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

Abstract

SUMMARY Diffractions of PKKPab (${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$) 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 ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ 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 ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves computed by numerical algorithms based on full-wave theory. We find that the diffraction effects for the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ 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 ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ 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 ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves are sensitive to large-scale P-wave anomalies (with Vp perturbations of ± 2 per cent and thicknesses of more than 100 km) and small-scale ultra-low velocity zones (ULVZs; with Vp reduction of 5 per cent or 10 per cent and thickness of tens of kilometers). However, the slownesses of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}\ $ 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 ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves in studying the structures at the base of the mantle.