On the inversion of Love‐ and Rayleigh‐wave dispersion and implications for Earth structure and anisotropy

Summary. Various factors can make it difficult to explain observations of Love‐ and Rayleigh‐wave dispersion with the same relatively simple isotropic model. These factors include systematic errors which might occur in determinations of observed group and phase velocities, lateral variations in structure along the path of travel, and the attempt to explain observations with a model comprised of only a small number of thick layers. The last of these factors is illustrated by an inversion of dispersion data in the central United States where shear‐wave anisotropy had previously been invoked as one way to explain incompatible Love‐ and Rayleigh‐wave velocities. It is shown that the data can be satisfied equally well by an isotropic model consisting of several thin layers. In cases where the incompatibility of Love‐ and Rayleigh‐wave data might be produced by intrinsic anisotropy, it is necessary to invert those data using an anisotropic theory rather than by separate isotropic inversions of Love and Rayleigh waves. Inversions of fundamental‐mode data for a region of the Pacific, assuming anisotropic media in which the layers are transversely isotropic with a vertical axis of symmetry, lead to models which are highly non‐unique. Even if the inversions solve only for shear velocities in the litho‐sphere and asthenosphere it is not possible, without supplementary information, to ascertain the depth interval over which anisotropy occurs or to determine the thickness of the lithosphere or asthenosphere with much precision.