Strong Serpentinization and Hydration in the Subducting Plate of the Southern Mariana Trench: Insights From <i>V</i> _p / <i>V</i> _s Ratios

Abstract The southern Mariana subduction zone, home to the Challenger Deep—the deepest known point on Earth—poses significant challenges for studying the hydration of the subducting plate due to its extreme depth. This study uses S‐wave seismic tomography and V p / V s ratios to investigate hydration and serpentinization at the Challenger Deep. We observe a low V p and V s layer in the upper mantle with V p / V s ratios exceeding 1.8, reaching up to 1.95 at the Moho. These high ratios indicate a strong serpentinized layer (&gt;15 vol%) with significant changes in the mechanical properties of the serpentinized peridotite. Additionally, V p / V s ratios in the crust and uppermost mantle increase from the outer rise to the trench axis, demonstrating that bending‐related faulting and hydration intensify as the plate approaches the trench. Our results suggest extensive faulting, hydration, and mantle serpentinization at the Challenger Deep, making this region an extreme example of water cycling in subduction zones.