Seismic anisotropy observed in upper oceanic crust

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Abstract
Seismic anisotropy in the upper 1500m. of oceanic basement has been observed by polarization analysis of three‐component borehole seismometer records. The most convincing evidence for the anisotropy is shear wave splitting for explosive sources at four azimuths. Compressional wave particle motion deviations suggestive of anisotropy are also observed but they may be caused by lateral inhomogeneities. The anisotropy was not resolved by travel‐time analysis. The observed velocities and particle motions in the horizontal plane can be modelled to within a standard deviation by assuming a perfectly elastic, homogeneous, anisotropic layer 2 with hexagonal symmetry and a horizontal symmetry axis. The most probable cause of the anisotropy is preferred crack orientation.