Depth dependent azimuthal anisotropy beneath the Juan de Fuca plate system

We use surface wave measurements to reveal anisotropy as a function of depth within the Juan de Fuca and Gorda plate system. Using a two-plane wave method, we measure phase velocity and azimuthal anisotropy of fundamental mode Rayleigh waves, solving for anisotropic shear velocity. These surface wave measurements are jointly inverted with constraints from shear wave splitting studies using a Markov chain approach. The resolved structure is consistent with previous SKS studies, but our inversions provide key missing information that holds clues to the vertical distribution of strain and spreading center processes. Anisotropy of the Juan de Fuca plate interior is strongest (~2.4%) in the low velocity zone between ~40-90 km depth, with ENE direction driven by relative shear between plate motion and mantle return flow from the Cascadia subduction zone. In disagreement with measurements, weak (~1.1%) lithospheric anisotropy in Juan de Fuca is highly oblique to the expected ridge-perpendicular direction, perhaps connoting complex intra-lithospheric fabrics associated with melt and/or off-axis downwelling. In the Gorda microplate, strong shallow anisotropy (~1.9%) is consistent with inversions and aligned with spreading, and may be enhanced by edge-driven internal strain. Weak anisotropy with ambiguous orientation in the low velocity zone can be explained by Gorda’s youth and modest motion relative to the Pacific. Deeper (≥90 km) fabric appears controlled by regional flow fields modulated by the Farallon slab edge: anisotropy is strong (~1.8%) beneath Gorda, but absent beneath the Juan de Fuca, which is in the strain shadow of the slab.