Laboratory measurements of elastic anisotropy parameters for the exposed crustal rocks from the Hidaka Metamorphic Belt, Central Hokkaido, Japan

Abstract

SUMMARY To study crustal rock seismic anisotropy and its effect on seismic wave propagation, we measure the seismic velocity anisotropy of two amphibolites, one biotite gneiss and one biotite schist from the Hidaka metamorphic belt in central Hokkaido, Japan, under confining pressures up to 150 MPa. The rock microstructures show foliation and lineation characterized by lattice preferred orientation (LPO) of hornblende or biotite. P- and two S-wave velocities are measured along the direction perpendicular to the foliation plane and two directions in the foliation plane: perpendicular and parallel to the lineation. We assume orthorhombic symmetry based on the rock microstructures and obtain Tsvankin’s anisotropic parameters (an extension of Thomsen’s parameters for orthorhombic symmetry). P- and S-wave phase velocity surfaces are calculated from anisotropy parameters and compared with the measured velocities along particular directions and with the velocity contour maps calculated from the Voigt averages of singlecrystal elastic constants based on the orientation of measured LPO data. Qualitatively, the measured velocity anisotropy agrees with the velocity contour calculated from the LPO data, although large quantitative differences exist between them. All anisotropy patterns can be approximated as transverse isotropy or its modification, appearing as orthorhombic symmetry. Biotite schist (containing 30 per cent volume ratio biotite) shows strong S-wave anisotropy, and the phase velocity surfaces of P waves show a large deviation from ellipticity in the plane perpendicular to the foliation and parallel to the lineation. In the same plane, S waves show a singularity due to a large bulge of the SV velocity surface.