Seismic anisotropy in the uppermost mantle beneath oceanic regions from data of broadband OBSs

For improving vertical resolution of seismic-anisotropy structure at depths of 10-100 km beneath oceanic regions, we measured phase velocities of surface waves in a broadband frequency range by two methods: the ambient noise interferometry in frequency higher than 0.035 Hz, and array analysis of event waveforms in lower frequency. We use seismograms recorded by broadband ocean bottom seismometers (BBOBSs) in two regions: (i) the Shikoku Basin in the Philippine Sea by Stagnant Slab Project, and (ii) east of Tahiti Island by a project called the tomographic investigation by seafloor array experiment for Society hotspot (TIARES). The frequency ranges of phase-velocity measurements in each region are summarized in Table. For the case of Shikoku Basin, we invert phase velocities for radially anisotropic structure. The resultant structure shows decrease of shear-wave velocity by 6-8 % at depths of 50-70 km, and intensification of radial anisotropy (VSH>VSV) from 1-2 % at 10-20 km depth to 4-6 % at 40-70 km depth. These results indicate increasing amount of preferred-oriented olivine crystal, and/or horizontal layering of partial melt near the boundary between the lithosphere and the asthenosphere. The azimuthal anisotropy of phase velocity in the Shikoku Basin is also investigated by array analysis of event waveforms for the fundamental mode of Rayleigh wave at 0.03 Hz. The fastest direction is NW, and consistent with direction of present plate motion. The velocity difference between fastest and slowest directions is 1-2 %. These results mainly reflect shear-wave velocity at depth of 30-60 km, and imply that lattice preferred orientation is, at least, partly (though may not be fully) responsible for the anisotropy in the depth range. We will obtain radially anisotropic structure and azimuthal anisotropy in Tahiti region, and will present difference between two regions.<br />Frequency range of phase-velocity measurements for two regions of analyses.
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