EFFECTS OF WATER ON SEISMIC-WAVE VELOCITIES IN THE UPPER-MANTLE

Possible roles of water to affect seismic wave velocities in the upper mantle of the Earth are examined based on mineral physics observation s. Three mechanisms are considered: (i) direct effects through the cha nge 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 rang e of water content in the upper mantle, but the latter two indirect ef fects involving the motion of crystalline defects can be significant. The enhanced anelastic relaxation will significantly (several %) reduc e the seismic wave velocities. Based on the laboratory observations on dislocation mobility in olivine, a possible change in the dominant sl ip direction in olivine from [100] to [001] and a resultant change in seismic anisotropy is suggested at high water fugacities. Changes in s eismic wave velocities due to water will be important, particularly in the wedge mantle above subducting oceanic lithosphere where water con tent is likely to be large.