GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 by thick outwash fan deposits and massive moraines, lying on top of lacustrine deposits. Nevertheless, these preliminary investigations highlight that a global information coverage through fully 3-D geophysical imaging is fundamental to reconstruct the basin structure and its tectonic evolution. For this purpose, in 2021-2022 we carried out an extensive 3-D deep electrical resistivity tomography (DERT) survey using the innovative Fullwaver technology, a technique successfully used to image the Castelluccio Basin subsurface, in the epicentral region of the 30 October 2016 Mw 6.5 Norcia earthquake (Central Italy - Sapia et al., 2021). Our goals were threefold: (a) map the subsurface geometry of the pre-Quaternary substratum and the infill structure down to ∼ 800 m depth, b) image the overstep region between the two main fault segments bounding the basin, c) map possible blind fault splays and understanding their relations with the MCF and MOF faults. In particular, the 3-D survey covered a ∼ 30 km 2 wide area and it was designed to map the whole basin structure taking into account the topography and local logistics. To design the best acquisition geometry considering the survey goals, the size and the large-scale geometry of the basin, we performed forward modeling and synthetic tests. We designed the survey to achieve a high sensitivity down to about 600 m and a minimum spatial resolution of about 50 m over the entire basin. We divided the survey area into two sub-regions: Campo Felice Plain and Camardosa Lake, respectively. Within each area, we used 36 independent digital receivers to record the electrical field generated by a 5 kW time-domain induced polarization transmitter through several current injections. Dipolar receivers, 50 m long, were deployed along a pair of parallel lines with an average distance of about 200 m. Current transmissions were injected with spacings of about 500 m in forward and reverse configurations between each pair of receiver lines and the array was progressively laterally shifted with the same geometric configuration, to cover the entire study area. We collected a set of 104 current injections in the Campo Felice Plain and a total of 45 transmissions in the Camardosa Lake sub-area (Fig. 1). We applied a 25 x 25 x 12.5 m mesh size in xyz directions generating about 2 million cells (254x114x68 including background), for 650 electrode positions which gave rise to a total of ∼ 20 thousand measurement points. All datasets were then merged and used in the same 3-D inversion scheme, based on a smoothed least square routine with a uniform resistivity starting model (details in Sapia et al., 2021). Results confirm that the CFB has a complex structure mostly controlled by the two main left-stepping MCF and MOF faults. We recognize thick low-resistivity regions, interpreted as the response of alluvial and lacustrine infill elongated parallel to the faults and depict nearly elliptical depocentres, with maximum depths of about 300 m (Camardosa Lake) and 600 m (Campo Felice Plain). A N-W trending lineament, parallel to the MCF fault, cuts the basin in its central portion as evidenced by a sharp resistivity contrast in the 3-D model. This structure has no morphological expression on the surface and can be interpreted as a NE-dipping normal fault, possibly an antithetic splay of the MCF. Local complexities of the resistivity model suggest the occurrence of small faults with different orientations. For example, we recognize a N-S trending resistivity contrast bounding the Camardosa Lake depocenter, which we interpret as a relay fault in the overstep region between the MOF and MCF. The 3-D survey was then complemented by a ∼ 2 km high-resolution 2-D transect as well as ancillary 2-D ERT profiles, acquired in sectors of crucial