Upper bounds of seismic anisotropy in the Tonga slab near deep earthquake foci and in the lower mantle

Seismic anisotropy in and around subducting Tonga slab (latitude similar to 20 degrees S) is investigated by using three component broad-band seismograms of deep earthquakes at Tonga (h > 550 km) recorded at the F-net, Japan. In the study area, the slab becomes stagnant when approaching the upper- and lower-mantle boundary, and the mantle transition zone and the shallowest lower mantle have been claimed to be anisotropic both the backarc side and the ocean side.We analyse shear wave splitting of teleseismic direct S waves from the deep earthquakes, and investigate a slightly different part of the Tonga subduction zone from previous studies. We find that anisotropy of an observable degree exists neither in the slab near the bottom of the upper mantle (below 600 km) nor in the lower mantle beneath the foci. The shear wave splitting lag time (delta t) attributable to the anisotropy inside the slab around the foci is less than 0.15 s, and the corresponding maximum degree of anisotropy is 0.9 per cent. The result is consistent with recent mineralogical studies which indicate that ringwoodite does not acquire significant preferred orientation of crystal lattice due to the deformation near the bottom of the upper mantle. The maximum delta t by large-scale anisotropy in the lower mantle traversed by the S waves from Tonga to Japan does not exceed 0.05 s, suggesting the absence of significant shear deformation near the top of the lower mantle.