On the frequency dependence and spatial coherence of PKP precursor amplitudes

Abstract

AbstractStudies 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.