Effects of Water on Seismic Wave Velocities in the Upper Mantle
Effects of Water on Seismic Wave Velocities in the Upper Mantle
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DOI：
10.2183/pjab.71.61
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发表时间：
1995
期刊：
影响因子：
--
通讯作者：
S. Karato
中科院分区：
文献类型：
--
作者：
S. Karato
基于矿物物理观测，研究了水对地球上地幔地震波速的可能影响。考虑了三种机制：（i）由于水的存在导致键强变化而产生的直接影响；（ii）由于非弹性弛豫增强产生的影响；（iii）由于矿物择优取向变化产生的影响。结论是，对于上地幔合理的含水量范围，第一种直接机制产生的影响极小，但涉及晶体缺陷运动的后两种间接影响可能是显著的。非弹性弛豫增强将显著（百分之几）降低地震波速。根据对橄榄石位错迁移率的实验室观测，提出在高水逸度下，橄榄石的主要滑移方向可能从[100]变为[001]，并导致地震各向异性发生变化。由于水导致的地震波速变化将是重要的，特别是在俯冲大洋岩石圈上方的楔状地幔中，那里的含水量可能较大。
Possible roles of water to affect seismic wave velocities in the upper mantle of the Earth are examined based on mineral physics observations. Three mechanisms are considered: (i) direct effects through the change in bond strength due to the presence of water, (ii) effects due to the enhanced anelastic relaxation and (iii) effects due to the change in preferred orientation of minerals. It is concluded that the first direct mechanism yields negligibly small effects for a reasonable range of water content in the upper mantle, but the latter two indirect effects involving the motion of crystalline defects can be significant. The enhanced anelastic relaxation will significantly (several %) reduce the seismic wave velocities. Based on the laboratory observations on dislocation mobility in olivine, a possible change in the dominant slip direction in olivine from [100] to [001] and a resultant change in seismic anisotropy is suggested at high water fugacities. Changes in seismic wave velocities due to water will be important, particularly in the wedge mantle above subducting oceanic lithosphere where water content is likely to be large.