Three-dimensional image of the anisotropic bodies beneath central Honshu, Japan

Shear-wave splitting from local deep earthquakes is investigated to clarify the volume and the location cif two anisotropic bodies iii the mantle wedge beneath central Honshu, Japan. We observe a spatial variation in splitting parameters depending on the combination of sources and receivers, nearly N-S fast in the northern region, nearly E-W fast in the southern region and small time delays in the eastern region. Using forward modelling, two models with 30 and 10 per cent anisotropy ate tested by means of a global search for the locations crf anisotropic bodies with various volumes. The optimum model is obtained for 30 per cent anisotropy, which means a 5 per cent velocity difference between fast and slow polarized waves. The northern anisotropic body has a volume of 1.00 degrees (longitude) x 0.5 degrees (latitude) x 75 km (depth), with the orientation of the symmetry axis being N20 degrees E. The southern anisotropic body has a volume of 1.25 degrees x 1.25 degrees x 1.00 km with the symmetry axis along N95 degrees E. Our results show that the anisotropic bodies are located in low-velocity and low-Q regions of the mantle. This, together with petrological data and the location of volcanoes in the are, suggests that the possible cause of the anisotropy is the preferred alignment of cracks filled with melt.