Radially and Azimuthally Anisotropic Global Waveform Inversion

In this study, we propose new models of the distribution of shear-wave velocity and of its radial and azimuthal anisotropy in the crust and the upper mantle at the global scale. Seismic anisotropy is the consequence of the preferential orientation of minerals due to deformation. The reconstruction of both its radial and azimuthal components gives us insight into past and present deformation and flow in the lithosphere and the asthenosphere. Moreover, the full consideration of anisotropy also enables to accurately determine the isotropic shear-velocity average, and therefore to isolate the effects of thermal or compositional variations from those of anisotropic fabric. Our models are constrained by a large compilation of waveform fits for more than 750,000 vertical-component and 100,000 transverse-component seismograms. We follow a two-step partitioned inversion procedure that comprises the Automated Multimode Inversion of surface, S, and multiple-S waveforms in a period range from 10 s to 450 s, followed by a 3D tomographic inversion that reconstructs the vSH and vSV velocities and their 2ψ and 4ψ azimuthal dependencies. For a coherent joint inversion of vertical and transverse components, we regularize the tomographic inversion in terms of the linear isotropic average vS0=(vSH+vSV)/2 and radial anisotropy δ=(vSH-vSV)/2. Our models are consistent with published radially and azimuthally anisotropic models in the distribution of well-known, large-scale features, while also bringing new insights in regions where previous studies tended to disagree.