Volcano-tectonic, electrical and electromagnetic investigations to highlight the structure of the most active sector of Campi Flegrei caldera

The Solfatara-Pisciarelli area, within the Campi Flegrei caldera, represents the site of small phreatic, phreatomagmatic and effusive eruptions, which emplaced domes and crypto-domes. Despite a significant number of scientific studies devoted to such an area, the deeper feeding system of the Pisciarelli hydrothermal field, its relation to the Solfatara system, and the main structures governing the fluid rising still represent open problems.The present contribution aims to detail the surface and buried volcano-tectonic structures and their interaction with hydrothermal fluids in the Solfatara-Pisciarelli area and characterize the presently unknown feeding zones. Geological-structural field surveys permitted the reconstruction of the geological map of the area and the implementation of the fault and fracture orientation and kinematic dataset. Additionally, a series of Electrical resistivity tomographies (ERT), carried out along profiles of different lengths, detailed the structure up to about 100 m depth. The detected patterns of electrical resistivity anomalies helps to define the main structural lineaments of the investigated sector, particularly the presence of normal faults, which results in the presence of sub-vertical resistivity discontinuities. The combination of the ERT and the geo-volcanological and structural survey results allowed the reconstruction of geological sections showing the main structures that characterize the Solfatara and Pisciarelli area. Finally, an Audio-MagnetoTelluric (AMT) survey was carried out in the central sector of the Campi Flegrei caldera to obtain information on the deeper feeding system of the Pisciarelli fumarolic field and its relations with that of the Solfatara and the volcano-tectonic structures of the area. The AMT survey comprised a series of electromagnetic measurements in 47 different sites. The subsequent data inversion produced a 3D model, which identified the electrical resistivity pattern of the investigated structure down to a depth of 2.5 km below sea level. Such a 3D model, which represents the first three-dimensional electromagnetic image of the first few kilometres of the central sector of the Phlegraean area, highlights the presence of significant anomalies related to distinct processes and physical conditions in the system. Remarkably, the main volcano-tectonic structures already hypothesized by shallower electrical surveys are detected by the AMT survey, which results describe their development in depth, identifying at the same time the main structures playing a significant role in the ongoing dynamics of the investigated area.The proposed combination of shallow ERT, deeper AMT and geological-structural field surveys suggests a possible paradigm for studies on the volcano-tectonic characterization of hydrothermal systems,  due to the good capability to shed light on their evolution. Furthermore, although the intensive monitoring already realized by the INGV-OV surveillance system in the Solfatara-Pisciarelli area, the reiteration of the proposed combination of surveys could reveal helpful to detect changes in the relationships between the faults and the hydrothermal fluid circulation. Our approach could also be of interest to other similar systems, which could steer toward unrest states compatible with impulsive events, such as hydrothermal and phreatic explosions, as recorded worldwide in several cases, also recently.