Constraints on Seismic Anisotropy in the Mantle Transition Zone from Long-Period SS Precursors

Abstract

The mantle transition zone (MTZ) of Earth is demarcated by solid-to-solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660 km depths. Mineral physics experiments predict that wadsleyite can have strong single-crystal anisotropy at the pressure and temperature conditions of the MTZ. Thus, significant seismic anisotropy is possible in the upper MTZ where lattice preferred orientation (LPO) of wadsleyite is produced by mantle flow. Here, we use a body wave method, SS precursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. We stack the data to explore the azimuthal dependence of travel times and amplitudes of SS precursors, and constrain the azimuthal anisotropy in the MTZ. Beneath the central Pacific, we find evidence for ~4% anisotropy with a SE fast direction in the upper mantle, and no significant anisotropy in the MTZ. In subduction zones, we observe ~4% anisotropy with a trench-parallel fast direction in the upper mantle, and ~3% anisotropy with a trench-perpendicular fast direction in the MTZ. The transition of fast directions indicates that the LPO of wadsleyite induced by MTZ flow is organized separately from the flow in the upper mantle. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the upper mantle, but negligible anisotropy (< 1%) in the MTZ. Finally, we correct for the upper mantle and MTZ anisotropy structures to obtain a new MTZ topography model. The anisotropy correction produces ±3 km difference, and therefore has minor overall effects on global MTZ topography.