Abstract
Data from six three-component Ocean Bottom Seismographs situated on a 175 km long profile on the continental shelf off Lofoten, northern Norway, have been modelled using 2-D seismic ray-tracing, and P- and S-wave velocity models for the upper mantle are presented. A dipping structure about 10 km below the Moho causes strong P and S reflections as well as P-to-S conversions. This upper mantle reflection might be related to a possible seaward dipping master fault, and/or the presence of partially hydrated peridotite. The modelling of these arrivals reveals significant seismic anisotropy in the upper mantle. The maximum P-wave anisotropy is inferred to be in excess of 15%, the maximum S-wave anisotropy in the order of 5–10%, and the axis of the fast velocity is inferred to be inclined at least 45° from the horizontal. The strong anisotropy for P-waves indicates that its cause is related to alignment of minerals. We postulate that the inferred steep inclination of anisotropic minerals might be a result of shearing along faults and fractures during post-Caledonian extensional episodes. It must be emphasized, however, that it cannot be excluded that more complicated anisotropic models with other geological implications can also explain the observations satisfactorily. The study demonstrates the importance of incorporating S-waves in analysis of seismic data, because the strong P-wave anisotropy would not have been discovered without the constraints given by the S-waves.
Published Version
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