Shear Velocity Structure of the Incoming Plate Near the Mariana Trench Constrained by Short-period Love waves and First Overtone Rayleigh waves

The Mariana subduction zone is a water-rich system with some of the oldest oceanic incoming plate and several large forearc serpentinite seamounts. To evaluate the amount of water input into the Mariana trench, Cai et al. (2018) investigated the crustal and uppermost mantle seismic structure across the central Mariana Trench using 20 broadband ocean bottom seismographs (OBSs). Analysis of fundamental mode Rayleigh waves from both ambient noise and earthquakes showed slow velocities in the incoming plate extending to 24 km below the Moho, indicating extensive serpentinization of the incoming plate, and providing evidence that much more bound water subducts than previously estimated. However, fundamental mode Rayleigh waves have limited resolution for the structure and anisotropy of the oceanic crust and uppermost mantle, leaving considerable uncertainty in the estimates. In this study, we include the fundamental mode Love waves and first overtone Rayleigh waves to reduce the uncertainty in constraining the shallow shear velocity structure. Unlike fundamental mode Rayleigh waves that are highly affected by the ocean depths, the Love and first overtone Rayleigh waves at short periods can better constrain the shallow incoming plate structure, including the crust and uppermost mantle. Three-component ambient noise cross-correlations show that distinct fundamental-mode Love and higher-mode Rayleigh waves are observed along most station pairs on the incoming plate between about 5-10 s period. Station pairs very near the trench and those that cross the trench generally have lower phase and group velocities than those at greater distances on the seaward incoming Pacific plate, which is consistent with crustal alteration and mantle serpentinization resulting from water circulation along plate-bending faults. The short-period dispersion of the Love and the first overtone Rayleigh waves, and the fundamental Rayleigh wave dispersion used by Cai et al. (2018) will be jointly inverted using a Bayesian Monte Carlo algorithm to provide a better-constrained incoming plate structure of the Mariana subduction zone.