Depth‐Dependent Azimuthal Anisotropy Beneath the Juan de Fuca Plate System. Issue 8 (30th July 2020) - Eilon, Zachary C.
Abstract: 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 SKS splitting studies using a Markov chain approach. We show that the two data sets are consistent and present inversions that offer new constraints on the vertical distribution of strain beneath the plates and the processes at spreading centers. Anisotropy of the Juan de Fuca plate interior is strongest (~2.4%) in the low‐velocity zone between ~40‐ to 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 Pn measurements, weak (~1.1%) lithospheric anisotropy in Juan de Fuca is highly oblique to the expected ridge‐perpendicular direction, perhaps connoting complex intralithospheric fabrics associated with melt or off‐axis downwelling. In the Gorda microplate, strong shallow anisotropy (~1.9%) is consistent with Pn 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. Plain Language Summary: Seismic waves travel across the surface of the planet with speeds that vary depending on the temperature and composition of the Earth's interior in the regions they traverse. In detail, we find that they sometimes also exhibit directional dependence of wave speed—they travel across the same location faster going in one direction than going in another. This directional dependence arises from the alignment (or "fabric") of crystals that make up the rocks tens and hundreds of kilometers beneath the surface. The fabric, in turn, relates to how rocks have moved over the last few millions of years, thereby allowing us to peer into the history of tectonic motions on our planet. We use a technique that provides rare information about how this fabric changes with depth and find substantial variations in fabric at different depths and locations within the Juan de Fuca plate boundary region. The variations are linked to the differences in strength of drag between the tectonic plates and the underlying mantle. We also find evidence that subducting slabs—cold plates that are in the process of sinking into the hotter interior of the planet—can interrupt and strongly modify the flow of the hot mantle around them. Key Points: We use a two‐plane wave method to measure azimuthal anisotropy as a function of depth across the Juan de Fuca and Gorda plate system Results are consistent with measured SKS splitting; our study offers depth sensitivity crucial to interpretation of underlying dynamics Regionalized patterns of anisotropy reveal substantial differences in ridge‐derived fabric and how plates shear over underlying mantle