Constraints on Bend‐Faulting and Mantle Hydration at the Marianas Trench From Seismic Anisotropy

Subduction zones are a key link between the surface water cycle and solid Earth, as incoming plate carries pore hydrous minerals into subsurface. However, fluxes from to subsurface reservoirs over geologic time highly uncertain because volume of carried in slab mantle is poorly constrained. Estimates hydration based on seismic tomography assume bulk serpentinization, representing an upper bound volume. We measure azimuthal anisotropy near Marianas Trench, use spatial variations constrain extent geometry bend-related faulting, place lower content for case where serpentinization confined within fault zones. The observations can be explained by minimum ∼0.85 wt% mantle, compared ∼2 obtained tomography.