  Magnetars form a special class of neutron stars possessing superstrong
magnetic fields and demonstrating power flares triggered likely by these
fields. Observations of such flares reveal the presence of quasi-periodic
oscillations (QPOs) at certain frequencies; they are thought to be excited in
the flares. QPOs carry potentially important information on magnetar structure,
magnetic field, and mechanisms of magnetar activity. We calculate frequencies
of torsional (magneto-elastic) oscillations of the magnetar crust treating the
magnetic field effects in the first order of perturbation theory. The theory
predicts splitting of non-magnetic oscillation frequencies into Zeeman
components. Zeeman splitting of torsional oscillation spectrum of magnetars was
suggested, clearly described and estimated by Shaisultanov and Eichler (2009)
but their work has not been given considerable attention. To extend it we
suggest the technique of calculating oscillation frequencies including Zeeman
splitting at not too strong magnetic fields for arbitrary magnetic field
configuration. Zeeman splitting enriches the oscillation spectrum and
simplifies theoretical interpretation of observations. We calculate several
low-frequency oscillations of magnetars with pure dipole magnetic field in the
crust. The results qualitatively agree with low-frequency QPOs detected in the
hyperflare of SGR 1806--20, and in the giant flare of SGR 1900+14.
