Upper Mantle Anisotropy from Global Adjoint Tomography

Seismic anisotropy is one of the key parameters for understanding the deformation and the flow in the lithosphere and the mantle. First-generation global adjoint models are transversely isotropic in the upper mantle. However, there is strong evidence for azimuthal anisotropy in the upper mantle as well. To this end, we continue global adjoint iterations based on 3D spectral-element wave simulations by also taking surface-wave azimuthal anisotropy into account. Starting from the transversely isotropic GLAD-M25~(Lei et al. 2020), we iteratively update vertically and horizontally polarized shear wavespeeds (transverse isotropy), and the density normalized azimuthal anisotropy parameters Gc' & Gs'. We invert both minor- and major- arc surface waves within the period bands of 40-100 s & 90-250 s from a dataset of 263 earthquakes. During the first 12 iterations, we used double-difference multitaper phase measurements. We continue our iterations with the exponentiated-phase misfit (Yuan et al. 2020), a variant of the instantaneous phase misfit (Bozdag et al. 2011), to better capture higher-mode surface waves and increase the resolution in the mantle transition zone. Our initial large-scale results are consistent with previous global azimuthally anisotropic models and plate motions. We also approach continental-scale resolution in densely covered regions in our global inversion. We performed the first ten iterations on Oak Ridge National Laboratory's Summit and continue the inversion on Texas Advanced Computer Center's Frontera system.