Teleseismic recordings of direct S and sS body wave phases, and their core‐reflected counterparts ScS and sScS, from intermediate and deep focus earthquakes are used to analyze the lowermost mantle shear velocity structure beneath Alaska. A model with a 2.75% shear velocity increase 280 km above the core‐mantle boundary accurately matches waveform complexities in both the S and sS wavetrains. Variations in source depth produce systematic shifts in the timing of the triplication arrivals between the S and sS travel time branches that are readily observed in long‐period WWSSN tangential component recordings. The systematic range‐ and depth‐dependence of the observed shifts are well‐predicted by the discontinuity model, and preclude explanations of the waveform complexity as resulting from multiple ruptures at the source, receiver reverberations, or near‐source scattering from s