Searching for seismic scatterers at the top of the lower mantle in northeast China using P-wave receiver functions

The lower mantle is the layer with the largest volume and mass proportion among Earth′s internal layers. Studies on the detection, distribution patterns, and heterogeneity of lower mantle heterogeneities will help reveal the mantle composition, mineral phase transitions, and thermochemical structure of Earth′s interior, thereby deepening our understanding of geodynamic processes such as mantle rheology and mantle convection. Northeast China is located at the subduction front of the northwestern Pacific Plate and has undergone multiple large-scale tectonic events since the Mesozoic, including the subduction of the Paleo-Pacific Plate (Izanagi Plate) and the Pacific Plate. The studies of deep structures beneath Northeast China indicate that subducted slabs have stagnated in the mantle transition zone or penetrated into the lower mantle. Due to method limitations, the detection of small-scale lower mantle scatterers has mostly been confined to regions with intermediate- and deep-focus earthquakes in subduction zones. In this study, we extend the receiver function analysis to detect the lower mantle scatterers, and successfully find the scatterers beneath the continental region of Northeast China. Based on dense broadband seismic arrays deployed on Northeast China, we extract P-wave receiver functions and cross-validated the reliability of lower mantle scatterer′ signals previously detected in intermediate- and deep-focus earthquake zones through slowness analysis, 3D velocity correction, and frequency-dependent analysis. Additionally, we newly identified multiple lower mantle scatterers at depths of 900 ~ 1000 km beneath the intracontinental region of Northeast China. Combined tomography models, we interpret these scatterers as remnants of the Izanagi Plate subduction, suggesting that the influence of the Izanagi Plate subduction extends at least 1000 km west and north of the Sea of Japan. Our study extends the traditional receiver function analysis to the deeper lower mantle, confirming its effectiveness in detecting lower mantle scatterers. It provides novel, robust, and independent seismic evidence for observations of scatterers in the uppermost lower mantle. Furthermore, it proposes a new approach for tracking remnants of ancient subducted slabs in the deep mantle, offering fresh insights into the dynamics of plate subduction processes.