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Topography of the 410 km and 660 km discontinuities beneath the Japan Sea and adjacent regions by analysis of multiple‐ScS waves

The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Due to the sparse distribution of seismic stations in the sea, previous studies mostly focus on mantle transition zone (MTZ) structures beneath continents or island arcs, leaving the vast area of the Japan Sea and Okhotsk Sea untouched. In this study, we analyzed multiple‐ScS reverberation waves, and a common‐reflection‐point stacking technique was applied to enhance consistent signals beneath reflection points. A topographic image of the 410 km and 660 km discontinuities is obtained beneath the Japan Sea and adjacent regions. One‐dimensional and 3‐D velocity models are adapted to obtain the “apparent” and “true” depth. We observe a systematic pattern of depression (~10–20 km) and elevation (~5–10 km) of the 660, with the topography being roughly consistent with the shift of the olivine‐phase transition boundary caused by the subducting Pacific plate. The behavior of the 410 is more complex. It is generally ~5–15 km shallower at the location where the slab penetrates and deepened by ~5–10 km oceanward of the slab where a low‐velocity anomaly is observed in tomography images. Moreover, we observe a wide distribution of depressed 410 beneath the southern Okhotsk Sea and western Japan Sea. The hydrous wadsleyite boundary caused by the high water content at the top of the MTZ could explain the depression. The long‐history trench rollback motion of Pacific slab might be responsible for the widely distributed depression of the 410 ranging upward and landward from the slab.