Abstract The Mariana subduction zone, with its simple oceanic structure and water‐rich environment, provides a natural laboratory for studying mantle hydration and the Lithosphere–Asthenosphere Boundary (LAB). Here, we analyze Ocean Bottom Seismometer (OBS) deployed across both the forearc and incoming plate regions to investigate the S‐wave velocity structure beneath the central Mariana subduction zone. Using multi‐frequency receiver functions and surface‐wave dispersion data, we apply a transdimensional Bayesian joint inversion that accounts for water‐layer effects to obtain high‐resolution images of the subsurface structure. Our results confirm significant mantle hydration and reveal a distinct low‐velocity zone at the LAB. Unlike previous studies that depict the LAB as a single boundary, our results indicate a rapid velocity decrease followed by an equally sharp increase, delineating a ∼15 km thick melt‐rich zone. The existence of such a melt‐rich zone may reduce mantle viscosity and facilitate decoupling between the lithosphere and asthenosphere.

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