Abstract: In this chapter, we review the state-of-the-art of the Campi Flegrei caldera (Naples) hydrothermal system, and its behaviour during the last decades. The Campi Flegrei caldera has been undergoing unrest since 1950, as evidenced by recurrent bradyseismic episodes accompanied by manifest changes in the degassing budget, degassing patterns and in the composition of the fumarolic fluids. In-depth analysis of geochemical and geophysical datasets acquired over decades has allowed identification of the mechanisms driving volcanic unrest at the Campi Flegrei caldera. We propose a conceptual model of the hydrothermal system feeding Solfatara fumaroles, where geochemical information is integrated with Audio Magneto Telluric measurements, which yields a realistic picture of the geometry of the system up to a depth of 2.5 km. The model identifies a ~2 km elongated vertical high resistivity structure in axis with the Solfatara fumaroles, which represents a relatively high permeability zone allowing hot fluid ascent from depth to the shallower portions of the hydrothermal system. Pulsed injections of hot magmatic fluids (CO2-rich and CH4-poor oxidised fluids) at the bottom of the hydrothermal system is thought to be one of the key processes that has controlled the evolution of the system during the last 40 years. The episodes of injection of magmatic fluids changed in frequency and intensity during time, ultimately causing an overall heating and pressurisation of the system since the early 2000s, as reflected by escalating degassing flux, increase in areal extension of the degassing areas, and in the composition of the fumaroles. In particular, the CO2/CH4 and He/CH4 ratios of fumarolic fluids exhibited recurrent peaks, marking the episodes of injection of magmatic fluids. Moreover, the quasi-monotonic increasing trend of the fumarolic CO2/H2O ratio, from 0.15 to 0.18 in 2000 to ~0.4 in 2018–2019, has been interpreted as due to the combined action of partial steam condensation, and CO2 addition from a magmatic source and possibly from de-carbonation of hydrothermal calcite favoured by the heating of the hydrothermal reservoir. These changes strongly suggest that the ongoing (since 2000) unrest is triggered by a degassing magma source, but also that the system’s response is modulated by dynamics and structures of the overlying hydrothermal envelope. This evolution clearly requires careful scientific scrutiny and intensified monitoring in the years to come. © 2022, Springer-Verlag GmbH Germany, part of Springer Nature

Abstract: Modeling seismicity at volcanoes remains challenging as the processes that control seismic energy release due to fluid transport, heat flow, and rock deformation are firmly coupled in complex geological media. Here, we couple fluid-flow and mechanical (deformation) simulators (TOUGHREACT–FLAC3D) to reproduce fluid-induced seismicity at Campi Flegrei caldera (southern Italy) in isothermal (HM) and nonisothermal (THM) conditions. The unique ability of the Campi Flegrei caprock to withstand stress induced by hot-water injections is included in the model parametrization. After pore pressure accumulation is guided by a combination of thermal and hydromechanical interactions, contrasting compressive and extensional forces act on the basal and top parts of the caprock, respectively. Then, pressure perturbation and caprock deformation induce fractures that allow hot fluids uprising to pressurize the overlying fault, driving it toward failure and triggering seismicity. Under similar mechanical boundary conditions, the induced thermal effects prompt seismic slip earlier but with higher seismic magnitudes when (1) thermal equilibrium is preserved and (2) the thermal contrast is enhanced due to increased fluid injection temperatures. The results indicate that numerical models of volcano seismicity must consider the influence of rock-sealing formations to obtain more robust, accurate, and realistic seismic predictions at volcanoes. The proposed models satisfactorily reproduce the magnitude–depth distribution of the swarm (October 5, 2019), preceding the two strongest earthquakes recorded in 35 years at the caldera (3.1 and 3.3—on December 6, 2019, and April 26, 2020, respectively) using hot-water injection from depth. © 2021. The Authors.

Abstract: Temperature and composition at fumaroles are controlled by several volcanic and exogenous processes that operate on various time‐space scales. Here, we analyze fluctuations of temperature and chemical composition recorded at fumarolic vents in Solfatara (Campi Flegrei caldera, Italy) from December 1997 to December 2015, in order to better understand source(s) and driving processes. Applying the singular spectral analysis, we found that the trends explain the great part of the variance of the geochemical series but not of the temperature series. On the other hand, a common source, also shared by other geo‐indicators (ground deformation, seismicity, hydrogeological and meteorological data), seems to be linked with the oscillatory structure of the investigated signals. The informational characteristics of temperature and geochemical compositions, analyzed by using the Fisher–Shannon method, appear to be a sort of fingerprint of the different periodic structure. In fact, the oscillatory components were characterized by a wide range of significant periodicities nearly equally powerful that show a higher degree of entropy, indicating that changes are influenced by overlapped processes occurring at different scales with a rather similar intensity. The present study represents an advancement in the understanding of the dominant driving mechanisms of volcanic signals at fumaroles that might be also valid for other volcanic areas. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.