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 themineral olivine that produce seismic discontinuities at 410 and 660‐km depths. Mineral physics experimentspredict that wadsleyite can have strong single‐crystal anisotropy at the pressure and temperatureconditions of the MTZ. Thus, significant seismic anisotropy is possible in the upper MTZ wherelattice‐preferred orientation of wadsleyite is produced by mantleflow. Here, we use a body wave method, SSprecursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. Westack the data to explore the azimuthal dependence of travel‐times and amplitudes of SS precursors andconstrain the azimuthal anisotropy in the MTZ. Beneath the central Pacific, wefind evidence for ~4%anisotropy with a SE fast direction in the upper mantle and no significant anisotropy in the MTZ. Insubduction 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 directionsindicates that the lattice‐preferred orientation of wadsleyite induced by MTZflow is organized separatelyfrom theflow in the upper mantle. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the uppermantle but negligible anisotropy (<1%) in the MTZ. Finally, we correct for the upper mantle and MTZanisotropy structures to obtain a new MTZ topography model. The anisotropy correction produces ±3 kmdifference and therefore has minor overall effects on global MTZ topography.