Three-dimensional seismic refraction data reveal the structure of the plate boundaries at the Mendocino triple junction. First arrival traveltimes were inverted to obtain a three-dimensional seismic velocity model. The tomography results indicate a strong lateral velocity contrast in the lower crust. This velocity contrast is beneath the surface trace of the San Andreas fault and tracks along the fault to the triple junction and then along the Mendocino fracture zone. Velocity values appropriate for mafic rocks are more than 5 km shallower on the Pacific plate than east of the San Andreas fault, and 1–3 km shallower than under the Cascadia accretionary prism. The model suggests that the San Andreas fault, which is growing as the triple junction migrates northward, cuts steeply through the entire crust. Pacific oceanic crust does not extend beneath North America at this location. The slabless window south of the triple junction and east of the San Andreas fault must be filled with upwelling asthenosphere. The sharp corner of the Pacific plate migrates northward, displacing the Cascadia accretionary prism and forcing the prism material to move north or east.