  Recent observations of quasi-periodic oscillations in the aftermath of giant
flares in soft gamma-ray repeaters suggest a close coupling between the seismic
motion of the crust after a major quake and the modes of oscillations in a
magnetar. In this paper we consider the purely elastic modes of oscillation in
the crust of a neutron star in the relativistic Cowling approximation
(disregarding any magnetic field). We determine the axial crust modes for a
large set of stellar models, using a state-of-the-art crust equation of state
and a wide range of core masses and radii. We also devise useful approximate
formulae for the mode-frequencies. We show that the relative crust thickness is
well described by a function of the compactness of the star and a parameter
describing the compressibility of the crust only. Considering the observational
data for SGR 1900+14 and SGR 1806-20, we demonstrate how our results can be
used to constrain the mass and radius of an oscillating neutron star.
