Yield-stress granular media flow if a sufficient shear stress is applied to them. Under certain conditions this flow can be spatially heterogeneous and intermittent in time. Although such behavior is widely observed in nature during avalanches, landslides and earthquakes, there are few quantitative ex-perimental measurements of these intermittent dynamics at the local scale in frictional systems.

We present the experimental and numerical studies of a two-dimensional sheared amorphous material composed of bidisperse photoelastic disks. We analyze the statistics of avalanches during shear including the local and global fluctuations in energy and changes in particle positions and orientations. We find scale-free distributions for these global and local avalanches denoted by power laws whose cutoffs vary with interparticle friction and packing fraction. Different exponents are found for these power laws depending on the quantity from which variations are extracted. An asymmetry in time of the avalanche shapes is evidenced along with the fact that avalanches are mainly triggered by the shear bands. A simple relation independent of the intensity is found between the number of local avalanches and the global avalanches they form. We also compare these experimental and numerical results for both local and global fluctuations to predictions from mean-field and depinning theories