A Simple Model for Sheared Granular Layers

We phenomenologically investigate stick-slip motion of sheared granular layers. Our phenomenology is constructed in the context of nucleation-and-growth of the fluidized area which is triggered by collapsing of stress chains. Based on this picture, we give a simple friction model by introducing the degree of the fluidization. It is found that the present model can successfully reproduce major features of the experimental results reported by Nasuno {\it et al.} (Phys. Rev. E {\bf 58}, 2161 (1998)) with quantitatively good agreement.