A global study of the lowermost mantle using short and long period scattered PKKP waves (PK●KP)

The short-period (1 Hz) seismic wavefield shows strong evidence for scattered energy from the interior of the Earth. This energy mainly arrives in the coda following major seismic phases; however, several ray configurations exist in which seismic energy from the lowermost mantle arrives as precursors to main phases, allowing analysis of heterogeneities in the deep Earth, undisturbed by crustal interference. Here we use the phase PKKP to infer small-scale structure at the Core-Mantle Boundary (CMB) and in the D″ layer. PKKP back-scattered at, or above, the CMB (PK●KP where the ● represents the location of scattering) is observed in a time window starting about 1720 s after origin (for a surface focus) and can be observed from 0° to greater than 60° epicentral distance. This time and distance window is free from other seismic arrivals thus allowing identification of the scattered PKKP energy, despite its relatively low amplitude. The ray path of PK●KP is complicated with scattering occurring off great-circle path, thus avoiding the attenuating inner core. Due to its raypath, PK●KP waves sample regions of the Core-Mantle Boundary inaccessible to most other scattering probes. Back-scattering in this frequency range is controlled by the acoustic impedance of the scattering heterogeneity. This method, therefore, could resolve density contrasts in the deep Earth. When combined with forward scattering probes, such as PKP, which are sensitive to elastic moduli, all the material properties of the scattering heterogeneity could be resolved. Here we use the dense, small to medium aperture arrays of the International Monitoring System of the CTBTO to extract the small amplitude PK●KP from seismic noise. Directivity information from the arrays and ray tracing allows us to infer the location of heterogeneity in the deep Earth. We use the frequency-wavenumber (fk) analysis in conjunction with the F-statistic coherency measure, commonly used in forensic seismology, to greatly increase the slowness vector resolution of the small aperture arrays of the International Monitoring System. The dataset comprises 653 earthquakes, with magnitudes larger than 6.0, resulting in 2094 source-receiver pairs. This allows unprecedented coverage of the CMB, particularly of the area beneath the Atlantic ocean and surrounding continents. We find strong lateral variation in scattering height and amplitude, indicating varying distribution of heterogeneities in the lowermost mantle. The location of these anomalies, both in terms of lateral distribution and height, is related to larger scale mantle structure and flow as shown by comparisons with tomographic and dynamical models. Short period observations of PK●KP energy at 1-2 Hz indicate that the scatterers are discrete heterogeneities with a scale length of ∼10 km. Using single broadband stations from the Global Seismic Network we probe the nature of the PK●KP wavefield. By analysing PK●KP at a range of frequencies we resolve the dominant size of the heterogeneity in the lowermost mantle. We aim to test the hypothesis that small-scale heterogeneities are derived from larger scale debris from the convection process.