Local rigidity and self-organized criticality for avalanches

The general conditions for a sandpile system to evolve spontaneously into a critical state characterized by a power law distribution of avalanches or bursts are identified as: a) the existence of a stationary state with a global conservation law; 6) long-range correlations in the continuum limit (ie. Laplacian diffusive field); c) the existence of a local rigidity for the microscopic dynamics. These conditions permit a classification of the models that have been considered up to now and the identification of the local rigidity as a new basic parameter that can lead to various possible scenarios ranging continuously from SOC behaviour to standard diffusion.