Mapping the lithosphere and asthenosphere with surface waves: Lateral structure and anisotropy
A review of the basic theory and practice of mapping lateral variations in surface wave phase velocities is presented, with some examples from the work of Ekstrom et al. [1997]. The importance of crustal corrections is demonstrated by comparison of observed phase velocities with those calculated from the recent crustal model of Mooney et al. [1998]. Phase velocity maps including terms of azimuthal anisotropy are presented, and an experiment suggests that in areas of poor path coverage, a bias is introduced when azimuthal effects are neglected in the inversion. The inversion procedure for obtaining a model of three-dimensional velocity heterogeneity in the mantle from surface wave dispersion data is discussed in the context of the new mantle model S20A [Ekstrom and Dziewonski, 1998]. This model exhibits significant lateral variations in radial anisotropy in the top 200-300 km of the mantle, resulting from the incompatibility of Rayleigh and Love wave dispersion data. This result suggests an important new direction for tomographic imaging of the upper mantle.