A compressional velocity model for the northwest Atlantic Ocean is derived by modeling waveform and travel time data from long-period and short-period World-Wide Standard Seismograph Network and Canadian network station recordings. A 90-km-thick lid with a velocity of 8.1 km/s at the top gradually increasing to 8.3 km/s at the bottom is obtained by fitting the travel time data of first arrivals and waveform data of almost purely oceanic paths at distances of 8¿--20¿. Triplication P waveform data constrain the structure below the lid. A distinct but smooth low-velocity zone is located at a depth of 170 km. Combining these results with the shear wave structure derived by Grand and Helmberger (1984a) for the same region, we suggest an olivine-rich mineralogy in the upper 100 km and partial melting possibly extending as deep as 300 km based on low seismic velocities and high VP/VS. ¿ American Geophysical Union 1993