We have determined three-dimensional P wave velocity structure up to a depth of 296 km beneath northern Tohoku, Honshu, Japan, by inverting arrival times of local and teleseismic events simultaneously. Velocities in the upper crustal layers correlate well with the surface geology and Bouguer gravity anomalies. Low-velocity zones are ubiquitous in the crust and the uppermost mantle beneath the Quarternary volcanic areas. A well-known feature of a pronounced low-velocity mantle wedge underlain by the dipping high-velocity (high-V) Pacific plate is generally recovered over the study area. Strong heterogeneity within the high-velocity Pacific plate is suggested by the presence of a pronounced low-velocity anomaly confined to the depths of 130--200 km. The decreased seismicity of intermediate-focus events may be correlated with the low-velocity zone. A two-dimensional inversion scheme was invoked to determine stably the average mantle structure perpendicular to the island arc. In the depths of 50--150 km, the inclined high-V zone has a thickness of 80--100 km with its top coinciding with the upper plane of the double-planed deep seismic zone. Below the depth of 200 km, the high-V zone is shifted toward the bottom of the 80-km-thick reference slab. In the intermediate-depth layer (150--200 km), the high-V zone is not clearly visible within the 80-km-thick reference slab. ¿ American Geophysical Union 1996