GLOBAL TOMOGRAPHY FROM RAYLEIGH AND LOVE WAVE-DISPERSION - EFFECT OF RAY-PATH BENDING

A large data set of fundamental-mode Rayleigh and Love wave phases has been employed for global tomographic inversions. These data represent observations of the first arriving surface waves R(1) and L(1) from a pproximately 850 seismic events, with about 10 observations of dispers ion per event. The inversion for laterally varying depth-dependent str ucture is performed in several steps. At discrete periods from 111 s-2 50 s for Love waves, and from 111s-200 s for Rayleigh waves, we first determine simultaneously the global distribution of phase-velocity ano maly and the relocations of the seismic events. Each phase-velocity di stribution is then corrected for laterally varying Moho depth and bath ymetry, followed by inversion for 3-D earth structure in the depth ran ge 0-240 km. In order to lie within the limits of ray theory we restri ct all model perturbations to a degree 0-16 spherical-harmonic expansi on. A second-order scattering (ray path) correction is included in the inversions. The phase-velocity perturbations show a high correlation with surface tectonics at shorter periods. Comparison of inversions sh ows that those performed without the ray-path correction exhibit a com plicated pattern of fast- and slow-velocity bias. The only common feat ure in the patterns of bias (with respect to period or wave type) is t hat fast velocity bias is concentrated in regions of large structural gradient. The amplitude pattern of the depth-dependent model has prono unced peaks in the intervals 0-70 km and 140-210 km. The deeper peak i s associated with lateral variations in asthenosphere structure. We de rive new estimates for the spherically averaged phase velocities of th e fundamental-mode Rayleigh and Love waves. The spherically averaged p hase velocities are decreased by about 0.1 per cent by including the r ay-path correction. These phase-velocity dispersions can be simultaneo usly fit well with an isotropic model either with or without the ray-p ath correction, but particularly well when the ray-path correction is included. In both cases a pronounced low-velocity zone of the global e xtent is required in the depth range 120-190 km.