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\begin{document}

\title{Splitting of ScS waves due to lowermost mantle anisotropy: Practical
challenges and new global measurements}



\author[1]{Jonathan Wolf}%
\author[1]{Maureen Long}%
\affil[1]{Department of Earth and Planetary Sciences, Yale University}%


\vspace{-1em}



  
  \date{May 19, 2024}


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\begin{abstract}
Many regions of the Earth's mantle are seismically anisotropic,
including portions of the lowermost mantle, which may indicate
deformation due to convective flow. The splitting of ScS phases, which
reflect once off the core-mantle boundary (CMB), is commonly measured to
identify lowermost mantle anisotropy, although some challenges exist.
Here, we use global wavefield simulations to evaluate commonly used
approaches to inferring a lowermost mantle contribution to ScS
splitting. We show that due to effects of the CMB reflection, only the
epicentral distance range between 60\textsuperscript{*} and
70\textsuperscript{*} is appropriate for ScS splitting measurements. For
this distance range, splitting is diagnostic of deep mantle anisotropy
if no upper mantle anisotropy is present; however, if ScS is also split
due to upper mantle anisotropy, the reliable diagnosis of deep mantle
anisotropy is challenging. Moreover, even in the case of a homogeneously
anisotropic deep mantle region sampled from a single azimuth by multiple
ScS waves with different source polarizations (in absence of upper
mantle anisotropy), different apparent fast directions are produced. We
suggest that ScS splitting should only be measured at ``null'' stations
and conduct such an analysis worldwide. Our results indicate that
seismic anisotropy is globally widespread in the deep mantle. ~%
\end{abstract}%



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