The Storegga event, which happened off the coast of Norway, is one of the largest underwater landslides known. It happened about 8,150 years ago after the last ice age ended and caused a huge tsunami that destroyed the North Atlantic and what was then the North Sea.
It was previously assumed that the slide displaced all of the sediments deposited during the last ice age over a 300-kilometre distance. The volume of sediments moved during the slide has been estimated to be between 2,400 and 3,200 cubic kilometres — enough to cover all of Germany with seven to nine meters of sediment.
Scientists from the GEOMAR Helmholtz Center for Ocean Research Kiel and the University of Bergen in Norway have discovered that much of the seafloor sediment moved 12,000 years ago, after the last Ice Age. That means the Nyegga landslide, named after the Nyegga area where the researchers discovered the first evidence of the event, occurred around 20,000 years ago.
The geophysicists and geologists can show that the Nyegga landslide is responsible for about one-third of the material that slid, or about 1,000 cubic kilometres. As a result, the Storegga landslide is smaller than previously thought.
Above all, submarine mass movements on the Norwegian central shelf are more complex and frequent than previously thought. According to conventional wisdom, such large submarine sope failures occurred in conjunction with glacial cycles.
According to this theory, the unstable material deposited by melting glaciers was removed by a single landslide and transported into the deep sea. This assessment must now be reevaluated.
“The Storegga event is one of the best-studied mega-slides in the world, and much of our understanding of large-scale landslides and related tsunami generation can be traced back to it,”
said lead author Dr. Jens Karstens, marine scientist in the Geodynamics Research Unit at GEOMAR.
The findings of this study indicate that some previous concepts may have been overly simplistic, and they are thus critical for assessing geohazards associated with landslides at continental margins. The new findings are based on ship-based echosounder surveys conducted during a research cruise in 2012 and an examination of dozens of sediment cores at the University of Bergen.
Age dating and sedimentological studies revealed unusual depositional profiles in seven of the sediment cores that were not explained by the previous landslide history. The researchers were able to explain the depositional layers with the Storegga event and the much earlier Nyegga landslide thanks to evidence of an earlier landslide event in the echosounder data.
Independent seismic reflection surveys show various large landslide events deposition in deeper sedimentary layers. More investigation is now required to provide a more precise geological understanding of these older events and to better assess the hazard potential of large submarine landslides.
Reference: Karstens, J., Haflidason, H., Berndt, C. et al. Revised Storegga Slide reconstruction reveals two major submarine landslides 12,000 years apart. Commun Earth Environ 4, 55 (2023).