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.