Large offset multichannel seismic reflection data from the subduction zone margin off Vancouver Island define the geometry, internal structure, and deformation style of the Cascadia accretionary prism. A very detailed seismic velocity analysis has been performed to quantitatively assess the velocity structure and thus to infer porosity variations and pore fluid expulsion across the deformation front. Within the sediments of Cascadia Basin approaching the deformation front, and within the frontal thrust zone of the accretionary prism, velocities increase landward probably as a result of sediment consolidation associated with horizontal compression. A remarkable conclusion is that more than one third of the initial pore fluid content of the incoming sediments is lost by the time they are incorporated into the wedge. In contrast, a pronounced velocity decrease is evident in the lower slope region 15--20 km landward of the deformation front where the wedge sediment thickness is more than doubled compared with the section in Cascadia Basin. The low velocity is explained by underconsolidation resulting from rapid horizontal shortening and vertical thickening of the sediment column, accommodated by displacements along thrust faults or by distributed deformation. As inferred from the velocity data, sediment porosities are high in this lower to midslope region of the wedge. The thickening of the wedge results in sediment elements being moved to greater depths landward with little porosity loss and velocity change. In the rapidly thickening section the overburden load stress increases faster than the sediments can respond by fluid expulsion and overpressures likely result.