  Here, we analyze in natural time $\chi$, the slow propagation of a
macroscopic crack in brittle heterogeneous materials through sudden jumps and
energy release events which are power law distributed with universal exponents.
This macroscopic crack growth is currently believed to exhibit similar
characteristics with the seismicity associated with earthquakes. Considering
that the crack front is self affine and exhibits Family-Vicsek universal
scaling, we show that the variance $\kappa_1 (\equiv\langle \chi^2\rangle
-\langle \chi \rangle^2)$ of natural time is equal to 0.0686, which almost
coincides with the value $\kappa_1\approx 0.07$ obtained from the seismicity
preceding major earthquakes. This sheds light on the determination of the
occurrence time of an impending mainshock.
