The time dependence of the parameter of the Gutenberg-Richter
(GR) magnitude distributions is computed for correlated foreshock
sequences of earthquakes, 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 a possibility of
estimating the moment of occurrence of a main shock by such an
analysis of a foreshock sequence. The discussion is applied to
two Vrancea main shocks. The appreciable limitations of such an
estimation are discussed.