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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 120; no. 32; p. e2221770120
• Main Authors: Liu, Chao, Yoshino, Takashi, Yamazaki, Daisuke, Tsujino, Noriyoshi, Gomi, Hitoshi, Sakurai, Moe, Zhang, Youyue, Wang, Ran, Guan, Longli, Lau, Kayan, Tange, Yoshinori, Higo, Yuji
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 08.08.2023
• Subjects: Asthenosphere; Attenuation; Cyclic loads; Frequency ranges; High temperature; Lava; Lithosphere; Mid-ocean ridges; Modulus of elasticity; Moisture content; Olivine; Physical Sciences; Ridges; Seismic properties; Upper mantle; Velocity; Water content; water; attenuation; lithosphere-asthenosphere boundary
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2221770120

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.