In‐situ evidence for a low degree of S‐wave anisotropy in the oceanic upper mantle

A seismic refraction survey using ocean bottom seismometers with one vertical and two horizontal component geophones was carried out off western Canada. This report presents one feature of a comprehensive interpretation of the data set, that dealing with shear-wave velocity anisotropy in the uppermost mantle. Amplitude-and traveltime-corrected SV record sections for two profiles, EX2 approximately parallel to the spreading ridge and EX3 approximately parallel to the direction of spreading, were compiled. From an interpretation based on comparisons with synthetic seismograms, the shear wave velocity in the upper mantle is 4.5 km/s for EX2 and 4.6 km/s for EX3. The corresponding compressional wave velocities are 8.3 km/s and 7.5 km/s. The significant difference in P-wave velocities is attributed to azimuthal velocity anisotropy of the type normally found in the Pacific basin. In contrast, the small difference in S-wave velocities suggests a very low degree of shear wave anisotropy. The combination gives a prominent anisotropy in values of Poisson's ratio for the upper mantle, the first in-situ evidence for this feature. These results are consistent with laboratory measurements on ultramafic rock samples from ophiolite complexes.