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Shallow magma transfer is difficult to detect at poorly monitored volcanoes. Magma transfer before the last 1538 eruption at Campi Flegrei caldera (Italy) was exceptionally tracked using historical, archeological, and geological data. Here, we extend that data set to 1650 to uncover any magma transfer during post‐eruptive subsidence. Results show two post‐eruptive subsidence phases, separated by a previously undocumented uplift during 1540–1582. Uplift highlights the pressurization of the central (∼3.5 km depth) and peripheral (∼1 km depth) pre‐eruptive sources, suggesting an aborted eruption. The subsidence events mainly require the depressurization of the central source and pressurization of a deeper magmatic layer (∼8 km depth). Therefore, despite the overall post‐eruptive deflation, after 1538 the deeper reservoir experienced continuous magma supply, with magma almost erupting between 1540 and 1582, challenging the common assumption of post‐eruptive deflation. This underlies the importance of monitoring the deeper magmatic systems, also after eruptions, to properly assess their eruptive potential. Plain Language Summary: Today, volcanic activity is monitored by satellite and ground networks. However, very little is known about the pre‐ and post‐eruptive behavior of volcanoes before the instrumental era. Here, we present a unique set of archeological records related to the elevation changes at Campi Flegrei caldera (Italy) during 1515–1650, mainly focusing on its behavior after the last 1538 eruption at Monte Nuovo. After the eruption, subsidence occurred from 1538 to 1540, followed, from 1540 to 1582, by a previously unreported uplift; the latter was followed by renewed subsidence until 1650 at least. Modeling the sources responsible for the deformation, we find that, despite the depressurization of shallow sources, after 1538, a deeper magmatic layer (∼8 km depth) experienced continuous magma supply, with magma almost erupting between 1540 and 1582. This questions the common notion of post‐1538 deflation at Campi Flegrei, where the depressurization of the shallower sources masks the pressurization of the deeper magmatic system for more than a century. Key Points: 20 archeological sites at Campi Flegrei show two post‐1538 eruption subsidence phases, separated by an undocumented uplift in 1540–1582During 1540–1582, a central sill‐like source (∼3.5 km depth) transfers magma below Monte Nuovo, representing an aborted eruptionFrom 1538 to 1650 a deeper magmatic layer (∼8 km depth) experienced continuous magma supply, also during caldera subsidence [ABSTRACT FROM AUTHOR]

Seafloor deformation monitoring is now routinely performed in the marine sector of the Campi Flegrei volcanic area (Southern Italy). The MEDUSA infrastructure is formed by four buoys deployed at a water depth ranging from 40 to 96 m, and equipped with cGPS receivers, accelerometers and magnetic compasses to monitor the buoy status and a seafloor module with a bottom pressure recorder and other onboard instruments. The analysis of the time series data acquired by the MEDUSA monitoring infrastructure system allows to study the seafloor deformation in the Campi Flegrei caldera with geodetic accuracy. In a previous work, we show that the time series acquired by the Campi Flegrei cGPS onland network and MEDUSA over the period 2017–2020 are in good agreement with the ground deformation field predicted by a Mogi model which is widely used to describe the observed deformation of an active volcano in terms of magma intrusion. Only for one of the buoys, CFBA (A), the data differ significantly from the model prediction, at a level of ≃ 6.9 σ and of ≃ 23.7 σ for the seafloor horizontal speed and direction, respectively. For this reason, we devised a new method to reconstruct the horizontal sea bottom displacement considering in the analysis both cGPS and compass data. The method, applied to the CFBA buoy measurements and validated also on the CFBC (C) buoy, uses compass data to correct cGPS positions accounting for the pole inclination. Including also systematic errors, the internal consistency, always within ∼ 3 σ for the speed and ∼ 2 σ for the angle, between the results derived for different maximum inclinations of the buoy pole (up to 3.5 ∘ ) indicates that the method allows to significantly reduce the impact of the pole inclination which, if not properly taken into account, can alter the estimation of the horizontal seafloor deformation. In particular, we find a good convergence of the retrieved velocity and deformation angle as we include in the analysis data from increasing values of the buoy pole inclination. Taking the result derived assuming the maximum allowed cutoff and accounting for statistical and systematic errors, we found a speed v = (3.521 ± 0.039 (stat) ± 0.352 (syst)) cm/yr and a deformation direction angle α = ( - 115.159 ± 0.670 (stat) ± 7.630 (syst)) ∘ (statistical errors at 1 σ quoted from the rms of their values, main systematic errors added linearly). The relative impact of the main potential systematic (statistical) effects increases (decreases) with the cutoff. Our analysis provides a horizontal speed consistent with the model at a level of ≃ 5.2 σ (stat only) or of ≃ 0.5 σ (stat and syst added linearly), and a deformation angle consistent with the model at ≃ 4.3 σ level (stat only) or at ≃ 0.3 σ level (stat and syst added linearly). Correspondingly, the module of the vectorial difference between the velocity retrieved from the data and the velocity of the adopted Mogi model diminishes by a factor of ≃ 7.65 ± 1.23 (stat) or ± 5.78 (stat + syst) with respect to the previous work. A list of potential improvements to be implemented in the system and instruments is also discussed. [ABSTRACT FROM AUTHOR]

