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

Abstract

AbstractWe analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 Ma 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 5–200 s. The obtained model shows a large velocity reduction at depths between 40 and 80 km, where the lithosphere‐asthenosphere boundary might exist. We then estimated shear wave azimuthal anisotropy at depths of 20–100 km 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 20–60 km. The fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60–100 km. 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 190–210 km.