  We study the time evolution of wavepackets of non-interacting electrons in a
two-dimensional disordered system in strong magnetic field. For wavepackets
built from states near the metal-insulator transition in the center of the
lowest Landau band we find that the return probability to the origin $p(t)$
decays algebraically, $p(t) \sim t^{-D_2/2}$, with a non-conventional exponent
$D_2/2$. $D_2$ is the generalized dimension describing the scaling of the
second moment of the wavefunction. We show that the corresponding spectral
measure is multifractal and that the exponent $D_2/2$ equals the generalized
dimension $\widetilde{D}_2$ of the spectral measure.
