In many areas the geological interpretation of seismic images can benefit from quantitative, high resolution velocity images. Waveform Tomography can yield the appropriate images provided the input data contain large offsets and low frequencies, such as those available from a suite of Ocean Bottom Seismographs (OBSs). We provide an illustration in the Nankai subduction zone, where a full interpretation requires both the delineation of detailed crustal fault structures and a quantitative velocity map. Previous prestack migrations have revealed key structures, including a megasplay fault, and two distinctive low velocity zones. However the migration sections are negatively affected by the influence of the sea bottom topographic relief. In this study we applied acoustic Waveform Tomography to the OBS data using the traveltime tomography to provide a suitable starting model; synthetic wavefields generated from the final model reproduced wide-angle reflections from the megasplay fault. Our image clearly delineated major geological features including the megasplay fault, thrusts within the accretionary prisms, and an apparent connectivity of two low velocity zones. By comparing the image with a slice from the 3D migration volume, we confirmed the reliability of our image, and the improvement enabled by the use of wide-angle data with Waveform Tomography.