In this paper we present a comprehensive database of 152 earthquake focal mechanism solutions for the Nordland area, northern Norway, and the adjacent offshore regions. 123 focal mechanisms are published for the first time. We developed an equation to objectively determine the quality of the focal mechanisms, while the equation was explicitly adjusted to the used dataset (not globally accepted). We used the equation to assign quality factors to the new focal mechanisms, while the quality of the earlier published focal mechanisms was kept as originally provided. About 20% of all focal mechanisms were set to the higher quality A and B, which was assigned to the onshore earthquakes only, whereas for offshore earthquakes the highest assigned quality was quality C, mostly due to larger azimuthal gaps. The results showed that seismicity onshore and offshore Nordland is different in type: with dominant shallow, normal-faulting earthquakes onshore and mostly deeper, mixed type faulting earthquakes offshore, while along the coast the faulting regime was mostly normal to strike-slip. The results indicated that maximum horizontal compressional stress, σH, directions in the offshore areas on a large scale originate from the plate-tectonic ridge push with NW–SE compression, whereas in the onshore regions σH directions are better explained through local stress-generating sources, such as topography. In our results we also recognised a possible relation between the Svartisen glacier massif and the enhanced seismicity in the surroundings, including an earthquake swarm activity to the west of the glacier. Moreover, in many onshore and offshore regions the nodal plane azimuths, obtained from the focal mechanisms, correlate well with the trends of geologically mapped onshore and offshore structures.
Earthquake source mechanisms in onshore and offshore Nordland, northern Norway