Spatial repartition of local plastic processes in different creep regimes in a granular material

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.