Surface waves in weakly anisotropic structures: on the use of ordinary or quasi-degenerate perturbation methods

Abstract

SUMMARY Both ordinary and quasi-degenerate perturbation methods are used to calculate velocities and polarizations of fundamental and higher surface wave modes, at first order in anisotropy, in structures containing preferred orientation of olivine crystals. It is shown that, except for the Rayleigh wave fundamental mode, the ordinary perturbation method becomes invalid at short period due to close spacing of the Rayleigh and Love wave-mode phase velocities. Strong coupling then occurs between adjacent modes which can only be accounted for with the more general quasi-degenerate perturbation method. This phenomenon weakens at longer periods, where the ordinary perturbation method can be used more efficiently for the fundamental and first higher modes of both Love and Rayleigh waves.