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\begin{document}

\title{Velocity Gradients of the African Large Low Velocity Province Boundary
Inferred from Backazimuth-Slowness Observations of Multipathing}



\author[1]{Jamie Ward}%
\author[1]{Andy Nowacki}%
\author[1]{Sebastian Rost}%
\affil[1]{University of Leeds}%


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  \date{June 18, 2024}


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\begin{abstract}
Large Low Velocity Provinces (LLVPs) are hypothesised to be purely
thermal features or possess some chemical heterogeneity, but exactly
which remains ambiguous. Regional seismology studies typically use
travel time residuals and multipathing identification in the waveforms
to infer properties of LLVPs. These studies have not fully analysed all
available information such as measuring the direction and inclination of
the arrivals. These measurements would provide more constraints of LLVP
properties such as the boundary velocity gradient and help determine
their nature. Here, we use array seismology to measure backazimuth
(direction) and horizontal slowness (inclination) of arriving waves to
identify structures causing multipathing and wavefield perturbation.
Following this, we use full-wavefield forward modelling to estimate the
gradients required to produce the observed multipathing. We use SKS and
SKKS data from 83 events sampling the African LLVP, which has been
extensively studied providing a good comparison to our observations. We
find evidence for structures at heights of up to 600 km above the
core-mantle boundary causing multipathing and wavefield perturbation.
Forward modelling shows gradients of up to 0.7\% \selectlanguage{greek}δ\selectlanguage{english}Vs per 100 km (0.0005
km /s km) are required to produce multipathing with similar backazimuth
and horizontal slowness to our observations. This is an order of
magnitude lower than the previous strongest estimates of -3\% \selectlanguage{greek}δ\selectlanguage{english}V per 50
km (0.0044 km /s km). As this is lower than found for both thermal and
thermochemical structures, gradients capable of producing multipathing
is not necessarily evidence for a thermochemical nature.%
\end{abstract}%



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