The largest earthquakes and the majority of the seismic energy are released on megathrust faults in subduction zones. The goal of this dissertation is to characterize the seismic behavior, structural and physical properties of the megathrust fault, so that we can better understand the controls on slip behavior and large earthquakes. To address this goal, I analyzed seismicity data collected by a local seismic network deployed in southern Alaska and multi-channel seismic (MCS) data from an active-source survey offshore of the Alaska Peninsula. This dissertation work revealed seismicity patterns associated with a large asperity, downdip transitions in megathrust fault structure, and along-strike variations in the properties of subducting sediment on the shallow part of the subduction zone. All of these observations have important implications for seismic behavior of the megathrust.