Storegga tsunami deposits, bracketed by peat, taken at the Montrose basin
A lot of work goes into understanding ancient tsunamis and their impacts.
Janine Krippner
The Storegga Slide is a giant submarine landslide that occurred thousands of years ago below the sea, offshore from southern Norway.
This is one of the largest landslides we know of – 2400 to 3200 cubic kilometres flowed out to around 800km from where it began.
The resulting tsunami impacted areas that now include Norway, Greenland, Scotland, and England. It took a lot of investigation work to understand this.
Storegga Slide. Creative Commons
Out in the field, once a tsunami deposit has been recognised and described similar deposits can be identified elsewhere. Lab work can then provide an age for when it formed. Identifying similar deposits of the same age builds a picture of what areas were hit.
For impacts, the tsunami run-up height is important; this is the height that the water reaches above sea level. If a tsunami hit today, we could walk up to the highest points on land and measure it as it varies along the coastline.
For ancient tsunamis this takes much more work.
One method is investigating tsunami deposits within old lake beds. Under normal conditions sediment settles down through water, forming a mud layer. This can be interrupted by large storms or a tsunami washing new material into the lake. Tsunamis can transport sand and gravel from the sea plus ripped-up clasts of land that the water crossed. Marine fossils can confirm the ocean source.
For the Storegga tsunami, researchers found deposits within lakes at increasing heights above sea level, then at lakes above a certain height they weren’t there. This gives a range of run-up heights between those lakes. Their age connects deposits across regions.
The Storegga Slide occurred during the end of the Last Glacial Maximum, when ice sheets covered large parts of Europe. This adds complexity.
Having so much water locked up as ice changes a few things. Sea level was not the same as it is now, so scientists must figure that out too. Part of understanding past sea level is finding evidence through the geologic record, building a global picture through time.
Another complexity is where the land was at this location, at that time. Thick ice is heavy and can push the land itself down. When those thick ice sheets melted, the land rebounded upwards to where it is now. The sea level was lower, and so was the land. The level of the tide at that time also matters. Many pieces of the puzzle must come together.
All this work led to run-up heights reaching 13 metres in Norway near the landslide, at least 20 metres in the Shetland Islands, around six in northeast Scotland, and around five in northeast England. A seemingly simple result required so much work.
Studies like this that help us understand what happened in the past show us what could happen in the future, and therefore, what actions we must take to protect ourselves. Figuring out geologic events across the ages contributes to building safer and more resilient communities.
Image showing the Storegga tsunami deposits, bracketed by peat, taken at the Montrose basin (Maryton). Creative Commons
