We conducted broadband dispersion survey by deploying two arrays of broadband ocean bottom seismometers in the northwestern Pacific Ocean at seafloor ages of 130 and 140 Ma. By combining ambient noise and teleseismic surface wave analyses, dispersion curves of Rayleigh waves were obtained at a period range of 5–100 s and then used to invert for one‐dimensional isotropic and azimuthally anisotropic βV (VSV) profiles beneath each array. The obtained profiles show ~2% difference in isotropic βV in the low‐velocity zone (LVZ) at a depth range of 80–150 km in spite of the small difference in seafloor ages and the horizontal distance of ~1,000 km. Forward dispersion‐curve calculation for thermal models indicates that simple cooling models cannot explain the observed difference and an additional mechanism, such as sublithospheric small‐scale convection, is required. In addition, the fastest azimuths of azimuthal anisotropy in the LVZ significantly deviate from the current plate motion direction. We infer that these observations are consistent with the presence of small‐scale convection beneath the study area. As for azimuthal anisotropy in the Lid, the peak‐to‐peak intensity is 3–4% at the depth from Moho to ~40 km. The fastest direction is almost perpendicular to magnetic lineation in area A at 130 Ma and oblique to magnetic lineations in area B at 140 Ma, suggesting complex mantle flow beneath the infant Pacific Plate surrounded by three ridge axes. The intensity of azimuthal anisotropy in the LVZ is ~2%, indicating that radial anisotropy is stronger than azimuthal anisotropy therein.

We have analyzed broadband surface wave data from ocean bottom seismometers deployed in the Shikoku Basin in the northeastern Philippine Sea to determine the radially anisotropic uppermost mantle structure beneath this oceanic basin. We first applied noise correlation method to continuous microseismic records to obtain phase velocities for fundamental‐mode and first higher‐mode Rayleigh waves and fundamental‐mode Love waves at periods of 7–29 s. At longer periods, we applied an array analysis method to teleseismic surface waves to obtain phase velocities of fundamental‐mode Rayleigh and Love waves at periods of 29–117 s. Using these broadband phase velocity measurements, we have determined the one‐dimensional radially anisotropic structure from the crust to the low velocity zone (LVZ) beneath the Shikoku Basin without assuming a priori structure in the uppermost mantle. The final structural model (SB‐RA10) has a high‐velocity lid from the Moho to a depth of ∼40 km, with an LVZ at greater depths. S wave velocities decrease by 6%–10% at a depth range of ∼40–70 km. This large decrease in velocity suggests that there is either a large difference in grain size between these layers or indicates the presence of partial melt or water in the LVZ. Furthermore, strong radial anisotropy of 4%–5% (VSH>VSV) is observed in the uppermost mantle, which may be stronger in the LVZ.

We 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.