GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.2 114 around the present Solfatara maar-diatreme volcano (Isaia et al., 2009; 2015). Eruptive activity consists both of effusive events such as testified by Mt. Olibano and Accademia lava domes and by phreatic, phreatomagmatic and strombolian eruption as Santa Maria delle Grazie and Solfata- ra (Fig. 2). Finally, two large volcanoes (Astroni and Fossa Lupara; Fig. 1), located along a NW-SE trending fissure a few kilometers northward of the Solfatara crater, formed immediately after the Solfatara eruption (Di Vito et al., 1999; Isaia et al., 2004; 2009). The last historical eruption occurred in 1538 CE, forming the Monte Nuovo cone (Fig. 1; Di Vito et al., 1987; Guidoboni & Ciuccarelli, 2011). At the present, this sector of the Campi Flegrei caldera, is also characterized by intense degassing and hydrothermal activity (e.g., Cardellini et al., 2017; Chiodini et al., 2012) and by the highest values of surface fractures and fault density, particularly around the town of Pozzuoli and in the Solfatara area (Bevilacqua et al., 2015, 2020). Furthermore, ground move- ments (called bradyseism) on the order of 1-10 m have been detected since Roman times (more than 2,000 years ago; e.g., Bellucci et al., 2006; Parascandola et al., 1947) and have continued to the present (Del Gaudio et al., 2010; INGV, 2020) associated with the reactivation of main faults (e.g., Di Luccio et al., 2015; La Rocca and Galluzzo, 2019; Vitale et al., 2019). Most of the volcano- tectonic earthquakes (with Md>0.5; OV-INGV, 2020) of the 1982-1984 unrest event, as well as the seismic events that occurred from 2000 to 2020, were localized in the Solfatara sector, object of this study. Results and Conclusion Our geo-volcanological and structural studies, of this sector of Campi Flegrei caldera, reveal the occurrence of a great number of faults, which localize the main fumaroles and mud pools. More detailed structural study allowed us to identify two fault systems. The first is characterized by two orthogonal sets with Apennine and anti-Apennine trends, whereas the second by two orthogonal sets E-W and NNE-SSW oriented. Furthermore, the former fault system deforms the oldest deposits up to Agnano-Monte Spina succession, while the second affects the younger deposits. These structures localize on the surface the main fumaroles and mud pool such us Fangaia, Bocca Grande and Bocca Nuova in the Solfatara crater and the Pisciarelli mud pool. The structural survey together with the reconstruction in depth of faults resulting from geo- logical cross-section and electrical resistivity tomography allow us to produce fault displacement and fracture density maps. The fault displacement map shows high values (tens of meters) for the master faults with Apennine and anti-Apennine trend (NW-SE and NE-SW); whereas low va- lues of displacement (few meters) were recorded for N-S and E-W trending faults. The fractu- re density map shows few fractures for meter in sectors far from faults (low fracture density); whereas tens of fractures for meters (high fracture density) were recorded close to fault planes and mostly at their intersection. Further surficial analyses were carried out by means of a self- potential survey, that led to a self-potential (SP) map showing widespread anomalies, related to fluid circulation, characterized by high values; however low values of SP were detected for the north-western sector including part of the Solfatara volcano. Abrupt changes between positive and negative anomalies follow the same trends of faults detected by means of structural and geoelectrical surveys. The results of the electrical investigations allowed to further highlight the complex structu- ral array consisting of normal faults with surficial expression, blind normal faults, and related damage zones, as well as to disclose the relationships among rising hydrothermal fluids and the mapped structures. ERT results have been used to distinguish between fluids storage zones and upstream-flowing areas. The reconstructed structural images (Fig. 3) allow to interpret the complex hydrothermal