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Oceanic lithosphere moves over a mechanically weak layer (asthenosphere) characterized by low seismic velocity and high attenuation. Near mid-ocean ridges, partial melting can produce such conditions because of the high-temperature geotherm. However, seismic observations have also shown a large and sharp velocity reduction under oceanic plates at the lithosphere–asthenosphere boundary (LAB) far from mid-ocean ridges. Here, we report the effect of water on the seismic properties of olivine aggregates in water-undersaturated conditions at 3 GPa and 1,223 to 1,373 K via in-situ X-ray observation using cyclic loading. Our results show that water substantially enhances the energy dispersion and reduces the elastic moduli over a wide range of seismic frequencies (0.5 to 1,000 s). An attenuation peak that appears at higher frequencies (1 to 5 s) becomes more pronounced as the water content increases. If water exists only in the asthenosphere, this is consistent with the observation that the attenuation in the asthenosphere is almost constant over a wide frequency range. These sharp seismic changes at the oceanic LAB far from mid-ocean ridges could be explained by the difference in water content between the lithosphere and asthenosphere.

Seismic anisotropy has been widely observed near the subducting slabs in the lower mantle transition zone (MTZ) and is often interpreted by the lattice preferred orientation (LPO) of constituent minerals. Akimotoite is one of the dominant minerals near the cold subducting slabs. Therefore, we conducted the well‐controlled uniaxial and shear deformation experiments on the MgSiO3 akimotoite aggregates at 21–23 GPa and 900–1300°C by using the D111‐type Kawai‐type multianvil apparatus. We observed strong LPOs and the most dominant slip system of akimotoite is suggested to be <10true1‾0> $< 10\overline{1}0 > $(0001). The elastic wave velocities of deformed samples were calculated to be strong azimuthal and polarization anisotropy with the velocities of horizontally polarized shear waves greater than that of vertically polarized shear waves for the horizontal mantle shearing. Our results provide important implications for the origin of observed seismic anisotropies and the mantle flow directions in the lower MTZ.

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