Abstract
A large Love-Rayleigh wave discrepancy of phase velocity has been observed for mantle overtones in the Pacific Ocean. To test if preferred orientation of olivine crystals can be the cause for such an anisotropic behaviour, we have considered that upper mantle material might be represented by a medium with hexagonal symmetry and a horizontal symmetry axis oriented in the direction of preferred orientation of the olivine crystal a-axis. A set of Love and Rayleigh wave data that are related to paths crossing the whole Pacific plate have been inverted within this framework considering three parameters varying with depth: S-velocity, density and the proportion of oriented crystals. Crystals are found to be preferentially oriented to 400 km depth. Different orientations of the symmetry axis have been tested. A good fit to the data is obtained for an axis trending E-W to SE-NW, in qualitative agreement with the average trend of the absolute plate motion along the investigated paths. The model presented in this study predicts azimuthal variations of fundamental mode Rayleigh wave velocities in good agreement with estimates published in the literature.
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