S. Karato, EFFECTS OF WATER ON SEISMIC-WAVE VELOCITIES IN THE UPPER-MANTLE, Proceedings of the Japan Academy. Series B Physical and biological sciences, 71(2), 1995, pp. 61-66
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.