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3-D Synthetic Modeling of Anisotropy Effects on SS Precursors: Implications for Mantle Flow in the Transition Zone
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  • Quancheng Huang,
  • Nicholas Charles Schmerr,
  • Caroline Beghein,
  • Lauren Waszek,
  • Ross Maguire
Quancheng Huang
University of Maryland, College Park

Corresponding Author:[email protected]

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Nicholas Charles Schmerr
University of Maryland, College Park
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Caroline Beghein
University of California Los Angeles
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Lauren Waszek
New Mexico State University
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Ross Maguire
University of Maryland
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The Earth’s mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. However, seismic anisotropy in the MTZ is difficult to obtain from surface wave or shear wave splitting measurements. Here, we investigate the sensitivity to anisotropy of a body wave method, SS precursors, through 3-D synthetic modeling. Our study shows that the SS precursors can distinguish the anisotropy originating from three depths: shallow upper mantle (80-220 km), deep upper mantle above 410-km, and MTZ (410-660 km). Synthetic resolution tests indicate that SS precursors can resolve 3% azimuthal anisotropy where data have an average signal to noise ratio (SNR=7) when azimuthal coverage is sufficient. To investigate regional sensitivity, we apply the stacking and inversion methods to two densely sampled areas: Japan subduction zone and a central Pacific region around the Hawaiian hotspot. We find evidence for a trench-perpendicular fast direction (Θ=87°) of MTZ anisotropy in Japan, but the strength of anisotropy is poorly constrained due to limited azimuthal coverage. We attribute the azimuthal anisotropy to lattice-preferred orientation of wadsleyite induced by trench-parallel mantle flow near the stagnant slab. In the central Pacific study region, there is a non-detection of MTZ anisotropy, although modeling suggests the data coverage should allow us to resolve up to 3% anisotropy. Therefore, the Hawaiian mantle plume does not produce detectable azimuthal anisotropy in the MTZ.