Effect of water on seismic attenuation of the upper mantle: The origin of the sharp lithosphere-asthenosphere boundary

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, seis-mic 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 obser-vation 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 asthe-nosphere 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.

Automated summary

This study investigates the effect of water on the seismic properties of olivine aggregates in the oceanic lithosphere. The results show that water substantially enhances energy dispersion and reduces elastic moduli, with an attenuation peak at higher frequencies. The findings suggest that the sharp seismic changes at the lithosphere-asthenosphere boundary far from mid-ocean ridges could be attributed to the difference in water content between the lithosphere and asthenosphere.