GNGTS 2021 - Atti del 39° Convegno Nazionale

61 GNGTS 2021 S essione 1.1 to consider these slopes as the result of repeated coseismic surface faulting events over the Quaternary. The long-term, Mid Pleistocene to Present slip rates for the faults of the Rieti basin and for the Leonessa fault have been estimated to be of the order of 0.2 - 0.5 mm / year, in good agreement with the Holocene slip rates measured through paleoseismic analysis (Michetti et al ., 1995; Roberts and Michetti, 2004; Cowie et al ., 2017; Archer et al ., 2019; Faure-Walker et al ., 2021). The historical and instrumental database records only moderate seismicity with maximum intensities MCS of about VIII (Bernardini et al ., 2013). Poorly documented events with a larger epicentral intensity (IX-X MCS) occurred in 76 and 1298 AD. Methods: Geological investigations We mapped the area in Figure 1b and 1c at 1:5000 scale, based on fieldwork and available literature data. We compiled information on over 38 (Cantalice; Fig. 1b) and 27 (Rivodutri; Fig. 1c) observations point, including structural analyses, when possible. We conducted mor- photectonic analyses through the interpretation of aerial photo coverages (1944, 1954 and 1988-89) and the processing of Lidar data (1 m resolution). After detailed site selection for exploratory trenching, we excavated FAC traces at a) the Apoleggia site (two trenches, 30 + 20 m long; Fig. 1c and Fig. 3), and b) the Cruciano site (three trenches, 30 + 30 + 35 m long). There we conducted microtopographic mapping of the fault scarp, using also Geodetic GPS (Topcon GB1000) and iPadPro LiDAR scanner. The structure from motion(SFM) method was used to photograph the trench walls with an overlap rate between photos of >70%. Trench wall stratig- raphy was mapped also through paleopedology analysis. We collected 13 radiocarbon samples from the faulted and unfaulted strata of the five trenches and sent them for AMS dating, which is in progress. Geophysical investigation The protocol adopted provided for the use of three survey techniques: Electrical Resistivity Tomography – ERT to investigate, at different resolutions, the subsurface down to a depth of maximum 50 m; while Capacitive-Coupled Resistivity Tomography (CCR) and Ground Penetrating Radar (GPR) have been used to analyze the shallow subsurface in great detail, down to a depth of ca. 5-6 m. Ideally, this should allow to obtain an exhaustive geophysical image of the terrain and reduce the natural uncertainty that characterizes each individual survey technique. Along the San Liberato Fault (Fig. 1b) we acquired 8 ERT profiles (total length ca. 980 m); the profiles at the Cruciano trench site have been repeated with the GPR technique (total length ca. 170 m). Along the Villaggio Santa Maria – Apoleggia – Piedicolle fault segments (Fig. 1c) 16 ERT profiles were acquired (total length of ca. 2460 m); we also acquired 1 CCR profile (length of ca. 380 m) along the Piedicolle alluvial fan; at the Apoleggia trench site and near Piedicolle we also acquired GPR profiles for a total length of ca. 2420 m. For the ERT we adopted the dipole - dipole and the Wenner-Schlumberger electrode configu- rations, in order to better emphasize both vertical electrical passages and sub-horizontal geom- etries, while still guaranteeing a good depth of investigation. A single-channel Syscal R2 georesis- tivity meter from IRIS Instrumentum was used for classical electrical tomography, equipped with a set of 2 m pitch cables capable of handling up to a maximum of 64 electrodes. For CCR Tomog- raphy we used the OhmMapper system of Geometrics, equipped with a set of 5 dipole receivers and a 5 m transmitter capable of quickly mapping the resistivity of the subsoil up to a depth of about 6 m with respect to the pc. As for the GPR technique, we used a GSSI Sir4000 instrument, equipped with a digital antenna with a central frequency of 200 MHz and coupled with a geodetic GPS (Topcon GB1000) in differential set-up.