Active faulting and seismogenic potential in low-strain rate southern Tuscany (Italy)

We explored the behaviour and earthquake potential of an active fault system in the slowly deforming part of southern Tuscany. This region corresponds to the eastern margin of the Siena Basin, a Neogene structural depression that developed during the extensional tectonics that affected the inner northern Apennines. Here, N-, NE- and WNW-trending faults were active during the Zanclean-Latest Quaternary. Clear evidence of the activity of these faults, particularly the most recent WNW-striking ones, is represented by faulted Late Pleistocene-Holocene travertine deposits that preserve also evidence of active seismogenic faulting. Indeed, this area in recent times was mostly interested by low-magnitude seismic sequences that occurred in the uppermost 10 km of the crust, mainly characterized by transcurrent and transtensive faulting mechanisms. However, the historical record includes also damaging earthquakes in the 5.0-6.0 Mw range, such as the 7 August 1414, Mw 5.7, Colline Metallifere, 13 April 1558, Mw 6.0, Valdarno Superiore, and 25 August 1909, Mw 5.3, Crete Senesi events, but, to date, very little is known about the geometry, maximum earthquake potential and slip rate of their causative faults.In this study, we characterize an active, capable, and seismogenic fault system identified in the saw-cut walls of an active travertine quarry near Serre di Rapolano, a few kilometres south-east of the city of Siena. To document the geometric and kinematic features of the active faults, we carried out a detailed geological and structural field survey of the quarry outcrop, collected samples for U-Th dating and constructed a virtual outcrop model. We found compelling evidence for nearly SW-NE surface-breaking faulting, perpendicular to the main structural fabric of the central and northern Apennines, whose activity extends at least into the Upper Pleistocene. The peculiar geology of the area also suggests that these faults have generated earthquakes associated with surface faulting, namely the occurrence of clastic dykes injected within the fault zones during earthquake-induced liquefaction.Our results may help to address the current lack of understanding concerning earthquake activity in the slowly deforming region, and improve the current knowledge of the seismotectonic setting of the Siena Basin and the corresponding part of the inner northern Apennines. Our findings hint to a still unexplored tectonic mechanism, which suggests that the earthquakes affecting this part of southern Tuscany may be caused by segments of rather elusive, very long, SW-NE and WNW-ESE lineaments crossing the entire Apennine stack.