Abstract We review pre-eruptive dynamics and evidence of open-system behavior in the volcanic plumbing system beneath Campi Flegrei Caldera, together with estimates of magma residence time, magma ascent, and mixing-to-eruption timescales. In detail, we compile pre- and syn-eruptive dynamics reported in the literature for (a) the Campanian Ignimbrite ~ 40 ka, (b) the Neapolitan Yellow Tuff (~ 15 ka), and (c) the recent activity within the Phlegrean area. We first summarize geochemical and textural evidence (e.g., magma mixing, crystal disequilibria, vertical zonings, and isotopic records) of open-system behavior for the pyroclasts erupted in the last 40 ky at Campi Flegrei Caldera. We show that the fingerprint of open-system dynamics is ubiquitous in the deposits associated with the volcanic activity at the Campi Flegrei Caldera in the last 40 ky. Then, we describe the results of geophysical and petrological investigations that allow us to hypothesize the structure of the magma feeding system. We point to a trans-crustal magmatic feeding system characterized by a main storage reservoir hosted at ~ 9 km that feeds and interacts with shallow reservoirs, mainly placed at 2–4 km. Finally, we define a scenario depicting pre-eruptive dynamics of a possible future eruption and provide new constraints on timescales of magma ascent with a physical model based on magma-driven ascending dyke theory. Results show that considerably fast ascent velocities (i.e., of the order of m/s) can be easily achieved for eruptions fed by both shallow (i.e., 3–4 km) and deep (i.e., ~ 9 km) reservoirs. Comparing the results from experimental and numerical methods, it emerges that mixing-to-eruption timescales occurring at shallow reservoirs could be on the order of minutes to hours. Finally, we highlight the volcanological implications of our timescale estimates for magma ascent and mixing to eruption. In particular, explosive eruptions could begin with little physical ‘warning’, of the order of days to months. In this case, the onset of volatile saturation might provide pre-eruptive indicators. Graphical Abstract

Abstract Shallow magma transfer is difficult to detect at poorly monitored volcanoes. Magma transfer before the last 1538 eruption at Campi Flegrei caldera (Italy) was exceptionally tracked using historical, archeological, and geological data. Here, we extend that data set to 1650 to uncover any magma transfer during post‐eruptive subsidence. Results show two post‐eruptive subsidence phases, separated by a previously undocumented uplift during 1540–1582. Uplift highlights the pressurization of the central (∼3.5 km depth) and peripheral (∼1 km depth) pre‐eruptive sources, suggesting an aborted eruption. The subsidence events mainly require the depressurization of the central source and pressurization of a deeper magmatic layer (∼8 km depth). Therefore, despite the overall post‐eruptive deflation, after 1538 the deeper reservoir experienced continuous magma supply, with magma almost erupting between 1540 and 1582, challenging the common assumption of post‐eruptive deflation. This underlies the importance of monitoring the deeper magmatic systems, also after eruptions, to properly assess their eruptive potential.

