Shear waves from three-component ocean bottom seismographs off Lofoten, Norway, indicative of anisotropy in the lower crust

Abstract

SUMMARY Data from four three-component ocean bottom seismographs (OBS) situated on a 145 km long profile on the Rost High on the continental shelf off Lofoten, northern Norway, have been modelled using 2-D seismic ray tracing, and a P- and S-wave velocity model along the profile is presented. The S-wave model has been obtained from a study of the high quality, horizontal component data. A Vp/Vs ratio of 1.95–2.15 is found in the sedimentary layers on the Rost High. These high values are consistent with shaly sediments, which are identified in dredged bedrock samples. South of the Rost High a considerably lower Vp/Vs ratio (ca. 1.6) is estimated for the sediments, which can be attributed in part to a higher degree of compaction in the deepest sediments and in part to a more sandy lithology. In the upper and middle crystalline crust a Vp/Vs ratio of 1.75 is obtained. Strong S reflections and P-to-S conversions at the Moho are observed on all four instruments, and the modelling of these arrivals indicates that the S-wave velocity in the lower crust varies with angle of incidence, from 3.5 km s−1 (Vp/Vs= 1.95) at vertical incidence to 4.0 km s−1 (Vp/Vs= 1.7) at 60–70d. This angle of incidence dependent S-wave anisotropy (the order of 10 per cent) is consistent with some horizontal or subhorizontal layering in the lower crust.