GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 strong historical earthquakes. This allowed previous authors to hypothesize different seismogenic behaviours of the ATF in the Sansepolcro basin and south of it. Brozzetti et al (2009) suggested that the ATF could be creeping, in the southern sector and able to nucleate large earthquakes in the northern sector. According to the damage distribution, these authors suggest that the 1389, 1458 and 1789 (Fig. 1) earthquakes could have nucleated along the Città di Castello seismogenic source, which includes the AF. However, there is no palaeoseismological evidence to support these assumptions and there are no field-based chronologic constraints on the AF activity during the late Quaternary. Results We mapped the fault using a 1 m DSM from Pleiades images and we carried out a GPR and seismic reflection survey to select suitable sites for paleoseismological trenching across different splays of AF. More precisely we selected two sites across the upper splay (Villa Sterpeto and Todari rod-cut sites) and one site across the basal AF splay (Anghiari site). The Villa Sterpeto site At the Villa Sterpeto site we dug a 50 m-long trench across the upper splay of the AF. The Orto-photomosaic of the trench, limited to the fault zone exposure, is shown in fig. 2.. A narrow fault zone ( ≥ 15 cm thick) juxtaposes the Citerna unit against a colluvial unit overlaying the Monterchi unit. The colluvial unit is made up of a sequence of stacked coarse-grained colluvial wedges overlapped with each other and interfingered with fine-grained colluvium. We mapped at least 4 coarse-grained colluvial wedges. A fifth possible colluvial wedge is partially visible in the lower part of the trench wall. The maximum vertical thickness of the wedges is 45 cm on average. We interpreted these wedges as scarp-derived colluvial units sourced by surface faulting episodes. In order to bracket the ages of the earthquakes, we sampled the fine-grained pedogenized colluvium beneath each wedge. Only the radiocarbon dating of the bulk organic fraction of the sediments was possible due to the absence of charcoals. According to the results of the radiocarbon dating the colluvial wedges are between 23 and 4 ky BCE. The two events responsible for the formation of the two youngest colluvial wedges occurred between 11 and 4 ka BCE (CW1, E1 VS) and between 13 and 11 ka BCE (CW2, E2 VS), respectively. Since CW1 is faulted, at least one event occurred after the formation of CW1. Since the age of two samples below the wedges CW3 and CW4 are reversed, we propose two different hypotheses for the age of the two responsible events. According to the hypothesis 1, the two events responsible for the formation of the wedges CW3 (E3 VS) and CW4 (E4 VS) occurred both between 14.1 and 13.3 ka BCE, without better time constraints. According to the hypothesis 2 E3 VS occurred between 15.2 and 13.3 ka BCE (E3 VS) and E4 VS occurred between 23.1 and 15.2 ka BCE.