GNGTS 2014 - Atti del 33° Convegno Nazionale

of this migration on the extensional tectonic zone that develops along the inner side of the above wedge (NCGGA fault system) has so far reached the Colfiorito zone in 1997,1998 and partially the Gubbio fault segment (1984). Thus, if a progressive northward development of such extensional fault system could be tentatively assumed, as suggested by the behaviour of previous seismicity in the study area, (Viti et al ., 2012, 2013), one might expect the next seismic activations to concern the northern segments (Gualdo Tadino, Gubbio and Alta Valtiberina) of the above fault system. Other possible insights into the next development of seismic activations in the study area could be achieved by the spatio-temporal distribution of background seismicity (Fig. 3). The annual number of events in the periods not following strong earthquakes (M ≥ 5.5) is generally lower than 100-300. In 1997, after the Colfiorito M= 5.7, 6.0, 5.5, 5.7 shock, the number of events reached about 1800. In 2009, after the L’Aquila M=6.3 shock, about 3400 events took place. Other significant increases of background seismicity, although less pronounced, occurred in 2003 (Bologna area and Romagna Apennines) in 2013 (Toscana-Emilia and Umbria-Marche Apennines), and in 2013 -2014 (UmbrianApennines), with more than 400 events. In the first two cases, the seismicity increases were preceded by moderate earthquakes, while in the last case (Umbria) the increase was not preceded by any major event. Furthermore, one can note that in Fig. 3 – ���� �������� �� ��� ������ �� ���������� ����������� ����� ���� ��� ������� �� ���������������������� � Time patterns of the number of background earthquakes (M>2, from CSI archive at http://csi.rm.ingv.it/) in the various zones of the central and northern Apennines. The data overcoming 800 events are indicated by a red number at the top of the respective bar. GNGTS 2014 S essione 1.2 185