The detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate, and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones. It is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism. In this study we have estimated the morphology of the upper surface of the subducting Pacific plate under northeastern (NE) Japan by using 1372 SP converted waves at the slab boundary, and 666 earthquakes beneath the Pacific Ocean that are well located with sP depth phases. We found that the Pacific slab beneath NE Japan subducts at a small dip angle of less than 10° for the first descent to about 20km depth and has a steeper dip angle of 15°-25° at 30-50 km depth and about 30° at deeper areas. The slab depth is approximately 50km under the Pacific coast, 150km under the Japan Sea coast, and nearly 100km beneath the volcanic front. At 60-150 km depths, earthquakes in the lower plane of the double seismic zone occur in the central part of the slab, while the upper-plane events occur within 10-15 km beneath the slab boundary. There is a change in the slab surface geometry around 38°N which corresponds to the landward extension of an oceanic fracture zone. The strike of the slab is NNE in the north and becomes NE in the south. The southern part of the slab has a steeper dip angle than the northern part. We believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of NE Japan. Many great (M 8.0-8.5) interplate earthquakes have occurred in northern NE Japan during recorded history (830-1995), and microearthquakes show a wide distribution in the forearc region from the Japan Trench to the Pacific coast, suggesting strong interplate seismic coupling in that region. In contrast, only a few M 7.0-7.5 class earthquakes have occurred in southern NE Japan in the same period, and microearthquakes tend to cluster in many small areas, suggesting weaker interplate coupling there. The subducted slab may be segmented by the oceanic fracture zones, and the degree of seismic coupling changes from segment to segment. The behavior of the segment in southern NE Japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle. This interpretation is supported by geomagnetic, topographic and geological studies which show that many small-sized seamounts were subducted in the past from the southern part of the Japan Trench, but almost no seamounts were subducted beneath the northern part.