Surface wave tomography for the Pacific Ocean incorporating seafloor seismic observations and its age-dependence

Using broadband seismic waveforms recorded on the seafloor by more than 200 broadband ocean bottom seismometers, as well as those on land, we measure phase speed dispersion of Love and Rayleigh waves up to the 4th higher mode to determine the three-dimensional radially anisotropic shear-wave speed structure in the upper mantle beneath the Pacific Ocean. The fastest anomalies at depths shallower than 100 km are located beneath southeast of the Shatsky Rise and strong radial anisotropy is located in the central Pacific at depths of 100 -200 km. Isotropic shear wave speed structures show age dependence. From age-bin-averaged shear wave speed profiles and a half-space cooling model, we estimate a thermo-speed relationship for the Pacific Plate to construct a reference age-dependent shear-wave speed model, which is further used as an initial model for tomography iteration. Deviation maps in the Pacific Ocean from the reference model indicate that large negative residuals, which may be due to partial melt, anelasticity, and/or added heat from mantle plumes, are located along the ridges and beneath hotspots, and that large positive residuals are found beneath the northwestern Pacific Ocean. The use of an age-dependent reference model, as well as the incorporation of OBS data, greatly improves the accuracy of local phase speed estimates in tomography, as evident from a direct comparison with in situarray measurements.