THE INTERMOUNTAIN BASINS WITHIN THE APENNINES (ITALY): LANDSCAPES RESULTING BY REPEATED STRONG EARTHQUAKES

The epicentral areas and the most relevant ground effects of strong historical to modern earthquakes that occur in the Apennines – with typical hypocentral depths of 5 – 15 km - are systematically located within the intermountain extensional basins that characterize the landscape of the inner part of Central and Southern Italy. This study aims at showing A) why we can assume that these basins are the result of repeated strong earthquakes over a geologic time interval, that is the last ca. 3 Ma, B) how this assumption can be used for seismic hazard assessment in Italy, and C) on which basis this approach should be used for understanding the seismic potential of others regions of the Earth. In particular, we focus here on the Fucino, Colfiorito, Bojano and Rieti basins, where our analysis clearly indicate that several geologic and geomorphic features, including the size of the basin and the thickness of the filling deposits, appear to be directly related with the source parameters of the associated “typical” earthquake, that is the dominant seismic event in the recent seismic history of the area. This indicates that the growth of the Apennines intermountain basins is due to the repeated occurrence of “characteristic” surface faulting earthquakes along the causative normal fault segments. Thus, a relation exists between seismic potential, length and displacement of the surface rupture (paleoseismic evidence), fault slip-rates, and dimensions of the associated intermountain basin. This relation, or “seismic landscape”, is apparent for all the Quaternary normal fault basins of the Apennines, also where local factors such as very high rates of depositional/erosional processes due to the drainage network (Rieti basin) or pre-existing structural complexities (Bojano basin) tend to mask the real shape and size of the growing geologic structure. Conversely, this is no more true for older, Pliocene basins, located on the Tyrrhenian side of the Apennines belt, which show a much more subdue seismicity and low-energy geomorphic relief. In this line, we propose a model illustrating the expected occurrence and characteristics of coseismic ground effects for the Fucino-like (M7) and the Colfiorito-like (M6) “seismic landscapes”, which summarize the basic criteria for evaluating seismic hazards in the extensional setting of the Apennines.