We investigated by means of nonlinear tools the temporal distribution of 1983-2003 seismicity of the Umbria-Marche region, Central Italy, that was struck by a strong event (M=5.8) on 26 September 1997. Information entropy analysis has been performed over the full and the aftershock-depleted catalogue. We analysed the series of the interevent times between successive earthquakes and the series of their magnitudes. The time evolution of this measure shows a clear anomalous behaviour in correspondence with the stronger event, more evident in the full catalogue than in the aftershock-depleted one.