Plain Language Summary
We use 398 portable seismometers that were deployed as part of the Alaska Amphibious Community Seismic Experiment to image the boundary between the subducting Pacific plate and the base of the North American plate. This seismic array with ~200 m spacing was deployed on Kodiak Island in 2019 within the southern rupture area of the 1964 Mw9.2 Great Alaska earthquake. We analyze conversions from compressional to shear waves from distant earthquakes to understand the conditions of the plate interface. Our results show a dipping velocity increase at ~30-40 km depth at the expected location of the Pacific slab crust-mantle boundary. In contrast to prior results from the northern 1964 rupture zone, we do not find a low-velocity layer atop the subducting plate. Our results indicate that the 1964 rupture connected segments of the Alaskan subduction zone with different plate interface properties.