GNGTS 2016 - Atti del 35° Convegno Nazionale

14 GNGTS 2016 S essione A matrice seismology investigations triggered some discussion whether the observed ground breaks were generated by tectonics or, conversely, landslides or deep seated mass movements (e.g., Gruppo di Lavoro INGV, 2016). This reminds us a long-debated issue, following the September 26, 1997, Colfiorito earthquake, a very similar seismic event in terms of both source parameters and structural framework (Cello et al. , 1998; Mildon et al. , 2016). Similarly to what happened for theAmatrice event, surface ruptures associated with the Col orito earthquake showed coseismic surface slip occurs along pre-existing Quaternary faults responsible for the recent tectonic evolution of the area. Along the reactivated fault segments, surface offset and slip vectors were remarkably constant over tens to several hundreds of metres, while the thickness of slope deposits varies from a few centimetres to tens of metres. All these indicate that the observed ground displacement was related to fault slip rather than ground failure due to shaking. It was also important to note that coseismic displacement of 4 to 8 cm over lengths of few kilometres was exactly what one should have expect in average for Mw = 5.7 to 6.0 earthquakes, based on the empirical relations (e.g., Wells and Coppersmith, 1994). Therefore, following the Amatrice earthquake, we believe it is useful to quote what was written after the Colfiorito event: “… the discussion on the amount of tectonic versus gravity component in the surface displacement in similar conditions is just useless […]. The reactivation of faults already mapped as having potential for coseismic surface displacement, because of the close resemblance with bedrock fault scarps and slickensides ruptured during the Fucino, 1915 […], and the Irpinia, 1980 […] earthquakes is the demonstration that these faults positively t the de nition of capable faults […]. This is what is needed in order to de ne geological criteria Fig. 1 – Trace of continuous surface faulting along Mt. Vettore slope; the arrow shows intersection with the SP34 road near Forca di Presta; note that the fault rupture at this site downthrows the uphill side of the road.