Either thrusting of higher over lower impedance sediment or reduction of impedance locally in the fault zone can produce negative-polarity reflections along the protothrusts and the frontal thrust of the Oregon accretionary prism. The vertical displacement of protothrusts averages only 27 m, is uncorrelated to the occurrence of polarity reversals, and is insufficient to generate the impedance contrasts associated with the negative-polarity reflections. The negative-polarity reflections from the protothrusts are probably due to localized impedance reduction in the fault zone. Waveform modeling of these negative-polarity reflections indicates they develop due to a single interface with negative impedance contrast. This seismic signal could be produced by a fault zone with a sharp upper boundary with low impedance fault material and a transition down section to sediment of higher impedance. An impedance inversion due to the 1-km throw on the frontal thrust could create the observed negative-polarity reflections. Interval velocities do not decrease across the frontal thrust, but core-scale velocity measurements indicate a velocity decrease in the frontal thrust. Therefore the negative-polarity reflections are apparently produced by a localized decrease in impedance. Waveform modeling of frontal thrust reflections suggests the low impedance interval is about 20 m thick with discrete upper and low boundaries. Moderately dipping faults are an efficient path for fluid expulsion across the turbidites with low equivalent vertical permeability. Fluid migration from the over-pressured section up the faults can locally reduce the velocity sufficiently to cause the negative-polarity reflections. ¿ American Geophysical Union 1995