Stress state due to the recent seismic events in the central Apennines (Italy)

The Central Apennines, Italy, are characterized by moderate seismic activity on normal faults, oriented in directions parallel to the Apenninic chain. The subject of this study is the Umbria-Marche Apennines, a segment approximately 200-km long, where three main seismic events occurred in the last three decades. The 1979 Norcia earthquake was a M-w = 5.8 event, taking place at the south end of the considered segment. The 1984 Gubbio earthquake was a M-w = 5.6 event which took place at the north end. The 1997-1998 Colifiorito sequence constituted 8 main shocks with magnitudes M-w between 5 and 6 and epicenters located between the Gubbio and the Norcia earthquake areas. A model made of an elastic half-space is considered, in which the seismic sources are represented by rectangular dislocations which have the appropriate values of source parameters, and in which the static stress field produced by each event is calculated. The analysis of the Coulomb stress change (DC) as a function of time shows that the coseismic stress transfer and fault interaction played an important role in the region during the past three decades: 7 earthquakes of the 9 considered took place where Delta C> 0. Such an interaction has been confirmed by the analysis of the aftershocks in the Colfiorito zone post September 26, 1997: about the 61% of the aftershocks considered took place where Delta C> 0. The comparison between the Delta C-s due to the coseismic stress transfer and the rate DCt due to the tectonic stress allows us to quantify the time advance of the earthquakes. The Delta C-s pattern shows positive values in two areas that can be regarded as historical seismic gaps.