The two possible causes, crack alignment and olivine-crystal alignment, of the observed anisotropy in the wedge portion of the upper mantle between the earth's surface and a subducting plate are discussed on the basis of recent results from the analyses of shear-wave anisotropy and 3-D P-velocity inversion. At present, the observed anisotropy can be explained equally well by the two models. The former mechanism, possibly related to magma-filled cracks, seems plausible because the study area is located beneath active volcanoes landward of the volcanic front. A crack density of 0.05 and an aspect ratio of 0.1 are obtained for the anisotropic and low-velocity mass with vertical and horizontal extents of about 100 km at depths 50 to 150 km assuming lengths and aspect ratios of the cracks within the mass to be uniform.
