Probing into the Upper Mantle Using Surface Waves

Beyond the Geometrical Ray Theory

Abstract

A variety of methods of surface wave inversion, which enables us to investigate detailed images of the upper mantle on a regional scale, are reviewed. The study of surface wave tomography beginning in the 1980's has brought us with a significant jump in our understanding of the Earth's interior, particularly the upper mantle. Most of the studies of surface wave tomography have been based upon a geometrical ray theory, working with either dispersion curves or waveforms of surface waves. Such a simple representation of surface wave propagation has allowed us to treat a greater number of data sets, which are indispensable for obtaining high resolution tomography models. However, the ray theory, which is relying upon the high-frequency approximation, is no longer valid when the scale-length of heterogeneity is comparable to the wavelength of waves to be considered. The effects of finite frequency are particularly important for the higher-frequency surface waves, which mainly sample the crust and uppermost mantle where very strong lateral heterogeneity is likely to exist. Recent development of the 3-D sensitivity kernels allows us to treat the effects of finite frequency in the tomographic inversions. The use of such finite frequency theory will further advance the methods of surface wave tomography.