Robustness of scale invariance in models with self-organized criticality

A random-neighbor extremal stick-slip model is introduced. In the thermodynamic limit, the distribution of stares has a simple analytical form and the mean avalanche size, as a function of the coupling parameter, is exactly calculable. The system is critical only at a special point J(e) in coupling parameter space. However, the critical region around this point, where approximate scale invariance holds, is very large, suggesting a mechanism for explaining the ubiquity of power laws in nature. [S1063-651X(99)06105-X].