Seismic evidence linking dehydration to the low-velocity layer above the 410 km discontinuity

The low-velocity layer above the 410 km discontinuity is widely attributed to dehydration-induced melting in the upper mantle; however, direct seismic evidence for this linkage remains scarce. Situated at the leading edge of the Northwest Pacific subduction zone, northeast China provides an exceptional natural laboratory owing to the deep subduction and stagnation of the Pacific slab. Using teleseismic waveform data from 327 broadband stations, we extracted 40,236 high-quality P-wave receiver functions, applied common conversion point (CCP) stacking along the 42°N profile (115°E–130°E) to image mantle discontinuities, and probed the origin of the low-velocity layer. Our results revealed pronounced lateral heterogeneity: regions with a well-developed low-velocity layer exhibited sharp and strong P410s phases, whereas adjacent regions lacking such a layer showed markedly weakened or absent signals. Forward modeling demonstrates that the transitional thickness of the 410 km discontinuity, which is closely governed by water content, exerts first-order control on P410s amplitudes, with broader discontinuities progressively diminishing converted-wave energy and weak-P410s regions yielding an average P410s/P660s amplitude ratio of 0.46 ± 0.07. Modeling based on the IASP91 velocity structure suggests an ~11 ± 3 km gradient layer at the 410 km discontinuity. These spatial correlations provide direct seismic evidence that dehydration at the 410 km discontinuity sharpens the phase boundary and enhances the converted phases, whereas persistent hydration broadens the transition zone and weakens the P410s signals. This study offers new constraints on deep-water circulation, mantle material transport, and the role of Pacific slab subduction beneath the East Asian continental margin.