On 26 September 1997, two seismic events of magnitude (Mw) 5.7 and 6.0 struck the Colfiorito area and the surrounding areas (on the border between Umbria and Marche), nine hours apart from each other (02:33 and 11:40 Italian time).
The 1997 seismic sequence on the border between Umbria and Marche represents a watershed for Italian seismology. It was the first earthquake in our country for which high-quality data collected by ground monitoring networks and satellites were simultaneously available. The picture that these data provided made it possible to outline with a precision never achieved before the characteristics of the faults that had been activated and the mechanisms of generation of the Apennine earthquakes. The seismic events of the following years, those of 2009 in L'Aquila and the recent sequence of 2016-2017, confirmed many of the interpretations drawn from the studies on the 1997 earthquakes, highlighting further characteristic elements. The photo gallery shows some images of the Mobile Seismic Network of the ING (National Institute of Geophysics, later merged into the National Institute of Geophysics and Volcanology), installed in the first hours after the earthquakes of 26 September, which represented one of the most important instruments for seismological research, as well as a very important information reference point for the local community affected by the earthquake in 1997.
Epicentral map of seismic sequences in central Italy from 1997 to 2017. The 1997 earthquakes are represented in blue. The three blue stars are the epicenters of the two earthquakes of 26 September and that of 14 October 1997. In yellow the sequence of the Aquila of 2009, in orange and red the seismicity of 2016-2017
An important trait that emerged from the studies on the 1997 sequence is the tendency of Apennine earthquakes to manifest themselves with the migration of activity between neighboring fault segments, as happened precisely on September 26, 1997. At the first earthquake of magnitude Mw 5.7, which occurred in the night at 02:33 Italian time, a second stronger event followed nine hours later, magnitude Mw 6.0, at 11:40 Italian time, which caused further collapses and casualties. Subsequent studies made it possible to understand the causes of this seismic migration (Cocco et al., 2000; Miller et al., 2004; Antonioli et al., 2005), even if a single model capable of explaining the variegated cases recorded in all subsequent Apennine earthquakes (e.g. L'Aquila, 2009; Amatrice-Norcia-Visso, 2016; Campotosto, 2017) has not yet been defined.
The collapse of the Basilica of Assisi on the morning of September 26, 1997
The sequence manifested itself with seven major earthquakes of moment magnitude (Mw) between 5.0 and 6.0 in the first month of activity and thousands of earthquakes of minor magnitude which in 40 days activated a fault system extending for about 45 kilometers along the Apennines.
Link to the full article on the INGVterremoti blog:
https://ingvterremoti.wordpress.com/2017/09/26/i-terremoti-del-900-la-sequenza-sismica-in-umbria-marche-del-1997/