Abstract
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 Møre and Vøring 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 deg 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.
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