Investigating Ultra-Low Velocity Zones as Sources of PKP Scattering Beneath North America and the Western Pacific Ocean: Potential Links to Subducted Oceanic Crust

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.