  Recent observations of giant flares from soft gamma-ray repeaters have
exhibited multiple 25-150 Hz oscillations. Frequencies in this range are
expected for toroidal shear waves in a neutron star (NS) crust, lending support
to Duncan's proposal that such modes may be excited in these events. This
motivates a reassessment of how these waves reflect the NS structure and what
role the magnetic field plays in setting their frequencies. We calculate the
eigenfrequencies and eigenfunctions of toroidal oscillations for a realistic NS
crust, including a vertical magnetic field at magnetar strengths
($B\sim10^{14}-10^{15} {\rm G}$). The lowest radial-order mode has a
red-shifted frequency of $\approx28 {\rm Hz}[l(l+1)/6]^{1/2}$, with the
prefactor depending on the NS's mass and radius, and its crust's depth and
composition. This mode is independent of the magnetic field for
$B\lesssim4\times10^{15} {\rm G}$, a limit much greater than the inferred
dipole magnetic fields for these objects. Though this is a good fit to the
observed oscillations, only rather loose constraints can be made for the NSs'
properties because all that can be fit is this prefactor (a single parameter).
Modes with shorter radial wavelengths are more sensitive to the magnetic field
starting at $B\sim2\times10^{14} {\rm G}$ and have higher frequencies (600-2000
Hz). The discovery of these modes, coupled with the oscillations observed thus
far, would provide a powerful probe to the NS crustal structure.
