A destructive earthquake, of magnitude Mw 6.1 to 6.2, hit Central Italy in the night of 23-24 of August at about 3:30 local time. The epicenter is located at ~10km SE of the town of Norcia, in the core of the Apennine mountain belt. Many aftershocks have been registered, two of them with magnitude between 5 and 5.5 one hour after the main shock. Focal mechanism, and repartition of aftershocks (a 25 km NW-SE zone), shows that the earthquake occurred on a ~20km long NW-SE normal fault parallel to the Apennine belt.
This type of seismic rupture and magnitude are typical of seismic events that regularly affect central Italy. The rupture of August 24th is between the fault zones activated by the earthquake sequences of l’Aquila (2009, M6.3) and Colfiorito (1997, M6 for the main shock). Several historical earthquakes, with magnitude larger that 6.5, are also known in the region. Note more specifically two events in 1703, one close to Norcia, the other close to l’Aquila, both with magnitude estimated to 6.7. More to the south, massive destructions and death-toll followed the Irpinia earthquake of magnitude 6.9 in 1980. The risk of earthquakes of this type was thus already identified in the region affected by August 24th event. Sadly, old monuments and buildings typical of towns and villages in central Italy are highly vulnerable, which explains the severity of destructions.
Central Italy seismic history, and the presence of many, very segmented, active faults shows that earthquakes often occurs as sequences or swarms in the region. That was the case in 1997 and 2009 as well as in 1703. This implies that the risk of aftershocks with magnitude between 5 and 6 cannot be excluded as well as the triggering of other earthquakes breaking adjacent faults and comparable to today’s event.
R. Lacassin, IPGP, 2016 Aug. 24, 10:00