Upper Mantle Hydration Indicated By Decreased Shear Velocity Near The Southern Mariana Trench From Rayleigh Wave Tomography

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 similar to 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.Plain Language Summary Estimating the amount of water carried by the subduction zone into Earth's interior plays an important role in the global water cycle, but is poorly constrained at subduction zones due to the lack of near-field observations. We utilize data recorded by OBSs near the Southern Mariana Trench. Using seismic imaging methods, we construct an SV-wave velocity model beneath the southernmost Mariana subduction zone. Our model demonstrates low-velocity anomalies within the upper similar to 25 km mantle near the trench, suggesting a large amount of water input in southern Mariana. Furthermore, our seismic observations suggest that low degree serpentinization of the forearc mantle in southern Mariana than central Mariana, agree with the lacking of serpentinite mud volcanoes in the forearc.