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Small‐scale scatterers in the lower mantle contain key information on the recycling process in the whole mantle. Resolving the fine structures of these scatterers is crucial for understanding the dynamic evolution in the deep Earth. Here, we search for scatterers in the lower mantle beneath Central America and the northwestern Pacific Ocean by events in South America and PKP recordings in Japan. In this work, we perform migration of PKP precursors on the source and receiver sides by raytracing to obtain the distribution map of scatterers, after considering 3D heterogeneous structures. Furthermore, we locate a distinct scatterer at the core‐mantle boundary (CMB) beneath Central America that generates strong PKP precursors with unique slowness and back‐azimuth by array processing, which is compatible with the migration result. Based on waveform modeling, this scatterer is essentially a localized ultra‐low velocity zone (ULVZ) with approximately P‐wave velocity reduction of 6% and a lateral extent of 3°. Heated up of the subducted lithosphere by the core or partial melting of the subducted oceanic crust are plausible sources of the localized ULVZ at the CMB we detect here in the subduction region. Plain Language Summary: Small‐scale scatterers in the deep Earth contain key information on the recycling process of materials, so it is necessary to obtain the fine structure of these scatterers to understand the evolution of our Earth. A scattered wave type, PKP precursors, is often used to detect scatterers in the lowermost mantle. This study maps the scatterer distributions in the lower mantle beneath Central America and the northwestern Pacific Ocean from dense recordings in Japan. Moreover, we find a localized scatterer at the base of the mantle which generates strong PKP precursors recorded by a dense array. We determine the seismic velocity and geometry of this localized scatterer by modeling the seismic recordings. Several mechanisms related to the subducted materials can explain the localized scatterer with large seismic velocity reductions imaged in this work. Key Points: Migration and array processing of PKP precursors suggest an intense scattering region at the core‐mantle boundary beneath Central AmericaThe localized scatterer is essentially a ULVZ with approximately P‐wave velocity reduction of 6% and a lateral extent of 3°The subducted materials are the plausible sources of the imaged ULVZ [ABSTRACT FROM AUTHOR]

Abstract Seismic energy arriving before the compressional (P) wave passing through the core (PKP), called PKP precursors, have been detected for decades, but the origin of those arrivals is ambiguous. The largest amplitude arrivals are linked to scattering at small‐scale lowermost mantle structure, but because these arrivals traverse both source and receiver sides of the mantle, it is unknown which side of the path the energy is scattered from. To address this ambiguity, we apply a new seismic array method to analyze PKP waveforms from 58 earthquakes recorded in North America that allows localization of the origin of the PKP precursors at the core‐mantle boundary (CMB). We compare these measurements with high frequency 2.5‐D synthetic predictions showing that the PKP precursors are most likely associated with ultra‐low velocity zone structures beneath the western Pacific and North America. The most feasible scenario to generate ULVZs in both locations is through melting of mid‐ocean ridge basalt in subducted oceanic crust.

Abstract: Well separated individual PKP precursors observed at the Yellowknife seismic array (YK) for a high quality doublet of earthquakes provide an opportunity to study the location of the corresponding scatterers and assess the stability of the location estimation. Based on the comparison of the waveforms of stacked individual precursors and those of PKIKP phases, we are able to determine that most of these precursors originate from scattering of the PKPbc (rather than the PKPab) branch above the B caustic on the receiver side. This allows a reliable location of the scatterers in the lower mantle. Their depths range from 2890 km (the CMB) to 2270 km, scattering angles range from 45.8° to 16.0°, and surface projections range from southern Ontario to northern Saskatchewan in Canada. These locations are associated with transitions from slow to fast velocities in mantle tomographic models and follow the expected general dip direction of fossil slabs under north America. This suggests that the subducted slab remnants under north America have retained their compositional signature. The fact that we can essentially treat these scatterers as reflections from plane boundaries suggests that the remnant fragments of slab may be spatially extended, which should be confirmed using broadband data. Average differences in measured slowness and back-azimuth of the doublet precursors are as small as 0.08 s/deg and 1.4 deg, respectively. Our study indicates that it may be possible to locate such scatterers using single earthquakes and small aperture seismic arrays. [Copyright &y& Elsevier]