Shear-wave splitting in the upper-mantle wedge above the Tonga subduction zone

Intermediate-period seismograms of shear waves from deep earthquakes are examined for shear-wave splitting diagnostic of seismic velocity anisotropy in the upper-mantle wedge between the subducting Tonga slab and stations in the Fiji Islands. Earthquakes with near-vertical incidence angles are selected to minimize complications of converted phases and the free surface. Horizontal-component seismograms for 25 deep earthquakes recorded at station LAK on the Lau Ridge show splitting of up to 0.9 s with an average of 0.45 ± 0.25 s between fast and slow S-wave components. Anisotropy in the upper mantle is the most likely cause of the splitting. The ESE orientation of the fast velocity direction at station LAK corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the Lau Basin and the northeastern portion of the South Fiji Basin. The observations are consistent with a model of flow alignment of olivine (100) and orthopyroxene (001) axes parallel to the direction of basin extension. Seismograms for eight deep earthquakes at station SVA on Viti Levu show up to 1.0 s splitting with an average of 0.65 ± 0.29 s and a weak NNE preferred orientation of fast velocity. Polarizations of shear-wave particle motions corrected for anisotropy are within 36° ± 26° of polarizations predicted for nine moment tensor solutions. The magnitude of shear-wave splitting and estimates of the thickness of the lithosphere in the Fiji region imply that the anisotropy cannot be contained in the lithosphere alone. Either the anisotropy is distributed in both the lithosphere and asthenosphere, or the asthenosphere alone is anisotropic.