We use double-difference tomography and waveform-derived cross-correlation differential times to estimate earthquake locations and P and S wave velocity structure in the subduction zone under Ibaraki and neighboring prefectures of north central Honshu. We find evidence in both earthquake hypocenters and the velocity structure that the Philippine Sea plate, or perhaps a microplate fragment, may be caught between the subducting Pacific plate and overriding Okhotsk plate in this region. In the southern part of the study area, we find a zone of interplate events extending as deep as 60 km, forming a distinct lineation in cross section. Focal mechanisms support the interpretation that these are low-angle, subduction interface events. We infer that these events probably occur on the interface between the Pacific and Philippine Sea plates rather than between the Pacific and Okhotsk plates. In the upper part of the downgoing Pacific plate, we find a zone of high Vp/Vs ratio (~1.95), which may be explained by high pore fluid pressures within the subducting crust. At a depth of 60--80 km, this region of high Vp/Vs appears to diffuse into the overlying mantle wedge, possibly indicating the upward release of fluids from the slab.