We review pre-eruptive dynamics and evidence of open-system behavior in the volcanic plumbing system beneath Campi Flegrei Caldera, together with estimates of magma residence time, magma ascent, and mixing-to-eruption timescales. In detail, we compile pre- and syn-eruptive dynamics reported in the literature for (a) the Campanian Ignimbrite ~ 40 ka, (b) the Neapolitan Yellow Tuff (~ 15 ka), and (c) the recent activity within the Phlegrean area. We first summarize geochemical and textural evidence (e.g., magma mixing, crystal disequilibria, vertical zonings, and isotopic records) of open-system behavior for the pyroclasts erupted in the last 40 ky at Campi Flegrei Caldera. We show that the fingerprint of open-system dynamics is ubiquitous in the deposits associated with the volcanic activity at the Campi Flegrei Caldera in the last 40 ky. Then, we describe the results of geophysical and petrological investigations that allow us to hypothesize the structure of the magma feeding system. We point to a trans-crustal magmatic feeding system characterized by a main storage reservoir hosted at ~ 9 km that feeds and interacts with shallow reservoirs, mainly placed at 2–4 km. Finally, we define a scenario depicting pre-eruptive dynamics of a possible future eruption and provide new constraints on timescales of magma ascent with a physical model based on magma-driven ascending dyke theory. Results show that considerably fast ascent velocities (i.e., of the order of m/s) can be easily achieved for eruptions fed by both shallow (i.e., 3–4 km) and deep (i.e., ~ 9 km) reservoirs. Comparing the results from experimental and numerical methods, it emerges that mixing-to-eruption timescales occurring at shallow reservoirs could be on the order of minutes to hours. Finally, we highlight the volcanological implications of our timescale estimates for magma ascent and mixing to eruption. In particular, explosive eruptions could begin with little physical 'warning', of the order of days to months. In this case, the onset of volatile saturation might provide pre-eruptive indicators. [ABSTRACT FROM AUTHOR]

Despite its known reconstructed volcanic history, the structural setting and present state of the Astroni Volcano of the Campi Flegrei caldera in Italy are still poorly defined. Through structural, geophysical, and geochemical investigations, we elucidate the structure and present volcanic activity of the Astroni Volcano, which hosts tuff cones, scoriae cones, lava domes, and lakes on the crater floor. A volcano‐tectonic analysis focused on the entire volcano edifice, coupled with electrical resistivity tomography of the shallower part of the Astroni crater, revealed the main rock formations, faults, and possible fluid patterns within the first 150 m depth. Two main NE–SW and NW–SE trending fault sets were imaged using electrical resistivity modeling and measurements along the wall of the volcanic edifice; they likely delimit a maar‐like structure resulting from the highest energetic subplinian Astroni 6 eruption event and acted as magma pathways during the late eruptive activity stage. A 3D view of the reconstructed resistivity model revealed both deep root‐conduit‐like structures and shallower dome‐like shapes for volcanic edifices on the crater floor. Gas and carbon compositions in the NNE sectors of the Astroni Lago Grande are similar to those of the Solfatara fumarole fluids, suggesting common hydrothermal origin and a possible link with a deep hydrothermal reservoir. This fluid‐emission area along the border of the younger volcanic structure exhibits a +40°C maximum soil‐temperature anomaly. The proposed volcano‐tectonic architecture should improve the unrest scenarios in case of reactivation in this Campi Flegrei caldera sector and the monitoring strategies for the Astroni Volcano. Plain Language Summary: The Campi Flegrei caldera (Southern Italy) is one of the most productive explosive volcanic systems worldwide. The central‐eastern sector of the caldera has recently been the location of several major volcanic vents, including the Astroni Volcano. It is very close to the Solfatara‐Pisciarelli area, where the most significant energy release and largest variation of the volcanic and seismic activities characterize the ongoing unrest crisis of the Campi Flegrei. In this study, we aim to provide new insights regarding the structure of the Astroni Volcano and elucidate the present volcanic activity through a multidisciplinary approach that merges volcanological, structural, geophysical, and geochemical data. A volcano‐tectonic analysis, coupled with a 3D electrical resistivity tomography, allowed us to define in detail the relation between the volcano‐tectonic structures, rock bodies, and fluid circulation within the first 150 m depth of the Astroni crater. We describe for the first time the hydrothermal emission activity and thermal anomaly within the Astroni crater and suggest the possible involvement of the geothermal reservoir. These results will help better evaluate the unrest scenarios in case of reactivation in this Campi Flegrei caldera sector and implement effective monitoring strategies for the Astroni Volcano. Key Points: Structural, geophysical, and geochemical investigations elucidate the structure and present volcanic activity of the Astroni VolcanoWe indicate the locations where we first detected hydrothermal emissions and temperature anomaliesResistivity anomalies and identified structural lineaments suggest the presence of active structures within the Astroni Volcano [ABSTRACT FROM AUTHOR]

