We analyzed 49,592 teleseismic receiver functions recorded by 278 CEArray stations to image the mantle transition zone (MTZ) beneath the South China Block to understand origins of deep velocity anomalies and their potential links to subduction and intraplate volcanism. We employed a fast-marching method and a high-resolution 3-D velocity model (FWEA18) derived from full waveform inversion in computing P-to-S conversion times to better image the 410-km and 660-km discontinuities. Our results indicate that the common-conversion-point stacking of receiver functions using 3-D conversion times yielded better migration images of the two discontinuities. The images revealed a slightly depressed 410-km with a few small uplifted patches, and showed that the 660-km beneath the western Yangtze Craton is depressed by 10-25 km, which is likely caused by the stagnant Paleo-Pacific slab. The 660-km beneath the southern Cathaysia Block has a 5-15 km high plateau with a topographic low at its central part. The lateral dimension of the topographic low is ~150 km and located beneath the central Pearl River Mount Basin near Hong Kong. We speculate the topographic low occurs within the Hainan plume with a temperature excess of ~300-400 K and is caused by the garnet phase transition. The displaced deep plume enters the MTZ and spreads nearly horizontally at the base. The plume evolves into two channels with a minor one toward the northeast and a major one toward the southwest, which keep moving upward to the 410-km. The southwest channel is likely the source that feeds the Hainan volcanoes.

Body-to-surface wave scattering, originated from strong lateral heterogeneity, has been observed and modeled for decades. Compared to body waves, scattered surface waves propagate along the Earth’s surface with less energy loss and, thus, can be observed over a wider distance range. In this study, we utilize surface waves converted from teleseismic SH or Sdiff wave incidence to map strong lateral heterogeneities across the entire contiguous US. We apply array-based phase coherence analysis to broadband waveforms recorded by the USArray Transportable Array and other permanent/temporary networks to detect coherent signals that are associated with body-to-surface wave scattering. We then locate the source of the scattering by back-propagating the beamformed energy using both straight-ray and curved-ray approximations. Our results show that the distribution of scatterers correlates well with known geological features across the contiguous US. Topographic/bathymetric relief along the continental slope off the Pacific Border is the major source of scattering in the western US. On the other hand, sedimentary basins, especially their margins, are the dominant scatterers in the central US. Moho offsets, such as the one around the periphery of the Colorado Plateau, are also a strong contributor to scattering, but isolating their effect from that of other near-surface structures without any additional constraints can be complicated. Finally, we demonstrate the possibility of using scattered surface waves to constrain subsurface velocity structures, as complementary to conventional earthquake- or ambient-noise-based surface wave tomography.