Depending on the area in which the earthquakes are sampled, the exponent b/c of the distribution of seismic moment or energy of earthquakes fluctuates from one region to another in a range typically between 0.5 and 1. To explain this observation, we propose a general mechanism which accounts for fluctuations in the b/c-value as being related to the variations of the fractal dimension of the set of faults on which the Gutemberg-Richter law is measured. ¿American Geophysical Union 1991