A simple model is presented in which a fault is represented by a one-dimensional array of elements to which a remote stress is applied. The fracture toughnesses of the fault elements are initially generated by a random walk function, but the fracture system quickly becomes organized into a fractal set characterized by a (box-counting) fractal dimension. The slope, b, of the frequency-magnitude distribution and the fractal dimension of the crack population are measured at each step, and found to vary in a similar manner to the seismic b-value and fractal dimension of earthquake epicenters. The model also predicts the observed variation of b-value of acoustic emissions in laboratory experiments. ¿ American Geophysical Union 1992