GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Figure 1. Map view of the Sansepolcro basin fault system with the locations of the paleoseismic investigations sites and historical earthquakes macroseismic epicenters. Geologic Setting The AF is a segmented fault made up of at least 2 different splays (Fig. 1). The western splay is located within the Pleistocene gravels and sands of the NW-SE-elongated Anghiari ridge. The eastern splay runs at the base of the ridge, juxtaposing the Pleistocene units of the Anghiari ridge against late Quaternary colluvial and alluvial deposits. Thanks to the geomorphological expression and some displaced geological markers, such us the depositional unit outcropping in the Anghiari ridge (Citerna and Monterchi units) and an overlying paleosol, the upper splay is easily identifiable. These units are commonly attributed to the Middle Pleistocene (Cattuto et al, 1995; Pucci et al, 2014; Benvenuti et al, 2016), but their ages are poorly constrained. Moreover, in the northern sector of the ridge, the upper splay affects the outcropping Jurassic Ophiolite rocks belonging to the Ligurian tectonic unit. The lower splay is buried by colluvial deposits and alluvial fans and for this reason it is not easily recognizable. The capability of the ATF and of its splays, including the AF studied in this work, to nucleate strong earthquakes is debated in the literature. The Sansepolcro basin was struck by several moderate to strong historical earthquakes (Fig. 1) while the instrumental seismicity is relatively low. An opposite behaviour characterizes the southern Upper Tiber Valley, with very high background and microseismic activity and almost no