September 2023 Greenland Megatsunami: Analysis and Modeling of the Source and a Week‐Long, Monochromatic Seismic Signal
Article Publication Date
8-Aug-2024
Landslide triggers megatsunami in narrow fjord
Seismologists measure tremors up to 5000 km away.
GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre
image:
Overview of seismic stations on Greenland (black triangles), the location of the tsunami (red circle) and the nearest seismic station (red triangle), whose filtered signals are shown.
view moreCredit: Angela Carillo Ponce et al.
It was a monster wave that hit a fjord on Greenland's east coast on 16 September 2023: In certain places, the traces of the flooding reached 200 metres high. Researchers led by Angela Carrillo Ponce from the German Research Centre for Geosciences (GFZ) have now evaluated the seismic signals from earthquake measuring stations worldwide and discovered another unusual event: Triggered by the megatsunami, a standing wave sloshed back and forth in the narrow bay of the uninhabited Dickson Fjord for more than a week. The international team published their work in the current issue of the scientific journal “The Seismic Records”.
Rockslide as triggering event
The tsunami was triggered by a large landslide. Earthquake measuring stations up to 5,000 kilometres away registered the shaking caused by the landslide as a short signal. However, there was also a very long-period (VLP) signal that was recorded by the seismometers for more than a week. Angela Carrillo Ponce, who works as a doctoral student in the “Physics of Earthquakes and Volcanoes” section of the GFZ, says: “The mere fact that the VLP signal of a wave sloshing back and forth triggered by a landslide in a remote area of Greenland can be observed worldwide and for over a week is exciting. That's why we in seismology have been most concerned with this signal.” Fortunately, the researcher adds, no people were harmed. Only a military base, which was without personnel at the time of the tsunami, was devastated.
Analysis of the seismic signals – shock waves that travel thousands of kilometers in the earth's crust – showed that a so-called standing wave formed in the fjord after the landslide. Initially, the parts of the flank that fell into the water triggered a giant wave that spread through the entire fjord to the offshore island of Ella, more than 50 kilometres away. Near the point where the rockslide entered the fjord, the maximum height was more than 200 metres, along the coast an average of 60 meters. Parts of the wave apparently spilled back from the steep banks in the narrow fjord and a standing wave began to form, which undulated back and forth for more than a week. However, this wave measured only around 1 metre in height.
Standing wave persisted unusually long
Such standing waves and the resulting long-period signals are already known in research. Such VLP signals are normally associated with large break-offs from glacier edges. “In our case, we also registered a VLP signal”, says Angela Carrillo Ponce, “the unusual thing about it was the long duration”. What was particularly impressive was that the data from seismic stations in Germany, Alaska and other parts of North America were of very good quality for the analysis. A comparison with satellite images confirmed that the cause of the first seismic signals corresponded well with the strength and direction of the rockfall that triggered the megatsunami. In addition, the authors were able to model the slow decay and the dominant oscillation period of the VLP signals.
This gives the researchers hope that they will be able to detect and analyze other similar events from the past. It is obvious that the retreat of glaciers, which previously filled entire valleys, and the thawing of permafrost are leading to increased landslides. Climate change is accelerating the melting of glaciers and could therefore increase the risk of megatsunamis.
Seismic signals of the megatsunami
Depending on the frequency range filtered out, the rockfall triggering the tsunami can be seen as a single peak (top), the standing wave sloshing back and forth as an undulating pattern in the recordings (middle, with several hours depicted) or the overall signal of the rockfall and the tsunami over the course of a week with strongly decreasing intensity of the oscillations (bottom).
Credit
Angela Carillo Ponce et al.
Journal
The Seismic Record
Method of Research
Data/statistical analysis
Subject of Research
Not applicable
Article Title
The 16 September 2023 Greenland Megatsunami: Analysis and Modeling of the Source and a Week-Long, Monochromatic Seismic Signal
Article Publication Date
8-Aug-2024
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