Connecting seismically active normal faults with Quaternary geological structures in a complex extensional environment: the Colfiorito 1997 case history (Northern Apennines, Italy)

The Northern Apennines of Italy are a region interested by low extensional strain rate, characterized by a complex Tertiary tectonic history, where superposed compressional and extensional deformation occurred; co-seismic surface ruptures are rare and are often matter of much debate. In such regions the relationships between Quaternary faults (i.e. faults which can be mapped at the surface and possess long-term displacement) and seismogenic faults (i.e. faults which actually generate earthquakes) are not a trivial problem. The availability of detailed geological mapping and high resolution seismological data for the Colfiorito area, struck in 1997 by a sequence of six 5<Mw<6 normal faulting earthquakes, allow us to compare and verify the strict geometric and kinematics correspondence between the mapped Quaternary fault pattern and the fault segments activated during the 1997 sequence, as highlighted by the hypocenters distribution; also the length of the mapped and activated faults (8-10 km) is quite similar. The co-seismic subdued region imaged by InSAR data, is located in the hanging-wall of the mapped normal faults that bound the Quaternary basins. In section view, there is a convincing continuity between the Quaternary normal faults and the aftershock alignments used to image fault geometry at depth. Comparison of striated fault planes and aftershock focal mechanism solutions show a strong kinematic consistency. This study point out that the Quaternary tectono-sedimentary evolution and the present-day geological and geomorphologic setting of the Colfiorito area can be interpreted as the result of repeated, extensional earthquakes, similar to the 1997 events, occurring on NW-SE trending normal faults. Our data also show that the main shocks of the Colfiorito sequence nucleated close to the intersections between the normal faults and the pre-existing compressional/transpressional structures, which acted as lateral barriers to rupture propagation and consequently constrain the main fault segment size.