Full‐Wave SKS Splitting Intensity Tomography Suggests Depth‐Dependent Upper Mantle Fabrics and Anisotropy Beneath Alaska

Alaska's tectonic complexity makes it ideal for probing upper mantle deformation. We present a 3D shear-wave anisotropy model obtained by inverting 7,985 SKS splitting intensity measurements from 261 broadband stations using 941 events from 2000 to 2023. Our multi-scale model resolves lateral and vertical variations of shear-wave azimuthal anisotropy. In the subduction zone, the fast axis in the sub-slab mantle below the forearc and offshore regions changes with depth from trench-normal to trench-parallel, in agreement with predictions of geodynamic model for retreating slabs. In the asthenosphere below the backarc region, fast axes are mainly oriented northeast–southwest, which is consistent with southwestward mantle flow and extrusion tectonics, and suggests strong lithosphere–asthenosphere coupling. Anisotropy weakens below ∼200 km, indicating an increased vertical olivine alignment due to slab steepening. A two-layer anisotropic structure in the backarc region suggests the presence of lithosphere fabrics unrelated to recent dynamics.