Some of the results of the paper by von Seggern (1980), concerning the reddening of the wavenumber spectrum of the stress difference over the fault as the main shock approaches, depend on an assumption on the coefficient c of the relation log M0 = d+cM. It is shown here that, at least in California, the reddening of the spectrum of a vertical section of the fractal surface (Mandelbrot 1977) begins with the aftershocks sequence, as suggested by von Seggern (1980) but the normal state (k-2, k is the wavenumber) is in the period between aftershocks and the foreshock and, because of the reddening of the spectrum, it is followed by the abnormal state (k-3) associated to the foreshock sequence. Then the major shock introduces a disorder into the local stress field and gives rise to very irregular zeroset areas with ''antipersistent'' fractal function, with k-0.5 to k-1.4, of the aftershock sequence. At this stage the cycle starts again with the reddening of the power spectrum. In other words there is a reddening which begins with the aftershock sequence, but with k-0.5 to k-1.4 and it ends with the foreshock sequence with k-3. The statistical analysis of fifteen sequences of earthquakes is also presented and it is suggested that the change in the density distribution function of the stress drop may be an indicator of the preparatory phase of large earthquakes.