  We general relativistically calculate the frequency of fundamental torsional
oscillations of neutron star crusts, where we focus on the crystalline
properties obtained from macroscopic nuclear models in a way depending on the
equation of state of nuclear matter. We find that the calculated frequency is
sensitive to the density dependence of the symmetry energy, but almost
independent of the incompressibility of symmetric nuclear matter. By
identifying the lowest-frequency quasi-periodic oscillation in giant flares
observed from soft gamma-ray repeaters as the fundamental torsional mode and
allowing for the dependence of the calculated frequency on stellar models, we
provide a lower limit of the density derivative of the symmetry energy as
$L\simeq 50$ MeV.
