Possible shallow crustal shear wave anisotropy off Lofoten, Norway, inferred from three-component ocean-bottom seismographs

Abstract

Summary P- and S-wave velocity models along two perpendicular profiles 145 and 175 km long on the undrilled continental shelf off Lofoten, northern Norway, have been obtained from the study of nine three-component ocean bottom seismographs. A VP/VS ratio of 1.96–2.15 is found in the sedimentary layers along a NE-SW profile. These high values are consistent with shaly sediments, which are identified in dredged bedrock samples. Along a NW-SE profile the S-wave velocities in the sedimentary layers are 5-10 per cent higher than along a NE-SW profile. The most likely explanation for this velocity difference is seismic anisotropy, which might be caused by liquid-filled microfractures alligned along the direction of the present-day maximum horizontal compressive stress. The Vp/Vs ratio in the upper crystalline crust is found to be 1.75 along both profiles. If the inferred anisotropy described above is caused by aligned microcracks, the lack of anisotropy in the crystalline basement indicates that these cracks are restricted to the sedimentary layers. It must be emphasized, however, that the observed azimuthal difference in S-wave velocities could be caused by lateral velocity variations in the sediments, although the anisotropy hypothesis is considered more likely.