Abstract Despite its known reconstructed volcanic history, the structural setting and present state of the Astroni Volcano of the Campi Flegrei caldera in Italy are still poorly defined. Through structural, geophysical, and geochemical investigations, we elucidate the structure and present volcanic activity of the Astroni Volcano, which hosts tuff cones, scoriae cones, lava domes, and lakes on the crater floor. A volcano‐tectonic analysis focused on the entire volcano edifice, coupled with electrical resistivity tomography of the shallower part of the Astroni crater, revealed the main rock formations, faults, and possible fluid patterns within the first 150 m depth. Two main NE–SW and NW–SE trending fault sets were imaged using electrical resistivity modeling and measurements along the wall of the volcanic edifice; they likely delimit a maar‐like structure resulting from the highest energetic subplinian Astroni 6 eruption event and acted as magma pathways during the late eruptive activity stage. A 3D view of the reconstructed resistivity model revealed both deep root‐conduit‐like structures and shallower dome‐like shapes for volcanic edifices on the crater floor. Gas and carbon compositions in the NNE sectors of the Astroni Lago Grande are similar to those of the Solfatara fumarole fluids, suggesting common hydrothermal origin and a possible link with a deep hydrothermal reservoir. This fluid‐emission area along the border of the younger volcanic structure exhibits a +40°C maximum soil‐temperature anomaly. The proposed volcano‐tectonic architecture should improve the unrest scenarios in case of reactivation in this Campi Flegrei caldera sector and the monitoring strategies for the Astroni Volcano.

Understanding the characteristics of a volcanic system is always important and becomes crucial when the volcano is in pluri-decadal unrest and located in a densely populated area, such as Campi Flegrei caldera, Italy. Ground deformation is a powerful source of information for studying the activity of magmatic sources and hydrothermal systems, even when difficult to detect otherwise. Here, we use ground displacement from ERS-ENVISAT and Sentinel-1A SAR images to investigate the 1993–2000 subsidence and part (2015–2022) of the present unrest. For each of the two time periods, we combine the line-of-sight displacements to obtain vertical and eastward displacements, and apply the empirical orthogonal function analysis to these latter time series—treated as a single data set—to decompose space-time fields into separated modes, consisting of uncorrelated spatial patterns and associated temporal evolutions. We only retain the first mode, since it captures the main deformation during both investigated periods, is the sole mode related to long-lasting (years) processes, and is less affected by noise than original data. Our analyses: (i) Confirm that most of the deformation is related to the activity of a 3–4 km deep sill-like source, which is inflated by magma and/or magmatic fluids during periods of unrest and deflates during periods of subsidence; (ii) Evidence ongoing deformation linked to local fluid migration in the Solfatara area; (iii) Identify persistent deformation features where peculiar fluid migration processes occurred during the 1982–1984 unrest; (iv) Most importantly, provide direct evidence of deep magma inflation at least since 2015, thus giving a strong warning of increasing risk at Campi Flegrei. Results demonstrate the capability of our approach to disclose hardly detectable processes and suggest a tool to monitor the activity of the deep magmatic source. Our approach can be useful also in other volcanic systems. [ABSTRACT FROM AUTHOR]

