Lateral Variations in Upper Mantle Discontinuities Beneath Northeast China Revealed by Seismic Ambient Noise

Abstract Seismic ambient noise cross‐correlation is applied to retrieve the deep P‐wave reflections from the upper mantle discontinuities for a newly deployed dense seismic array in Northeast China. Our results reveal a ∼15 km depression of the 410‐km discontinuity and a “pit‐like” topography with a localized depression about 28 km on the 660‐km discontinuity, which is confirmed by receiver functions analysis and SdP converted waveforms. We analyze lateral variations in relative amplitude ratios of P660P to P410P, and identify three distinct regions characterized by radically decreased P660P energy. Numerical waveform modeling shows a strong correlation between the intermittently observed P660P and the dipping topography of the 660‐km discontinuity, while the change in seismic velocity gradient of the 660‐km discontinuity may be another possible reason for the observed decrease in amplitude. The refined “hinge” point, where the depth of the 660‐km discontinuity begins to deepen steadily, constrains a broad depression with a lateral extent >400 km, implying a progressively eastward lying of the Pacific slab above the 660‐km discontinuity. The recovered 660‐km discontinuity to the west corresponds to an interrupted high‐velocity anomaly, possibly suggesting a spatial slab break or gap west of the profile.