Body wave arrivals at SOSUS hydrophone arrays are used to determine the seismic velocity structure of the Juan de Fuca ridge and Cascadia subduction zone regions. Since 1991 NOAA has been recording data from the Navy's SOSUS hydrophone arrays. This has provided a unique set of continuously recorded hydrophone data from oceanic stations. Body wave or P-phase arrivals from large earthquakes are detected by the SOSUS hydrophones. Using body wave arrivals Pn velocities of 8.0 km/s are measured for paths purely beneath the oceanic plates and for paths originating beneath the North American continental plate the Pn velocity is 7.7 km/s. For paths along the subduction zone velocities of 7.6--7.7 km/s are measured. The data from the SOSUS arrays are combined with data from the Pacific Northwest Seismic Network (PNSN), a network of seismic stations in Washington, Oregon and northern California, to construct a tomographic image of the crust and uppermost mantle for the Cascadia subduction zone. The model extends from the oceanic plate (131(degrees)W) to the North American continental plate (120(degrees)W) and from 40(degrees)N to 45(degrees)N. The tomographic inversion is a joint inversion for seismic structure and epicenter location using both the body wave and T-phase data. The model images the transition from oceanic to continental plate and the initiation of subduction of the Gorda plate. The discrepancy between event epicenters located using T-phase arrivals alone with the same epicenters located with P- and T-phase arrivals are analyzed to learn more about the process of T-phase generation.