Abstract
The earthquake activity of Norway and nearby offshore areas is low
to intermediate, with few events above magnitude 5. Recent significant
improvements in instrumental coverage in parallel with a better utilization
of older (including historical) data have shown that the seismicity
in the south is predominantly confined to the coastal areas and to
the Viking Graben, while from the northern North Sea to Svalbard
the earthquakes in a broad sense follow the continental margin. Fifty-one
focal mechanism solutions from these areas, about half of them new,
reveal stress directions that clearly indicate a connection to the
plate tectonic 'ridge push' force, at least for the areas at a minimum
distance from the continental margin. Along the margin, stress directions
also indicate a possible connection to postglacial uplift as well
as to lithospheric loading effects. A dominance of normal faulting
on the landward side and reverse faulting on the oceanic side agrees
with this interpretation. On a regional level, the seismicity in
these areas correlate quite well with geologic features such as grabens,
fault zones, fault complexes, fracture zones, and the margin itself,
indicating that these structures act in a general sense as weakness
zones in the presence of a regionally more stable stress field. In
the northern North Sea, however, an area with quite anomalous stress
orientations, with strike-slip faulting, is found in a region transitional
between normal and reverse faulting. Most of the earthquake foci
are confined to the presumably brittle parts of the crust, but many
events are also located quite close to, and on both sides of, the
Moho discontinuity.
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