  The highly tangled magnetic field of a magnetar supports shear waves similar
to Alfv\'en waves in an ordered magnetic field. Here we explore if torsional
modes excited in the stellar interior and magnetosphere can explain the
quasi-periodic oscillations (QPOs) observed in the tail of the giant flare of
SGR 1900+14. We solve the initial value problem for a tangled magnetic field
that couples interior shear waves to relativistic Alfv\'en shear waves in the
magnetosphere. Assuming stellar oscillations arise from the sudden release of
magnetic energy, we obtain constraints on the energetics and geometry of the
process. If the flare energy is deposited initially inside the star, the wave
energy propagates relatively slowly to the magnetosphere which is at odds with
the observed rise time of the radiative event of $\lesssim 10$ ms. Nor can the
flare energy be deposited entirely outside the star, as most of the energy
reflects off the stellar surface, giving surface oscillations of insufficient
magnitude to produce detectable modulations of magnetospheric currents. Energy
deposition in a volume that straddles the stellar surface gives agreement with
the observed rise time and excites a range of modes with substantial amplitude
at observed QPO frequencies. In general, localized energy deposition excites a
broad range of modes that encompasses the observed QPOs, though many more modes
are excited than the number of observed QPOs. If the flare energy is deposited
axisymmetrically, as is possible for a certain class of MHD instabilities, the
number of modes that is excited is considerably reduced.
