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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 …