  Seismic anisotropy, arising from the crystallographic or lattice-preferred
orientation of anisotropic minerals or the shape-preferred orientation of melts
or cracks, can establish a critical link between Mars's past evolution and its
current state. So far, although seismic anisotropy in Mars has been proposed
due to different velocities of vertically and horizontally polarized shear
waves in the Martian crust, obtained from crustal converted waves, multiples,
and surface waves recorded by the InSight seismometer, the evidence is
plausible. Notably, the shear wave splitting, which stands out as a straight
indicator of seismic anisotropy, has not been reported using marsquake records.
In this study, we employ Low-frequency marsquakes detected by the InSight
seismometer to reveal shear wave splitting in Mars. We find that the direct S
waves of three marsquake recordings (S0173a, S0235b, and S1133c) with high
signal-to-noise ratios exhibit the splitting pheonmenon. We rule out the
possibility of apparent anisotropy through synthetic tests, affirming the
presence of seismic anisotropy in Mars. The delay time (about 1.33 s on
average) measured from the direct S wave splitting is too large to be solely
attributed to the seismic anisotropy in the upper crust (0 - 10 km) beneath the
InSight. Thus, seismic anisotropy in the deeper region of Mars is
indispensable. Combined with other geophysical evidence near the InSight
landing site, the strong seismic anisotropy observed in this study implies the
porous crust with aligned cracks being greater than 10 km beneath the InSight
and/or the presence of an active mantle plume underneath the Elysium Planitia
of Mars.
