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.