Granular packings under constant shear stress display below the Coulomb
limit, a logarithmic creep dynamics. However the addition of small stress
modulations induces a linear creep regime characterized by an effective viscous
response. Using Diffusing Wave Spectroscopy, we investigate the relation
between creep and local plastic events spatial distribution ("hot-spots")
contributing to the plastic yield. The study is done in the two regimes, i.e.
with and without mechanical activation. The hot-spot dynamics is related to the
material effective fluidity. We show that far from the threshold, a local
visco-elastic rheology coupled to an ageing of the fluidity parameter, is able
to render the essential spatio-temporal features of the observed creep
dynamics