Investigation of a Lower Mantle Shear Wave Triplication Using a Broadband Array

SH signals recorded by an array of four broadband (Benioff 1-90) instruments, run by Caltech, are inspected for evidence of a lower mantle shear wave velocity discontinuity previously detected using WWSSN data. Deep focus earthquakes beneath Argentina span the distance range 78 ø to 82 ø from the array, where the triplication in the travel time curve produced by the discontinuity is expected to result in an arrival between the direct shear wave, S, and the reflection off of the core-mantle boundary, ScS. The SH data for these events do show evidence of the arrival; however, the broadband signals also show significant receiver structure complexity. Empirical receiver functions are determined for each station by stacking observations for events in South America closer than 75 ø from the array. Incorporating these receiver functions into the synthetic modeling demonstrates that lower mantle shear velocity models with a discontinuity fit the observations better than models without one.