S-wave anisotropy in the upper mantle under a volcanic area in Japan

Abstract

Most ultra-basic nodules that probably originated in the upper mantle have anisotropic petrofabric and elastic properties^1,2. Within seismology, however, the Earth is generally assumed to be isotropic and this assumption is appropriate for most seismological studies. Thus, rocks can be considered isotropic on large scales although anisotropic on a scale of hand specimen³. This interpretation is questionable, however, in light of the increasing resolution of seismic observations. Many reliable reports on seismic anisotropies have been accumulating: for example, azimuthal anisotropy in P_n velocities in oceanic basins⁴, and in the continent⁵ and the back-arc basin⁶, and an S-wave splitting of the SH and SV waves revealing an anisotropic structure as deep as 190 km (ref. 7). These suggest that some unsuccessful detections of the Earth's anisotropies may be due to lack of appropriate observations. We describe here the recent discovery of shear wave anisotropy in a volcanic area in Japan, which was revealed by an improvement in the seismographic observation systems.