Second-mode seismic wave trains along many paths in Eurasia are observed to have a phase relationship between the vertical motion and the transverse horizontal motion which cannot exist for elastic waves in isotropic layered media. These coupled, or generalized, surface waves are caused by an anisotropic layer immediately beneath the crust. It is suggested that this anisotropy is the result of orientation of crystalline mantle material by convection currents.
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Stuart Crampin (1967) studied this question.