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

Abstract

SUMMAR YTo 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.

From crystal to crustal: petrofabric-derived seismic modelling of regional tectonics

“…5). It is clear that micas, and especially biotite, make the main contribution to the bulk seismic anisotropy (see also Takanashi et al 2001;Chlupacova et al 2003). Due to their generally weak CPO in these samples, feldspars and quartz impose a diluting effect on the anisotropy introduced by micas.…”

Seismic velocity anisotropy in mica-rich rocks: an inclusion model

“…Thomsen's parameter δ changes from a negative value to a positive value as the crack volume ratio increases. Takanashi et al (2000) measured the change of δ in biotite schist (Hidaka, Hokkaido, Japan, with 30 per cent volume ratio of biotite) under confining pressure. Since open cracks are closed with increasing confining pressure, the change in the anisotropic parameter with increasing confining pressure corresponds to the reduction in crack volume.…”

“…However, most experimental measurements are only performed in the axial directions assigned by the foliation and lineation in rocks. Recently, Takanashi et al (2000) measured Thomsen's anisotropic parameters for some metamorphic rocks and found a typical non‐elliptic P ‐velocity anisotropy and an S ‐wave singularity in biotite schist. Mica‐group minerals are very common in the shallower crust and their preferred orientation is also common.…”

“…Mica‐group minerals are very common in the shallower crust and their preferred orientation is also common. The volume fraction of mica‐group minerals sometimes exceeds 30 per cent (Siegesmund & Kern 1990; Jones & Nur 1984; Takanashi et al 2000). Thus, mica‐induced anisotropy is expected in many parts of the Earth's crust.…”

“…Recently, Takanashi et al (2000) found an interesting velocity anisotropy in biotite schist from Hokkaido, Japan. The anisotropy is characterized by a remarkable bulge for one of the S ‐wave phase velocity surfaces and a non‐elliptic shape of the P ‐wave phase velocity surface in the plane including the direction of lineation.…”