Full-wave <i>Pn</i> inversion for azimuthal anisotropy of the upper mantle

SUMMARY Azimuthal anisotropy of the upper mantle, resolved from seismic records, sheds light on the characteristics of lithospheric deformation and asthenosphere flow. However, our understanding of the 3-D structures of P-wave azimuthal anisotropy in the upper mantle remains limited, due to the lack of accurate and high-resolution tomography methods. Motivated by this necessity, we present a novel full-wave inversion method to simultaneously retrieve the 3-D structures of P-wave velocity and azimuthal anisotropy of the upper mantle. The inversion is parametrized by imposing five elastic coefficients representing Pn-wave azimuthal anisotropy on the isotropic elastic tensor based on the weak anisotropy assumption. We verify the accuracy of the full-wave sensitivity kernels of Pn waveform cross-correlation delay times to the azimuthal anisotropic coefficients. Synthetic inversions demonstrate that our method effectively resolves the anomalies of both P-wave velocity and azimuthal anisotropy including 2$\psi $ and 4$\psi $ terms. Moreover, we analyse the model resolution and trade-offs using the Hessian-vector product efficiently computed by the Scattering-Integral method. This study provides a novel and powerful physics-based tool to reveal accurate and high-resolution 3-D structures of azimuthal anisotropy in the upper mantle, which will facilitate our understanding of the modes of lithospheric deformation and mantle convection.