Probing depth and lateral variations of upper-mantle seismic anisotropy from full-waveform inversion of teleseismic body-waves

SUMMARY While seismic anisotropy can potentially provide crucial insights into mantle dynamics, 3-D imaging of seismic anisotropy is still a challenging problem. Here, we present an extension of our regional full-waveform inversion method to image seismic anisotropy in the lithosphere and asthenosphere from teleseismic P and S waveforms. The models are parametrized in terms of density and the 21 elastic coefficients of the fourth-order elasticity tensor. The inversion method makes no a priori assumptions on the symmetry class or on the orientation of the symmetry axes. Instead, the elasticity tensors in the final models are decomposed with the projection method. This method allows us to determine the orientation of the symmetry axes and to extract the contributions of each symmetry class. From simple synthetic experiments, we demonstrate that our full-waveform inversion method is able to image complex 3-D anisotropic structures. In particular, the method is able to almost perfectly recover the general orientation of the symmetry axis or complex layered anisotropic models, which are both extremely challenging problems. We attribute this success to the joint exploitation of both P and S teleseismic waves, which constrain different parts of the elasticity tensor. Another key ingredient is the pre-conditioning of the gradient with an approximate inverse Hessian computed with scattering integrals. The inverse Hessian is crucial for mitigating the artefacts resulting from the uneven (mostly vertical) illumination of teleseismic acquisitions.