GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.2 187 The Fucino Plain. The Fucino plain (Central Italy) is an intramontane basin formed by the extensional tectonics that affected the central-western Apennine range during Late Pliocene (Fig. 1). This tectonic depression is filled by about ~1000 m of Upper Pliocene-Holocene lacustrine and alluvial sediments that unconformably overlie Meso-Cenozoic carbonate substratum (Bosi et al. , 1995; Cavinato et al. , 2002). The plain is bordered and crossed by a network of buried and/or exposed faults and is characterised by high seismic activity (the plain was struck by the Avezzano earthquake of 13 January 1915). The fault activity is testified by the terraced Middle-Upper Pleistocene alluvial fan and fluvial-lacustrine deposits hanging for hundreds of meters above the present valley floor in the north and north-eastern borders of the basin (Galadini et al. , 1997; Cavinato et al. , 2002). The main faults recognized in the Fucino basin are: (1) the Avezzano-Celano (ACF-inferred) fault (ENE trending, SE dipping) in the northern sector; (2) the San Benedetto Gioia dei Marsi fault (SBGMF), reactivated during the 1915 Avezzano earthquake, and the Statale Marsicana (SMF) fault in the eastern sector; (3) the Ortucchio fault (OF-inferred) near the center of the basin; and (4) the Trasacco (TF) and Luco dei Marsi (LF) faults (NW trending, SW dipping) in the southwestern sector (Cavinato et al. , 2002). The vertical offset across the SBGMF in the Middle and Late Pleistocene very likely exceeds 300 m, providing a slip rate of about 1 mm yr-1 (Cara et al. , 2011). Other seismo- induced geological phenomena (i.e. liquefaction and gas and water emissions) were observed inside the plain (Fig.1). Results. A large number of soil gas samples (more than 1200) were collected in the plain and analysed for different gas species (Rn, CO 2 , He and CH 4 ). The soil gas sampling was accomplished by a consolidated procedure reported in Ciotoli et al., 2014. Collected samples are then analyzed by using statistical and geostatistical techniques in order to define the anomaly threshold and construct the soil gas distribution maps. Results, discussed in Ciotoli et al., 2007, highlighted the presence of linear gas anomalies both in correspondence of known and visible faults of the eastern border of the plain (SBGMF), as well as provided clear indication of the presence of buried faults (OF and TF) in correspondence of different cover thickness in the middle of the plain. Geostatistical analysis of radon data provided a correlation between the anisotropic shape and the orientation of radon anomalies, and the different geometry of the faults: broader low anomalies along the TF linked to a wider and low permeability fracture zone in the western sector; highest values and more sharp anomalies along the high permeability fractured zone of the SBGMF in the eastern side of the plain. The re-interpretation of the radon data in the light of the new samplings confirms the presence of anisotropic radon distribution in correspondence of exposed San Benedetto dei Marsi Fault (SBGMF), as well as provided clear indication of the presence of buried Ortucchio Fault (OF) and Trasacco Fault (TF) in the middle of the plain, and Avezzano-Celano Fault (AFC) to the north (Fig. 2). Discussion. The proximity to the fault plane and the bedrock lithology are the main factors controlling the shallow radon emissions. Radon anomalies in the Fucino plain produced a consistent and clear anisotropic distribution that enabled to infer the location of the fault zones (Ciotoli et al. , 2007) (Fig. 2). According to literature data, faults are accompanied by Rn anomalies having a simple shape with the maximum values above the main fault and the minimum values on the fault margins (King et al. , 1996). However, many studies highlight that Rn anomalies above faults vary in intensities and shapes, as well as radon peak values can assume different spatial position within the fault zone (Ciotoli et al. , 2016; Seminsky et al. , 2014). The spatial irregular distribution of radon concentrations is predetermined by the more or less complex geometry of the fault zones and their activity, as well as by the volume of fractured rock involved (Annunziatellis et al. , 2008). In general, the evolution of the fault zone is characterized by the stepwise development of a different number of fault planes and of a variable volume of fractured rocks across and along strike. In fact, the process of fault evolution provides a progressive linkage mechanism among many small fault segments, to satisfy a strain localization process. At the early stages, there