Seismicity related to a given fault area is governed by the distribution of the difference between shear stress and frictional stress over the fault surface. Extent of rupture in any given earthquake depends on the size of the area around the nucleation point for which a dynamic rupture criterion is satisfied. The distribution of this ''size'' follows a negative power law with an exponent which implies a certain falloff in the wavenumber spectrum of the stress difference over the fault. This exponent decreases as the time of a main shock approaches, with an associated reddening of the wavenumber power spectrum from k-1 to k-2 decay. The cause of such changes is the intervening seismicity between main shocks, including aftershocks. The observation of stresses at only sparse points along a fault or at points removed from the fault will be of little value in the prediction of the time or magnitude of a future earthquake.