Small-scale heterogeneities at the bottom of the lower mantle beneath the northern Bay of Bengal and the northern Gulf of Mexico by the analysis of PKP precursors

The bottom of the lower mantle is a key region for material circulation and energy exchange within the Earth, with extremely high heterogeneity and complex dynamics processes. Although tomography models have revealed the large-scale velocity structure at the bottom of the lower mantle, the nature of the small-scale lateral heterogeneity structure remains controversial due to resolution limitations. The scattering observations of PKP precursors have been widely used to constrain the small-scale structures at the bottom of the lower mantle due to their special sampling paths and arrival time characteristics. This study cross-validates the presence of scatterers at the bottom of the lower mantle in the northern Bay of Bengal and the northern Gulf of Mexico through migration and array analysis of PKP precursors sampled from seismic arrays in the Sichuan-Yunnan and adjacent areas in China. The forward modelling of the envelope of PKP precursors using the Monte Carlo seismic phonon method reveals that their P-wave velocity perturbations are 0.3 and 0.55 per cent in each area, respectively. Based on the distribution range of the small-scale scatterers, we infer that the northern Bay of Bengal scatterer lies within 200 km above the core-mantle boundary, whereas the thickness of the scattering layer in the northern Gulf of Mexico is approximately 250 km. We propose that the small-scale lateral heterogeneities observed in both regions originate from subducted slabs and may have been transitioned into post-perovskite.