In the Cascadia subduction zone, beneath southern Vancouver Island at 25–45 km depth, converted teleseismic waves reveal an ∼5-km-thick landward-dipping layer with anomalously high Vp/Vs averaging 2.35 ± 0.10 (2σ), interpreted as subducted oceanic crust of the Juan de Fuca plate. This layer is observed downdip of the inferred locked seismogenic zone, in the region of episodic tremor and slip. Laboratory velocity measurements of crystalline rock samples made at 200 MPa confining pressure and elevated pore pressures demonstrate that Vp/Vs increases with increasing fluid-filled porosity. The observed high Vp/Vs values are best explained by pore fluids under near lithostatic pressure in a layer with a high porosity of 2.7%–4.0%. Such large volumes of fluid take ∼1 m.y. to accumulate based on reasonable rates of metamorphic fluid production of ∼10−4 m3/(m2 yr) in subducting Juan de Fuca crust and mantle. Accordingly, the permeability of the plate interface at these depths must be very low, ∼10−24 to ∼10−21 m2, or the porous layer must have a permeability <3 × 10−20 m2.