Are there earthquakes in Sweden?
On average, a couple of earthquakes occur daily in Sweden. Most of them are so minor as to be detected by instruments only, but sometimes there are powerful tremors felt over large areas. This is related by Björn Lund, head of the Swedish National Seismic Network, in a short film.
“We can get quakes of up to magnitude six on the Richter scale. Five point four is the strongest we’re completely sure about. That happened near the Koster islands, off the west coast, in 1904. It was such a big one that it shook the whole of southern and central Sweden, Norway and the whole of Denmark. And it was even said to be felt all the way to Saint Petersburg,” Björn Lund says.
These earthquakes are due to the North American and Eurasian continental tectonic plates sliding apart in the mid-Atlantic and causing stresses to build up in the Swedish part of Earth’s crust. When these stresses cause a sudden break along a fault, the ground shakes.
In Sweden, most earthquake activity is around Lake Vänern, along the coast of Norrland and in northernmost Sweden, where postglacial rebound took place. This occurred at the end of the last ice age when Earth’s crust, after being weighed down by ice several kilometres thick, rose up again, activating fissures in the bedrock and, as a result, causing quakes.
But there are also earthquakes in Sweden that the scientists cannot entirely explain.
“In the county of Hälsingland, over the past 20 years, we’ve observed a cluster of seismic sites that don’t quite behave as we thought. We don’t know why they’re there. Geologically, for instance, you can’t see any major deformation zones. We wonder if it can be a hidden postglacial fault that’s still active.”
Using 13 temporary measuring stations that were installed in Hälsingland in summer 2021, Björn Lund and his colleagues now plan, over the next three years, to find out more about these quakes and try to clarify what causes them.
What is a fault?
A fault is a fissure (crack) in Earth’s crust where a marked shift has taken place. Along an active fault, stresses constantly build up until the land suddenly ruptures and an earthquake takes place. Lake Vättern, which formed 650 million years ago, and the Halland Ridge, which is some 80 million years old, are examples of old faults in Sweden.