Sensitivities of diffracted PKKPab waves to the velocity structures in the lowermost mantle

Yulin Chen, Sidao Ni, Baolong Zhang, Li Zhao, Xiaodong Song, Yi Wang

Research output: Contribution to journalArticlepeer-review


Diffractions of PKKPab (PKKPabdiff) along the core mantle boundary (CMB) have been observed well beyond its cutoff distance in recent studies, making it useful for improving the spatial sampling to constrain the lowermost mantle velocity structures. However, the diffractions of PKKPab waves may occur at one of the three CMB interaction points (core entry, underside reflection and exit), giving rise to uncertainties of the sampling region. Moreover, the sensitivity kernels of the non-geometrical PKKPabdiff in the lowermost mantle are difficult to obtain with classical ray theory and can be expected to be more complicated than the typical banana-doughnut shape for direct arrivals. In this study, we address these two issues by analysing the finite-frequency delay-time sensitivity kernels of the PKKPabdiff waves computed by numerical algorithms based on full-wave theory. We find that the diffraction effects for the PKKPabdiff waves are most significant near their core entry and exit regions. For a dominant period of 1 s, the estimated widths of the first Fresnel zones on the mantle side of these two areas are about 60 km. To further investigate the sensitivities of PKKPabdiff to different structures in the lowermost mantle, we conduct a series of 1D and 2D high-frequency (∼1 Hz) modelling experiments. Our results show that the travel times and amplitudes of the PKKPabdiff waves are sensitive to large-scale P-wave anomalies (with Vp perturbations of ± 2 percent and thicknesses of more than 100 km) and small-scale ultra-low velocity zones (ULVZs; with Vp reduction of 5 percent or 10 percent and thickness of tens of kilometers). However, the slownesses of the PKKPabdiff waves remain nearly unchanged in the perturbed models. We explain this unexpected result by the differential delay-time sensitivity kernels for stations at similar epicentral distances. Our results demonstrate both the advantages and limitations of the PKKPabdiff waves in studying the structures at the base of the mantle.

Original languageEnglish (US)
Pages (from-to)1617-1631
Number of pages15
JournalGeophysical Journal International
Issue number3
StatePublished - Jun 1 2023
Externally publishedYes


  • Body waves
  • Composition and structure of the mantle
  • Numerical modelling
  • Wave propagation

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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