Shear velocity structure of the Mariana mantle wedge from Rayleigh wave phase velocities

We examine the seismic structure of the Mariana mantle wedge using data from a combined deployment of ocean bottom seismographs and land stations in 2003-2004. We measure Rayleigh wave phase velocities and invert these results for the shear velocity structure and azimuthal anisotropy in the region. In the back-arc region low phase velocities indicate shear velocities as low as 3.9 ± 0.1 km/s at depths of about 60 km. Locations of the lowest seismic velocities at 18.0°N-18.5°N, 16.0°N-16.5°N, and 14.0°N-14.5°N correspond to the locations of gravity lows and probably indicate the presence of temperature and melt production maxima compared to elsewhere along the spreading center. The shape of the low velocity anomaly also indicates that there is a degree of asymmetry to the thermal structure across the spreading center. The fore arc exhibits low shear velocities (V_s ∼ 3.6 km/s) at depths shallower than 40 km. These low uppermost mantle velocities are likely due to serpentinization of the fore arc. Azimuthal anisotropy results suggest an average peak-to-peak anisotropy of about 1%-2% with a fast direction of NNE. Fore-arc anisotropy shows arc-parallel fast directions at shallow depth in agreement with previous shear wave splitting observations and deeper arc-perpendicular directions influenced by the slab. Back-arc anisotropy is largely arc perpendicular, consistent with splitting observations west of the spreading center.