Subduction zones worldwide exhibit remarkable variations in seismic activity and slip behavior along strike and down dip, and many factors have been invoked to explain this variability. The subduction zone off the Alaska Peninsula exhibits pronounced changes in earthquake behavior over relatively small along-strike distances, making it an excellent place to examine controls on seismogenesis. Marine seismic reflection and refraction data acquired across this subduction zone with the R/V Marcus Langseth reveal substantial along-strike changes in incoming sediment thickness, normal faulting at the outer rise and hydration of the plate prior to subduction. These variations in the properties of the subducting plate correlate with changes in plate boundary properties, seismicity and earthquake rupture history. We propose the amount of water and sediment delivered into the subduction zone on the incoming plate are influencing seismic behavior throughout this system.