GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Figure 3 . Interpreted orthophoto-mosaic of the Todari road-cut trench.. The Anghiari site We dug a 55 m-long trench across the lower splay of the AF, a few hundred meters north of the Anghiari town. Here there are two main synthetic fault zones and a secondary antithetic footwall splay. The fault zones are sealed by a colluvial deposit containing pottery shards the age of which ranges between XII and XIV century CE. At the hanging wall of the main fault zone there are two wedge-shaped colluvial units overlaying a colluvium dated around 3-3.3 ka BCE, thanks to radiocarbon dating of charcoals. In our interpretation, there is evidence of at least one, and possibly two, surface faulting event(s) during the last 3-3.3 ka, probably coincident with the youngest event(s) that ruptured the Todari road-cut site. Discussion and conclusions Putting together the novel data achieved thanks to the paleosismological investigations we rebuilt the slip history of the AF during the last 22 ka BCE, which includes at least seven surface faulting earthquakes. The resulting recurrence time ranges between 3200 and 2500 years and the vertical slip rate ranges from 0.2 and 0.3 mm/yr. At the Villa Sterpeto site, the minimum vertical displacement of each faulting event can be estimated thanks to the maximum vertical thickness of the wedges, which is on average up to 45 cm. Moreover, the age of the youngest earthquake derived by this study is close to the age of two historical earthquakes in the Sansepolcro basin: the 1458 (Mw 5.8, I 0 (MCS) VIII-IX) and the 1489 (Mw 5.1, I 0 MCS VII) earthquakes (Fig. 1). In conclusion we proved that the AF experienced repeated surface faulting events in the last 20 ka, demonstrating its capability to generate surface faulting earthquakes.