Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 10 1 2010 10.5194/nhess-10-19-2010 http://www.nat-hazards-earth-syst-sci.net/10/19/2010/ http://www.nat-hazards-earth-syst-sci.net/10/19/2010/nhess-10-19-2010.html http://www.nat-hazards-earth-syst-sci.net/10/19/2010/nhess-10-19-2010.pdf 19 24 2010-01-11 Strong foreshock signal preceding the L'Aquila (Italy) earthquake (<i>M</i>_w 6.3) of 6 April 2009 G. A. Papadopoulos papadop@gein.noa.gr M. Charalampakis A. Fokaefs G. Minadakis Institute of Geodynamics, National Observatory of Athens, Athens, Greece We used the earthquake catalogue of INGV extending from 1 January 2006 to 30 June 2009 to detect significant changes before and after the 6 April 2009 L'Aquila mainshock (M_w=6.3) in the seismicity rate, <i>r</i> (events/day), and in b-value. The statistical z-test and Utsu-test were applied to identify significant changes. From the beginning of 2006 up to the end of October 2008 the activity was relatively stable and remained in the state of background seismicity (<i>r</i>=1.14, <i>b</i>=1.09). From 28 October 2008 up to 26 March 2009, <i>r</i> increased significantly to 2.52 indicating weak foreshock sequence; the b-value did not changed significantly. The weak foreshock sequence was spatially distributed within the entire seismogenic area. In the last 10 days before the mainshock, strong foreshock signal became evident in space (dense epicenter concentration in the hanging-wall of the Paganica fault), in time (drastic increase of <i>r</i> to 21.70 events/day) and in size (b-value dropped significantly to 0.68). The significantly high seismicity rate and the low b-value in the entire foreshock sequence make a substantial difference from the background seismicity. Also, the b-value of the strong foreshock stage (last 10 days before mainshock) was significantly lower than that in the aftershock sequence. 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