Shallow structure of the Cascadia subduction zone beneath western Washington from spectral ambient noise correlation

High-resolution 3-D shear velocity (Vs) images of the western Washington lithosphere reveal structural segmentation above and below the plate interface correlating with transient deformation patterns. Using a spectral technique, phase velocities are extracted from cross-correlated ambient noise recorded by the densely spaced "CAFE" broadband array. The spectral approach resolves shear velocities at station offsets less than 1 wavelength, significantly shorter than typically obtained by standard group velocity approaches, increasing the number of useable paths and resolution. Tomographic images clearly illuminate the high Vs (> 4.5 km/s) subducting slab mantle. The most prominent anomaly is a zone of low Vs (3.0-3.3 km/s) in the middle to lower continental crust, directly above the portion of the slab expected to be undergoing dehydration reactions. This low-velocity zone (LVZ), which is most pronounced beneath the Olympic Peninsula, covers an area both spatially coincident with and updip of the region of most intense episodic tremor and slip (ETS). The low Vs and comparison with published P wave velocity models indicate that Vp/Vs ratios in this region are greater than 1.9, suggesting a fluid-rich lower crust. The LVZ disappears southward, near 47 degrees N, coincident with sharp decreases in intraslab seismicity and ETS activity as well as structural changes in the slab. The spatial coincidence of these features suggests that either underthrusting of hydrated low-velocity material or long-term fluid fluxing of the overriding plate via dewatering of a persistently hydrated patch of the Juan de Fuca slab may partially control slip on the plate interface and impact the rheology of the overriding continental crust.