  The anharmonic decay rates of atomic vibrations in amorphous silicon (a-Si)
and paracrystalline silicon (p-Si), containing small crystalline grains
embedded in a disordered matrix, are calculated using realistic structural
models. The models are 1000-atom four-coordinated networks relaxed to a local
minimum of the Stillinger-Weber interatomic potential. The vibrational decay
rates are calculated numerically by perturbation theory, taking into account
cubic anharmonicity as the perturbation. The vibrational lifetimes for a-Si are
found to be on picosecond time scales, in agreement with the previous
perturbative and classical molecular dynamics calculations on a 216-atom model.
The calculated decay rates for p-Si are similar to those of a-Si. No modes in
p-Si reside entirely on the crystalline cluster, decoupled from the amorphous
matrix. The localized modes with the largest (up to 59%) weight on the cluster
decay primarily to two diffusons. The numerical results are discussed in
relation to a recent suggestion by van der Voort et al. [Phys. Rev. B {\bf 62},
8072 (2000)] that long vibrational relaxation inferred experimentally may be
due to possible crystalline nanostructures in some types of a-Si.
