  In a realistic model of magneto-elastic oscillations in magnetars, we find
that crustal shear oscillations, often invoked as an explanation of
quasi-periodic oscillations (QPOs) seen after giant flares in soft gamma-ray
repeaters (SGRs), are damped by resonant absorption on timescales of at most
0.2s, for a lower limit on the dipole magnetic field strength of 5 x 10^13 G.
At higher magnetic field strengths (typical in magnetars) the damping timescale
is even shorter, as anticipated by earlier toy-models. We have investigated a
range of equations of state and masses and if magnetars are dominated by a
dipole magnetic field, our findings exclude torsional shear oscillations of the
crust from explaining the observed low-frequency QPOs. In contrast, we find
that the Alfv\'en QPO model is a viable explanation of observed QPOs, if the
dipole magnetic field strength exceeds a minimum strength of about several
times 10^14 G to 10^15 G. Then, Alfv\'en QPOs are no longer confined to the
fluid core, but completely dominate in the crust region and have a maximum
amplitude at the surface of the star.
