  A characteristic change of seismicity has been recently uncovered when the
precursory Seismic Electric Signals activities initiate before an earthquake
occurrence. In particular, the fluctuations of the order parameter of
seismicity exhibit a simultaneous distinct minimum upon analyzing the seismic
catalogue in a new time domain termed natural time and employing a sliding
natural time window comprising a number of events that would occur in a few
months. Here, we focus on the minima preceding all earthquakes of magnitude 8
(and 9) class that occurred in Japanese area from 1 January 1984 to 11 March
2011 (the day of the M9 Tohoku earthquake). By applying Detrended Fluctuation
Analysis to the earthquake magnitude time series, we find that each of these
minima is preceded as well as followed by characteristic changes of temporal
correlations between earthquake magnitudes. In particular, we identify the
following three main features. The minima are observed during periods when long
range correlations have been developed, but they are preceded by a stage in
which an evident anti-correlated behavior appears. After the minima, the long
range correlations break down to an almost random behavior turning to
anti-correlation. The minima that precede M$\geq$7.8 earthquakes are
distinguished from other minima which are either non-precursory or followed by
smaller earthquakes.
