North Jutland and East Denmark most prone to earthquakes due to tectonic zone

A magnitude 3.9 earthquake struck in Køge Bugt on Monday, the strongest tremor recorded in Denmark in 14 years, according to the Geological Survey of Denmark and Greenland (Geus). The quake’s epicentre lay in the bay south of Copenhagen, and its occurrence in eastern Denmark reflects a longstanding geological pattern, researchers told DR.

The earthquake-prone areas in Denmark are concentrated along the Sorgenfrei-Tornquist Zone, a 30-to-40-kilometre-wide belt that runs from southern Norway to the Carpathian mountains in southern Poland. In Denmark, the zone crosses North Jutland, passes north of Djursland, runs through Kattegat and along the coast of North Zealand, then continues through Skåne in Sweden and down past Bornholm.

“It is the transition to the Scandinavian bedrock – Norway, Sweden and down through Bornholm. It is a transition zone that has been thoroughly fractured with many faults over hundreds of millions of years,” said Søren Bom Nielsen, professor of geoscience at Aarhus University.

The zone is more susceptible to deformation than the rigid Scandinavian bedrock. When tectonic plates push and pull against each other, the stiff bedrock resists movement, while the weaker transition zone absorbs the stress until it is released as an earthquake, Nielsen explained.

Two large-scale forces drive the pressure. The spreading of the Atlantic seafloor, as the American and European continents drift apart, builds up stress. At the same time, Africa is moving northward, slowly closing the Mediterranean and causing frequent earthquakes in southern Europe, but also affecting Denmark. “The new ocean floor forming in the Atlantic increases the pressure. Meanwhile, Africa is gradually closing the Mediterranean by pushing up against Europe,” Nielsen said.

Tine Larsen, senior researcher at Geus, also pointed to pressure from the Mid-Atlantic Ridge as a contributing factor. She noted that hundreds of small tremors occur in Denmark every year. “And then once in a while, we have some strong enough that we can feel them,” she said.

Asked whether aftershocks are likely, Nielsen said they often follow but are usually weaker than the initial quake. “So it is possible that we will see aftershocks in Denmark as well, but they will probably not be anything you notice,” he said.

The last earthquake of comparable strength in Denmark was a magnitude 4.3 tremor in 2012 with an epicentre in Kattegat. Earlier events include a 4.7 quake in Thy in 2010 and a 4.8 quake in Skåne in 2008.