Seismic reflection profiles of many accretionary prisms, including the Oregon prism, exhibit high-amplitude, reversed-polarity reflections from the decollement and other thrusts. It has been suggested that these reflectors image fault zones with enhanced fluid content due to dilation by very high fluid pressure. We present measurements of velocity as a function of effective stress on samples of the Oregon frontal thrust that show that velocity decreases by up to 15% as fluid pressure rises from hydrostatic to lithostatic conditions, under constant confining stress, in fault zone and wall rock alike. Synthetic seismic modeling shows that the frontal-thrust reflector at this location is the result of a thin low-velocity zone in the plane of the fault, 100-300 m/s slower than the walls. Combining models with experimental results, we conclude that fluid pressure of 86% to 98% of lithostatic stress reduces velocity enough to generate the reflections.