  We present a simple model of a dynamical system driven by externally-imposed
coherent noise. Although the system never becomes critical in the sense of
possessing spatial correlations of arbitrarily long range, it does organize
into a stationary state characterized by avalanches with a power-law size
distribution. We explain the behavior of the model within a time-averaged
approximation, and discuss its potential connection to the dynamics of
earthquakes, the Gutenberg-Richter law, and to recent experiments on avalanches
in rice piles.
