We have modeled the SH motion from earthquakes in the northwest Atlantic Ocean to derive an oceanic upper mantle shear velocity model. The signals were recorded on long-period WWSSN and Canadian network stations on the east coast of North America. The travel times and waveforms of seismograms, in the distance range 11¿--16¿, were used to constrain the lid structure. These data indicate a fast (4.75 km/s) lid of about 100 km thickness in the older western Atlantic. From 16¿ to 28¿ the S waves pass through triplications due to velocity jumps near 400 and 660 km. The branches from both discontinuities are visible in the S wave data. These arrivals were modeled using synthetic seismograms to obtain accurate travel times. The two triplications are also apparent in SS data from 30¿ to 50¿. The triplication curves derived from the S waves agree with the S S data. given the lid structure, the waveforms and travel times from the more distant data put tight constraints on the shear velocities at greater depths. We find shear velocities at greater depths. We find shear velocities, from 200 to 400 km, to be slow in relation to a shear model of the Canadian shield, derived by a similar technique, indicating continent-ocean heterogeneity to about 400 km.