Constraining the 410-km Discontinuity with Triplication Waveform

The detailed structure near the 410-km discontinuity provides key constraints of the dynamic interactions between the upper mantle and the lower mantle through the mantle transition zone (MTZ) via mass and heat exchange. Meanwhile, the temperature of the subducting slab, which can be derived from its fast wave speed perturbation, is critical for understanding the mantle dynamics in subduction zones where the slab enters the MTZ. Multipathing, i.e., triplicated, body waves that bottom near the MTZ carry rich information of the 410-km discontinuity structure and can be used to constrain the discontinuity depth and radial variations of wave speeds across it. In this study, we systematically analyze the tradeoff between model parameters in triplication studies using synthetic examples. Specifically, we illustrate the necessity of using array normalized amplitude. Two 1-D depth profiles of the wave speed below the Tatar Strait of Russia in the Kuril subduction zone are obtained with our inversion approach applied to the dense broadband station waveforms recorded in China. We observe triplications due to both the 410-km discontinuity and the slab upper surface. Therefore, seismic structures for these two interfaces are simultaneously inverted. Our derived 410-km discontinuity depths are at 425±15km and 420±15km, with no observable uplift. The slab upper surface is inverted to be located about 50-70 km below the 410-km discontinuity, between the depths of the 1%-2% P-wave speed contours of a regional 3-D FWI model, but we find twice the wave speed perturbation amplitude. A wave speed increase of 3.9%-4.6% within the slab, compared to 2.0%-2.4% from the 3-D FWI model, is critically necessary to fit the waveforms with the shortest period of 2 seconds, indicating that high-frequency waves are required to accurately resolve the detailed structures near the MTZ.