  We present a statistical analysis of the acoustic emissions induced by
dislocation motion during the creep of ice single crystals. The recorded
acoustic waves provide an indirect measure of the inelastic energy dissipated
during dislocation motion. Compression and torsion creep experiments indicate
that viscoplastic deformation, even in the steady-state (secondary creep), is a
complex and inhomogeneous process characterized by avalanches in the motion of
dislocations. The distribution of avalanche sizes, identified with the acoustic
wave amplitude (or the acoustic wave energy), is found to follow a power law
with a cutoff at large amplitudes which depends on the creep stage (primary,
secondary, tertiary). These results suggest that viscoplastic deformation in
ice and possibly in other materials could be described in the framework of
non-equilibrium critical phenomena.