The aim of this research is to identify an efficient model to describe the fluctuations around the trend of the soil temperatures monitored in the volcanic caldera of the Campi Flegrei area in Naples (Italy). This study focuses on the data concerning the temperatures in the mentioned area through a seven-year period. The research is initially finalized to identify the deterministic component of the model given by the seasonal trend of the temperatures, which is obtained through an adapted regression method on the time series. Subsequently, the stochastic component from the time series is tested to represent a fractional Brownian motion (fBm). An estimation based on the periodogram of the data is used to estabilish that the data series follows an fBm motion rather than fractional Gaussian noise. An estimation of the Hurst exponent H of the process is also obtained. Finally, an inference test based on the detrended moving average of the data is adopted in order to assess the hypothesis that the time series follows a suitably estimated fBm. [ABSTRACT FROM AUTHOR]

The complex sequential compositional zoning of clinopyroxene crystals erupted <5 ka during the Agnano-Monte Spina (A-MS) eruption, which is considered to be the reference event for a future large-scale explosive eruption at Campi Flegrei caldera (Italy), has been characterized in detail. Concentration profiles (20–800 μm long, spacing 2.5–10 μm) of major (Si, Ti, Al, Fe, Mg and Ca) and minor (Mn, Na, Ni and Cr) elements were measured along different directions in the clinopyroxene crystals. The zoning patterns of Fe–Mg and selected elements (e.g., Al, Ti) often consist of two or more compositional plateaus with both sharp and/or slightly diffuse boundaries between the plateaus. Each compositional population results from a growth stage in a distinct Magmatic Environment (ME), which is identified as a defined set of intensive thermodynamic variables (pressure, temperature, bulk composition and fugacity of fluids including oxygen). The large range of chemical compositions of clinopyroxenes reveals the existence of at least four MEs that are characterized by different compositional populations; two of these are dominant. The variation in zoning pattern from one plateau composition to another fingerprints the transfer of the crystal from one ME to another. In combination with Sr isotopic data and thermobarometric estimates, our systematic characterization of clinopyroxene zoning patterns suggests recharge by deep mafic magmas (ME0: Mg# = 85–92) of an evolved shallow reservoir (ME2: Mg# = 70–78). Such a process also led to the formation of a compositionally intermediate environment (ME1: Mg# = 84–80), that is detected in the clinopyroxene zoning pattern. A new method has been developed in this work to evaluate the effective diffusive modifications that affect the concentration profiles of zoned crystals. The application of different diffusion modeling methods indicates that deep and shallow reservoirs beneath the Campi Flegrei caldera were connected to each other over several tens of years until the amount of mafic recharge increased during the last 10–15 years before the A-MS eruption. This study highlights the complex relationships between events of magma recharge and the onset of eruption. Our results provide a contribution to the knowledge of timescales of magmatic processes that have recently occurred beneath the Campi Flegrei caldera, which is useful for risk assessment. [ABSTRACT FROM AUTHOR]

