Analysis of teleseismic receiver functions recorded in the northern Cascadia subduction zone show that large P-to-S converted phases are generated at the boundaries of a dipping layer of deep crustal reflectors (the ''E'' zone) beneath central Vancouver Island. These data provide the first shear wave constraints on this layer which is clearly observed in seismic reflection sections and is coincident with a region of low electrical resistivity interpreted from a magnetotelluric survey. Modeling of the amplitudes, polarities, and arrival times of the P-to-S converted phases demonstrates that the E zone is a prominent low-velocity layer, with an S velocity 1.0¿0.2 km/s lower than the regions above and below. The E zone dips 7¿¿5¿ in the direction N10¿E¿20¿ and extends from depths of 36.5 to 41 km at ALB-B on central Vancouver Island. The estimated S velocity contrast, when considered with refraction P velocities, yields a Poisson's ratio of 0.34 for this zone. Given the uncertainty in ΔVp and ΔVs, this value may range from 0.27 to 0.38. The low P and S velocities and high Poisson's ratio combined with electrical resistivity measurements suggest a porosity of 0.1--1.9% and crack aspect ratios of 0.001--0.01, values consistent with a region containing thin, fluid-saturated cracks and thus supportive of the interpretation of this feature as a major shear zone. ¿American Geophysical Union 1991