大洋岩石圈在机械薄弱层(软流圈)上移动，具有低地震速度和高衰减的特点。大洋中脊附近，由于高温地热，部分熔融会产生这种情况。然而，地震观测显示，在远离大洋中脊的岩石圈—软流圈边界(LAB)，海洋板块下的速度大幅降低。
研究人员通过使用循环加载的原位X射线观察，报告了在3GPa和1223-1373K的水欠饱和条件下，水对橄榄石聚集体的地震性质的影响。研究结果表明，在很宽的地震频率范围内(0.5-1000 s)，水大大增强了能量分散，降低了弹性模量。随着含水量的增加，出现在较高频率(1-5 s)的衰减峰值变得更加明显。如果水只存在于软流圈中，这与软流圈中衰减几乎是恒定在一个宽的频率范围内的观测结果是一致的。远离大洋中脊的海洋LAB的这些剧烈地震变化，可以用岩石圈和软流圈之间的含水量的差异来解释。
附：英文原文
Title: Effect of water on seismic attenuation of the upper mantle: The origin of the sharp lithosphere–asthenosphere boundary
Author: Liu, Chao, Yoshino, Takashi, Yamazaki, Daisuke, Tsujino, Noriyoshi, Gomi, Hitoshi, Sakurai, Moe, Zhang, Youyue, Wang, Ran, Guan, Longli, Lau, Kayan, Tange, Yoshinori, Higo, Yuji
Issue&Volume: 2023-7-31
Abstract: 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.
DOI: 10.1073/pnas.2221770120
Source: https://www.pnas.org/doi/10.1073/pnas.2221770120