Global tomography from Rayleigh and Love wave dispersion: effect of ray-path bending

Abstract

SUMMARY 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 R1 and L1 from approximately 850 seismic events, with about 10 observations of dispersion per event. The inversion for laterally varying depth-dependent structure is performed in several steps. At discrete periods from 111s–250s for Love waves, and from 111s–200s for Rayleigh waves, we first determine simultaneously the global distribution of phase-velocity anomaly and the relocations of the seismic events. Each phase-velocity distribution is then corrected for laterally varying Moho depth and bathymetry, followed by inversion for 3-D earth structure in the depth range 0–240 km. In order to lie within the limits of ray theory we restrict all model perturbations to a degree 0–16 spherical-harmonic expansion. 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 shows that those performed without the ray-path correction exhibit a complicated pattern of fast- and slow-velocity bias. The only common feature in the patterns of bias (with respect to period or wave type) is that fast velocity bias is concentrated in regions of large structural gradient. The amplitude pattern of the depth-dependent model has pronounced peaks in the intervals 0–70 km and 140–210 km. The deeper peak is associated with lateral variations in asthenosphere structure. We derive new estimates for the spherically averaged phase velocities of the fundamental-mode Rayleigh and Love waves. The spherically averaged phase velocities are decreased by about 0.1 per cent by including the ray-path correction. These phase-velocity dispersions can be simultaneously fit well with an isotropic model either with or without the ray-path correction, but particularly well when the ray-path correction is included. In both cases a pronounced low-velocity zone of the global extent is required in the depth range 120–190 km.