The southernmost Mariana margin lacks a mature island arc and thus differs significantly from the central‐north Mariana and Izu‐Bonin margins. This paper presents a new P wave velocity model of the crust and uppermost mantle structure based on a 349‐km‐long profile of wide‐angle reflection/refraction data. The active source seismic experiment was conducted from the subducting Pacific plate to the overriding Philippine plate, passing through the Challenger Deep. The subducting plate has an average crustal thickness of ~6.0 km with Vp of 7.0 ± 0.2 km/s at the base of the crust and low values of only 5.5–6.9 km/s near the trench axis. The uppermost mantle of the subducting plate is characterized by low velocities of 7.0–7.3 km/s. The overriding plate has a maximum crustal thickness of ~18 km beneath the forearc with Vp of ~7.4 km/s at the crustal bottom and 7.5–7.8 km/s in the uppermost mantle. A zone of slight velocity reduction is imaged beneath the Southwest Mariana Rift that is undergoing active rifting. The observed significant velocity reduction in a near‐trench crustal zone of ~20–30 km in the subducting plate is interpreted as a result of faulting‐induced porosity changes and fracture‐filling fluids. The velocity reduction in the uppermost mantle of both subducting and overriding plates is interpreted as mantle serpentinization with fluid sources from dehydration of the subducting plate and/or fluid penetration along faults.

Reduction of seismic velocities has been employed to study the hydration of incoming plates and forearc mantle in recent years. However, few constraints have been obtained in the Southern Mariana Trench. We use an ocean bottom seismograph (OBS) deployment to conduct Rayleigh wave tomographic studies to derive the SV‐wave velocity structure near the Southern Mariana Trench. Measured group and phase velocities as a function of period are inverted to determine the SV‐wave velocity using a Bayesian Monte Carlo algorithm. The incoming Pacific Plate is characterized by low velocities (3.6–4.1 km/s) within the upper ∼25 km of the mantle near the trench, indicating extensive mantle hydration of the incoming plate in southern Mariana. The velocity reduction in the forearc mantle is not as large as in central Mariana, most likely indicating a lower forearc serpentinization in this region, which is consistent with the absence of serpentinite mud volcanoes.

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 (>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.