New seismic measurements of the repeated phenomenon of Episodic Tremor and Slip in northern Cascadia indicate identical source processes of tremor and slow slip. Predicted polarization directions of upgoing S-waves radiated from shear slip on the plate interface align with the relative motion between the Juan de Fuca and North American plates. Seismic observations from small-aperture array data on the Olympic Peninsula of the Cascadia subduction zone show uncharacteristically stable linear particle motion coincident with the passage of tremor sources beneath the array. The azimuth of this horizontal ground motion matches expected polarizations from slip on the plate interface. This finding suggests that Cascadia tremor is shear slip on the plate interface, implying that, as in Japan, geodetically observed slow slip and seismically observed tremor are manifestations of the same phenomenon.