CRUSTAL STRESS REGIME IN FENNOSCANDIA FROM FOCAL MECHANISMS

An extensive data set of earthquake focal mechanisms is now available for all of northern Europe and especially for Fennoscandia. These mechanisms are considered to provide representative coverage of the stress field. The maximum horizontal compressional stress orientations are internally very consistent over the area of northern Europe. The dominating NW-SE compressional stresses appear tied to relative plate motion, with Mid-Atlantic Ridge spreading and European-African collision in southern Europe. For Fennoscandia some exceptions to the regional stresS Pattern exist. The causes of these local anomalies have been investigated. No correlations with geological provinces or province boundaries were found. In addition, there does not appear to be any clear correlation between anomalous stress directions and postglacial uplift in the present earthquake activity. This lack of correlation is in sharp contrast to geological evidence in the form of large faults indicative of large postglacial earthquakes occurring right after the end of the latest ice age, 9000 years ago. Taken together. this evidence suggests a tremendous change of stress field in Holocene time, from one dominated by the postglacial unloading right after the ice age to one dominated by the present plate motion today.