Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 4 5/6 2004 10.5194/nhess-4-691-2004 http://www.nat-hazards-earth-syst-sci.net/4/691/2004/ http://www.nat-hazards-earth-syst-sci.net/4/691/2004/nhess-4-691-2004.html http://www.nat-hazards-earth-syst-sci.net/4/691/2004/nhess-4-691-2004.pdf 691 695 2004-11-03 Information entropy analysis of seismicity of Umbria-Marche region (Central Italy) L. Telesca V. Lapenna M. Lovallo Institute of Methodologies for Environmental Analysis, National Research Council, Tito, Italy 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.