Earthquake activity and its relationship with geologically recent motion in Denmark

Abstract

Maps of Scandinavian earthquake activity show a rather abrupt cut-off towards the southwest in the middle of Denmark. The southwestern part of Denmark belongs to an inactive area. While the Danish earthquakes occur in response to the same stress field as those in Fennoscandia, they are differently connected to the geologically recent motion. The 2–10 earthquakes per year in the Danish area, of magnitudes between 2 and 4.5, are located in the border zone between the Fennoscandian Shield and the Danish Basin and in the central northwestern part of the Danish Basin. The earthquakes in Kattegat, between Sweden and Denmark, are located in the Sorgenfrei-Tornquist Zone, which was uplifted in Late Cretaceous-Tertiary times. The earthquakes of this zone have depths between 0 and 15 km. They occur along the edges of blocks, which have experienced motion within Quaternary times. Contrary to this the most active area, a rather narrow NW-SE-trending zone from the shoulder of Jylland northwestwards into the Skagerrak and the North Sea, is located approximately in the axial part of the Danish Basin. A tentative correlation may be made with a fault zone which is thought to have been active from the Permian to the present. That fault zone has been a controlling factor for the distribution of the Tertiary and possibly also the Quaternary deposits. It seems though that no direct link exists between the faults observed in the pre-Upper Permian basement and shallow faults in the Tertiary and Quaternary deposits. The shallow faults are probably formed as response to movements in the Upper permian salt masses, which in turn may be activated by faulting at deeper levels. Further investigations are necessary to establish whether and how the shallow faults in the sediments are related to the earthquakes which occur deeper in the crystalline rocks, some as deep as 30–40 km, i.e. below the Moho.