Inferences on ultralow-velocity zone structure from a global analysis of SPdKS waves

Anomalous boundary layer structure at the core‐mantle boundary (CMB) is investigated using a global set of broadband SKS and SPdKS waves from permanent and portable broadband seismometer arrays. SPdKS is an SKS wave that intersects the CMB at the critical angle for ScP, thus initiating a diffracted P wave (Pdiff) along the CMB at the core entry and exit locations. The waveshape and timing of SPdKS data are analyzed relative to SKS, with some SPdKS data showing significant delays and broadening. Broadband data from several hundred deep focus earthquakes were analyzed; retaining data with simple sources and high signal‐to‐noise ratios resulted in 53 high‐quality earthquakes. For each earthquake an empirical source was constructed by stacking pre‐SPdKS distance range SKS pulses (∼90°–100°). These were utilized in our synthetic modeling process, whereby reflectivity synthetic seismograms are produced for three classes of models: (1) mantle‐side ultralow‐velocity zones (UVLZ), (2) underside CMB core rigidity zones, and (3) core‐mantle transition zones. For ULVZ structures, ratios of P‐to‐S velocity reductions of 1:1 and 1:3 are explored, where 1:3 is appropriate for a partial melt origin of ULVZ. Over 330 unique CMB boundary layer models have been constructed and tested, corroborating previous work suggesting strong trade‐offs between the three model spaces. We produce maps of inferred boundary layer structure from the global data and find evidence for extremely fine‐scale heterogeneity where our wave path sampling is the densest. While uncertainties are present relating to the source versus receiver sides of the SPdKS wave path geometry, our data are consistent with the hypothesis that ULVZ presence (or absence) correlates with reduced (or average) heterogeneity in the overlying mantle.