Colleagues at Stockholm, Lund and Uppsala Universities just published an exciting article in the journal Geology, where they describe a major earthquake that occurred some 11500 years ago! This is really a long time ago, and it is amazing that an area that has been stable for such a long time discloses its vivid past once geologists are at work! Using geophysical mapping and long sediment cores, the group of researchers could show that this paleo-earthquake must have had a seismic magnitude of around 7.5. Thus they argue that this earthquake is among the largest intraplate earthquakes that have occurred during the past 11500 years.
What caused this gigantic earthquake, which might even have led to huge tsunami waves? Most likely it was triggered by stress release. When the Scandinavian ice sheet melted, Earth’s crust gradually and slowly recovered (and is still recovering as seen by the ongoing land-uplift in Sweden). However around 11500 years ago, the ice sheet melted very rapidly, which means that a huge load was removed within a short time. This rapid “unloading” had an effect on the underlying bedrock, which happy to be relieved, started to crack and bend, and ruptured. There is however another important player in the game! At the same time as the ice front melted and disintegrated, the large Baltic Ice Lake found a sudden opening and emptied within 1-2 years into the North Sea! The sudden drainage of so much water, which lowered the level of the Baltic Ice Lake by around 20-25 m, could also have contributed to the sudden unloading and to the stress release in the bedrock.
For sure, Sweden is today a safe country when it comes to earthquakes and tsunamis. It can by no means be compared to the active plate boundaries in so many other parts of the world, where earthquakes occur daily and where some of the larger ones can lead to devastating tsunamis. However this study shows that many secrets are still buried deep down and that it needs a group of geologists to unravel these!