[1] The mantle beneath the Japanese islands is complex because of subduction of the Pacific and Philippine Sea plates and the deformation associated with it. Detection of seismic anisotropy should be useful for understanding the processes occurring in such mantle. Here we resolve seismic anisotropy of the mantle wedge from that of the underlying subducted Pacific slab using a large number of measurements of shear wave splitting for a family of core-reflected shear phases: ScS, sScS, ScS2, and sScS2. The anisotropy of the mantle wedge changes sharply across the volcanic front. On the Pacific side of the volcanic front a vertically propagating shear wave is polarized with the fast direction approximately parallel to the trench, whereas on the back-arc side it is polarized with the fast direction approximately parallel to the plate convergence direction. The Pacific slab is uniformly anisotropic with the NNW fast direction, consistent with the paleospreading direction of the northwestern Pacific seafloor. The preferential alignment of anisotropic crystals frozen in the plate at the time of its formation appears to be preserved down to depths beyond 400 km within the slab.
[1] A family of core-reflected shear waves generated by the 28 June 2002 Vladivostok deep earthquake, including near vertical reflections from the 410-and 660-km discontinuities, were recorded by $500 tiltmeters of the Hi-net and $60 broadband seismometers of the F-net in Japan. The observed upper mantle reflections were crosscorrelated with synthetics calculated on the basis of the spectral element method for a fully three-dimensional Earth model using the Earth Simulator supercomputer to accurately determine the depths of the reflection points. The mapped upper mantle discontinuities were compared with a high-resolution P wave tomographic image. The 660-km discontinuity is depressed at a constant level of $15 km along the bottom of the horizontally extending aseismic slab under southwestern Japan. The transition from the normal to the depressed level occurs within a lateral distance of less than $200 km. Observations suggest that the reflections from the 410-km discontinuity interfere with those from slab-related structures on top of this discontinuity, leading to a spuriously large elevation of the 410-km discontinuity in and near the subducted slab. Records at stations relatively free from such interference effects, however, still imply elevation of this discontinuity within the slab.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.