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\begin{document}

\title{Upper Mantle Seismic Velocity Estimates around the St. Paul Transform
System, Equatorial Atlantic}



\author[1]{Guilherme Weber Sampaio de Melo}%
\author[2]{Ross Parnell-Turner}%
\author[3]{Robert P. Dziak}%
\author[4]{Deborah Smith}%
\author[1]{Aderson do Nascimento}%
\author[5]{Marcia Maia}%
\affil[1]{Federal University of Rio Grande do Norte}%
\affil[2]{Scripps Institution of Oceanography}%
\affil[3]{NOAA}%
\affil[4]{National Science Foundation}%
\affil[5]{Université de Bretagne Occidentale}%


\vspace{-1em}



  
  \date{June 16, 2024}


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\begin{abstract}
The equatorial region of the slow-spreading Mid-Atlantic Ridge is
characterized by several major transform faults, which are some of the
longest on Earth. Among them, the St. Paul Transform system (SPTS) is a
complex group of four transform faults, bounding three short
intra-transform segments with total offset of 630 km. The northernmost
transform is the 200 km-long, 30 km-wide Atoba Ridge, which represents a
major topographic feature that rises above sea level at the St. Peter
and St. Paul islands (SPSPA). This push-up ridge formed from
transpressive stresses along several transform fault step-overs and
restraining bends, uplifting mantle rocks at a rate of
\textasciitilde{}1.5 mm/yr. Moderate-sized earthquakes
(\textgreater{}4.0 Mw) have been located by global teleseismic networks
along the SPTS and near region. These earthquakes are recorded at large
epicentral distances, and include raypaths that travel within the upper
mantle (Pn and Sn phases). Pn velocity estimates can help to understand
the dynamics of upper mantle structure around of the transform faults.
Here, waveforms recorded over \textasciitilde{}6 months of 2012 by two
autonomous hydrophones moored north and south of SPTS (EA-2 and EA-8),
and a seismographic station installed on SPSPA island (ASPSP station)
are examined. These data allow us to make Pn velocity estimates from 32
earthquakes that occurred in the SPTS region from 1.5\selectlanguage{ngerman}º S to 4.5º N. Pn
wave velocities are typically thought to be 8.0--8.2 km/s in upper
mantle, however we identify Pn velocities ranging from 7.5 to 9.0 km/s.
The slower velocities (7.5-8.0 km/s) are from ray paths oriented
parallel to the ridge axis and could be explained by elevated mantle
potential temperature and the presence of melt. Ray paths passing
through the transform fault system have Pn velocities from 8.1 to 9.0
km/s, indicating that upper mantle conditions are strongly affected by
the presence of the crustal fault system. We will also compare our
velocity estimates to global shear-wave tomographic models of the upper
mantle. Hence it is our goal to show that the availability of autonomous
hydrophones and a single island seismic station can be used to make rare
estimates of Pn velocities, as well as provide insights into upper
mantle structure, in this remote part of the Atlantic Ocean.%
\end{abstract}\selectlanguage{ngerman}%



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