Stress inversion of earthquake focal mechanism solutions from onshore and offshore Norway

A comprehensive compilation of 112 earthquake focal mechanism solutions in Norway and adjacent areas has been completed, including 7 previously unpublished solutions for recent earthquakes determined as part of the NEONOR (Neotectonics in Norway) project. Using the method of Gephart & Forsyth (1984), the 97 solutions on the Norwegian mainland and margin have been inverted with respect to primary stress directions, which on a regional basis and in the best possible way could satisfy the individual solutions. In doing this the Norwegian mainland and margin areas were divided into six zones containing between 5 and 34 earthquake focal mechanism solutions. Two additional zones containing only in situ measurements are also defined, and horizontal stress directions evaluated. The results show a tendency for oblique-slip reverse faulting offshore, for oblique-slip normal faulting onshore and for average stress directions which are in overall compliance with a NWSE oriented regional stress field assumed to be dominated by the mid-Atlantic ridge-push force. Local variations do exist, however, in particular for more shallow earthquakes in mid-Norway coastal areas, where the dominating compressive stress directions are NESW, implying coast-perpendicular extension. While this may be related to different kinds of local effects, it is also possible that deglaciation flexure can explain the observed stress directions in this region, since this is also where we find the maximum postglacial uplift gradients.