The Campi Flegrei caldera is considered the most dangerous volcano in Europe and is currently in a new phase of unrest (started in 2000 and still ongoing) that has persisted intermittently for several decades (main crisis occurred from 1950 to 1952, 1970–1972, and 1982–1984). Here, by combining the petrological and geochemical data collected in recent decades with numerical simulations, we place new constraints on the source(s) of the current dynamics of the volcano. In particular, we show that the measured (N2‐He‐CO2) geochemical changes at the fumaroles of Solfatara hydrothermal site are the result of massive (about 3 km3) magma degassing in the deep portion (≥200 MPa, 8 km of depth) of the plumbing system. This degassing mechanism would be able to flood the overlying hydrothermal system with hot gas, thus heating and fracturing the upper crust inducing shallow seismicity and deformation. This implies that the deep magma transfer process (≥8 km) has been decoupled from the source of deformation and seismicity, localized in the first kilometers (0–4 km) of caldera‐filling rocks. This information on magma transfer depth can have important implications for defining the best monitoring strategies and for forecasting a future eruption. Finally, this study highlights how petrological and geochemical data allow us to explore the dynamics of the deep portion of the plumbing system and thus trace the occurrence of recharge episodes, in a portion of the ductile lower crust where magma transfer occurs in the absence of earthquakes. Plain Language Summary: Calderas are volcanic depressions formed as the ground collapses during huge volcanic eruptions. They often exhibit pronounced unrest, with frequent earthquakes, ground uplift, and considerable heat and mass flux that are monitored by volcanologists for eruption forecasting. However, as this activity is due to the complex interactions among magma and hydrothermal system stored beneath the volcano, it is always difficult to predict the evolution of the unrest toward critical conditions until to eruption. The Campi Flegrei caldera is among the most dangerous volcanos in Europe and is currently in a new phase of unrest that has lasted for several decades, whose nature (magmatic or not magmatic) has remained unclear. Here, we combine petrological and geochemical observations collected in recent decades with numerical simulations to place new constraints on the source of the recent dynamics of the volcano. In particular, we show that new deep magma has recharged the shallow reservoir beneath the volcano and flooded the overlying hydrothermal system with hot gas; thereby weakening the upper rocks allowing deformation (ground uplift) and fracturing (seismicity). This information is particularly important in the case of high‐risk Campi Flegrei caldera, because it can help to improve defense strategies in case of future eruption. Key Points: We explore recent magma dynamics under Campi Flegrei restless caldera (Italy)New petrological and geochemical data trace mafic magma recharges in the lower crustDeep‐gas flooding of hydrothermal system justifies shallow seismicity and deformation [ABSTRACT FROM AUTHOR]

The eruptions of Campi Flegrei (Southern Italy), one of the most studied and dangerous active volcanic areas of the world, are fed by mildly potassic alkaline magmas, from shoshonite to trachyte and phonotrachyte. Petrological investigations carried out in past decades on Campi Flegrei rocks provide crucial information for understanding differentiation processes in its magmatic system. However, the compositional features of rocks are a palimpsest of many processes acting over timescales of 100–104 years, including crystal entrapment from multiple reservoirs with different magmatic histories. In this work, olivine, clinopyroxene and feldspar crystals from volcanic rocks related to the entire period of Campi Flegrei's volcanic activity are checked for equilibrium with combined and possibly more rigorous tests than those commonly used in previous works (e.g., Fe–Mg exchange between either olivine or clinopyroxene and melt), with the aim of obtaining more robust geothermobarometric estimations for the magmas these products represent. We applied several combinations of equilibrium tests and geothermometric and geobarometric methods to a suite of rocks and related minerals spanning the period from ~59 ka to 1538 A.D. and compared the obtained results with the inferred magma storage conditions estimated in previous works through different methods. This mineral-chemistry investigation suggests that two prevalent sets of T–P (temperature–pressure) conditions, here referred to as "magmatic environments", characterized the magma storage over the entire period of Campi Flegrei activity investigated here. These magmatic environments are ascribable to either mafic or differentiated magmas, stationing in deep and shallow reservoirs, respectively, which interacted frequently, mostly during the last 12 ka of activity. In fact, open-system magmatic processes (mixing/mingling, crustal contamination, CO2 flushing) hypothesized to have occurred before several Campi Flegrei eruptions could have removed earlier-grown crystals from their equilibrium melts. Moreover, our new results indicate that, in the case of complex systems such as Campi Flegrei's, in which different pre-eruptive processes can modify the equilibrium composition of the crystals, one single geothermobarometric method offers little chance to constrain the magma storage conditions. Conversely, combined methods yield more robust results in agreement with estimates obtained in previous independent studies based on both petrological and geophysical methods. [ABSTRACT FROM AUTHOR]

