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 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 lattice-preferred orientation 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. Show more