EVIDENCE OF A LOWER MANTLE SHEAR VELOCITY DISCONTINUITY IN S AND sS PHASES

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 rangeand 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 everberations, or near-source scattering from slab structure.