Nicholas Mancinelli

On the frequency dependence and spatial coherence of PKP precursor amplitudes

Studies now agree that small-scale (∼10 km) weak (∼0.1%) velocity perturbations throughout the lowermost mantle generate the globally averaged amplitudes of 1 Hz precursors to the core phase, PKP. The possible frequency dependence and spatial coherence of this scattered phase, however, has been given less attention. Using a large global data set of ∼150,000 PKP precursor recordings, we characterize the frequency dependence of PKP precursors at central frequencies ranging from 0.5 to 4 Hz. At greater frequencies, we observe more scattered energy (relative to the reference phase PKPdf ), particularly at shorter ranges. We model this observation by invoking heterogeneity at length scales from 2 to 30 km. Amplitudes at 0.5 Hz, in particular, suggest the presence of more heterogeneity at scales >8 km than present in previously published models. Using a regional bootstrap approach, we identify large (>20∘), spatially coherent regions of anomalously strong scattering beneath the West Pacific, Central/North America, and—to a lesser extent—East Africa. Finally, as proof of concept, we use array processing techniques to locate the origin of scattered energy observed in Southern California by the Anza and Southern California Seismic Networks. The energy appears to come primarily from out-of-plane scattering on the receiver side. We suggest that such improvised arrays can increase global coverage and may reveal whether a majority of precursor energy comes from localized heterogeneity in the lowermost mantle.

Scattered energy from a rough core–mantle boundary modeled by a Monte Carlo seismic particle method: Application to PKKP precursors

We stack a large global data set of 1 Hz PKKP waveforms to constrain globally averaged properties of PKKP precursors. We find that the precursor observations are better explained by scattering from core-mantle boundary (CMB) topography than by scattering from the near surface, lower mantle, outer core, or inner core. However, as previously noted, simple models of CMB topography and standard 1-D seismic velocity models fail to model the range dependence of the relative amplitude between PKKPbc and its precursors. We find that this systematic mismatch is due, at least in part, to the assumed velocity gradient in the lowermost 250 km of the outer core. Our globally averaged PKKP precursor observations are consistent with random CMB topography with RMS variations of ∼390 m and a horizontal correlation length of ∼7 km.