<p>Structural variability at the subduction interface imposes stress and strength heterogeneity that affects the behavior of megathrust earthquakes. Characterizing this variability can be challenging because the slab interface is often deep and seismically quiet. Seismic wavefields from intraslab earthquakes can contain high-frequency phase conversions from the megathrust fault that are delayed and distorted by interface topography and velocity heterogeneity. Distributed acoustic sensing (DAS) allows for long-term deployments of dense seismic arrays that can yield many observations of these converted phases at high spatial resolution over large areas. Here, we develop a technique to identify and enhance slab-converted phases in DAS data, and we pick the differential arrival times and relative polarities between the direct and converted phases at short wavelength along the fiber optic cable. We apply this framework to earthquake wavefields measured by a DAS array at the southern end of the Cascadia Subduction Zone, above the locked zone, which suggests kilometer-scale interface roughness and variations in crustal thickness and absolute depth of the Gorda slab. Converted phase wavefields also suggest sharp variability in the presence and position of fluids in the slab and forearc crust.</p>