Seismotectonics of the Norwegian continental margin

The Norwegian continental margin and surrounding areas are seismically less active than some other passive margins worldwide, indicating a potential earthquake deficit. The adjacent oceanic crust is mostly aseismic except for parts of the Lofoten and Norway Basins which have experienced rapid deposition of glacial sediments. The excess load has enhanced the local stress field and, in turn, the seismic activity. The marginal highs along the continent-ocean transition and the part of the More and Voring Basins which experienced crustal extension prior to the early Tertiary breakup are practically aseismic. This region is also underlain by high-velocity lower crust emplaced during breakup, suggesting crustal strengthening, which also may increase the return periods for the largest events. Farther landward, a spatial correlation of seismic activity and the glacial sediment wedge suggests a causal relationship, expressed through preferential rejuvenation of Late Jurassic-Early Cretaceous faults. The locally high seismic activity in the coastal region occurs in areas where the continental crust is relatively thick (25-30 km) but still thinner than in the shield area farther east (45-50 km). Although stress relations are complex, we suggest that increased stress due to appreciable postglacial rebound gradients in the coastal region may be a contributing factor. While the stress field along the margin complies in general with the ridge push force, we infer that regional and local stress enhancement factors are not only present in this region but also necessary for explaining the earthquake activity. The relative importance of local stress sources is supported by several cases of 90 degrees stress rotations relative to the ridge push direction. The superposition of regional and local stress, together with the existence of weakness zones and faults, yields potentials for earthquakes, primarily through structural reactivation.