  The time dependence of the parameter of the Gutenberg-Richter (GR) magnitude
distribution is computed for foreshock sequences of earthquakes, correlated
with the main shock, by using the geometric-growth model of earthquake focus,
the magnitude distribution of correlated earthquakes and the time-magnitude
correlations, derived recently. It is shown that this parameter decreases in
time in the foreshock sequence, from the background values down to the main
shock. If correlations are present, this time dependence and the time-magnitude
correlations provide a tool of monitoring the foreshock seismic activity. We
discuss the relevance of such an analysis for the occurrence moment and the
magnitude of a main shock. The discussion is applied to a few Vrancea main
shocks and the precursory seismic activity of the l'Aquila earthquake. The
limitations of such an analysis are discussed.
