  We systematically examine the fundamental frequencies of shear torsional
oscillations in neutron star crusts in a manner that is dependent on the
parameter $L$ characterizing the poorly known density dependence of the
symmetry energy. The identification of the lowest quasiperiodic oscillation
(QPO) among the observed QPOs from giant flares in soft-gamma repeaters as the
$/ell=2$ fundamental torsional oscillations enables us to constrain the
parameter $L$ as $L/ge 47.4$ MeV, which is the most conservative restriction on
$L$ obtained in the present work that assumes that the mass and radius of the
flaring neutron stars range 1.4-1.8 $M_{/odot}$ and 10-14 km. Next, we identify
one by one a set of the low-lying frequencies observed in giant flares as the
fundamental torsional oscillations. The values of $L$ that can reproduce all
the observed frequencies in terms of the torsional oscillations coupled with a
part of dripped neutrons via entrainment effects are then constrained as 101.1
MeV $/le L /le$ 131.0 MeV. Alternatively, if only the second lowest frequency
observed in SGR 1806$-$20 has a different origin, one obtains relatively low
$L$ values ranging 58.0 MeV $/le L /le$ 85.3 MeV, which seem more consistent
with other empirical constraints despite large uncertainties.
