Velocity anisotropy of biotite schist and amphibolite from the Hidaka metamorphic belt was measured under confining pressures of up to 150 MPa, where most of thin cracks are closed and intrinsic anisotropy of rocks appears. In both rocks, intrinsic velocity anisotropy is basically interpreted as the result of the lattice preferred orientation of horblende and biotite. The anisotropy of biotite schist can be interpreted well by TI anisotropy with a slight modification of orthorhombic symmetry caused by crenulation. However, amphibolite showed a confusing anisotropy which cannot be explained well by either TI or orthorhombic symmetry. Employing a model consisting of a TI matrix having oriented oblate-spheroidal cracks with short axes parallel to the symmetry axis of the matrix, we interpret the velocity changes in biotite schist as a function of confining pressure. The model explains changes of velocity anisotropy in biotite schist fairly well.
