Precursors to S resulting from S to P conversions at discontinuities beneath a receiver (Sp phases) can provide useful constraints on crustal and upper mantle structure. In particular, Sp/Sv amplitude ratios are sensitive to the shear velocity contrast across the discontinuity at which the conversion takes place. We have observed large-amplitude M-converted Sp phases on long-period seismograms from seven stations in eastern Canada following a deep-focus event in the Peru-Brazil region of South America. From the S-Sp time differences were determine that the average crustal shear velocity beneath the stations is 3.65¿0.15 km/s. To satisfy the average observed Sp/Sv amplitude ratios, the shear velocity contrast across the Mohorovi?ic (M) discontinuity must be at least 1.25 km/s. These constraints, together with the observed apparent velocity of Sn waves, require the existence of a thick low-velocity channel for shear waves at the base of the crust in eastern Canada. From average P velocities observed in refraction studies, it appears that the value of Poisson's ratio within this channel must be greater than 0.33. The high values of Poisson's ratio obtained for the lower crustal low-velocity channel indicate the presence of water or another volatile component. We suggest that the hydration state of the lower crust beneath eastern Canada exceeds that of the intermediate crust. We have also observed Sp phases converted at discontinuities as deep as 200 km.