  We perform extensive simulations and systematic statistical analyses of the
structural dynamics of amorphous silicon. The simulations follow the dynamics
introduced by Wooten, Winer and Weaire: the energy is obtained with the Keating
potential, and the dynamics consists of bond transpositions proposed at random
locations and accepted with the Metropolis acceptance ratio. The structural
quantities we track are the variations in time of the lateral lengths
($L_x$,$L_y$,$L_z$) of the cuboid simulation cell. We transform these
quantities into the volume $V$ and two aspect ratios $B_1$ and $B_2$. Our
analysis reveals that at short times, the mean squared displacement (MSD) for
all of them exhibits normal diffusion. At longer times, they cross over to
anomalous diffusion, with a temperature-dependent anomalous exponent
$\alpha<1$. We analyze our findings in the light of two standard models in
statistical physics that feature anomalous dynamics, $viz.$, continuous time
random walker (CTRW) and fractional Brownian motion (fBm). We obtain the
distribution of waiting times, and find that the data are consistent with a
stretched-exponential decay. We also show that the three quantities, $V$, $B_1$
and $B_2$ exhibit negative velocity autocorrelation functions. These
observations together suggest that the dynamics of the material belong to the
fBm class.
