Seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of OBS array records at 60Ma seafloor

We analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60Ma seafloor in the southeastward of Tahiti island, the South Pacific, by the Tomographic Investigation by seafloor ARray Experiment for the Society hotspot project. We first obtained one-dimensional shear wave velocity model beneath the array from average phase velocities of Rayleigh waves at a broadband period range of 5-200s. The obtained model shows a large velocity reduction at depths between 40 and 80km, where the lithosphere-asthenosphere boundary might exist. We then estimated shear wave azimuthal anisotropy at depths of 20-100km by measuring azimuthal dependence of phase velocities of Rayleigh waves. The obtained model shows peak-to-peak intensity of the azimuthal anisotropy of 2%-4% with the fastest azimuth of NW-SE direction both in the lithosphere and asthenosphere. This result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid-ocean ridge but is roughly parallel to the ancient plate motion at depths of 20-60km. The fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60-100km. Additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190-210km.