Understanding the characteristics of a volcanic system is always important and becomes crucial when the volcano is in pluri-decadal unrest and located in a densely populated area, such as Campi Flegrei caldera, Italy. Ground deformation is a powerful source of information for studying the activity of magmatic sources and hydrothermal systems, even when difficult to detect otherwise. Here, we use ground displacement from ERS-ENVISAT and Sentinel-1A SAR images to investigate the 1993–2000 subsidence and part (2015–2022) of the present unrest. For each of the two time periods, we combine the line-of-sight displacements to obtain vertical and eastward displacements, and apply the empirical orthogonal function analysis to these latter time series—treated as a single data set—to decompose space-time fields into separated modes, consisting of uncorrelated spatial patterns and associated temporal evolutions. We only retain the first mode, since it captures the main deformation during both investigated periods, is the sole mode related to long-lasting (years) processes, and is less affected by noise than original data. Our analyses: (i) Confirm that most of the deformation is related to the activity of a 3–4 km deep sill-like source, which is inflated by magma and/or magmatic fluids during periods of unrest and deflates during periods of subsidence; (ii) Evidence ongoing deformation linked to local fluid migration in the Solfatara area; (iii) Identify persistent deformation features where peculiar fluid migration processes occurred during the 1982–1984 unrest; (iv) Most importantly, provide direct evidence of deep magma inflation at least since 2015, thus giving a strong warning of increasing risk at Campi Flegrei. Results demonstrate the capability of our approach to disclose hardly detectable processes and suggest a tool to monitor the activity of the deep magmatic source. Our approach can be useful also in other volcanic systems.

Studying seismicity in a volcanic environment provides important information on the state of activity of volcanoes. The seismicity of the Neapolitan volcanoes, Campi Flegrei, Vesuvius, and Ischia, shows distinctive characteristics for each volcano, covering a wide range of patterns and types. In this study we relocated some significant volcano-tectonic earthquake swarms that occurred in Campi Flegrei and Vesuvius. Moreover, we compared the earthquake occurrence evolution, the magnitude and the seismic energy release of the three volcanoes. Also, we considered the results of seismic analysis in the light of geochemical and ground deformation data that contribute to defining the state of activity of volcanoes. In Campi Flegrei, which is experiencing a long term unrest, we identified a seismogenic structure at shallow depth in Pisciarelli zone that has been activated repeatedly. The increasing seismicity accompanies an escalation of the hydrothermal activity and a ground uplift phase. At Vesuvius a very shallow seismicity is recorded, which in recent years has shown an increase in terms of the number of events per year. Earthquakes are usually located right beneath the crater axis. They are concentrated in a volume affected by the hydrothermal system. Finally, Ischia generally shows a low level of seismicity, however, in Casamicciola area events with a moderate magnitude can occur and these are potentially capable of causing severe damage to the town and population, due to their small hypocentral depth (typically < 2.5 km). After the seismic crisis of August 21, 2017 (mainshock magnitude M = 4), the seismicity returned to a low level in terms of occurrence rate and magnitude of earthquakes. The seismicity of these three different volcanic areas shows some common aspects that highlight a relevant role of hydrothermal processes in the seismogenesis of volcanic areas. However, while the main swarms in Campi Flegrei and most of the Vesuvian earthquakes are distributed along conduit-like structures, the seismicity of Ischia is mainly located along faults. Furthermore, the temporal evolution of seismicity in Neapolitan volcanic area suggests a concomitant increase in the occurrence of earthquakes both in Campi Flegrei and Vesuvius in recent years.

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. Plain Language Summary: Can we model seismicity in a volcanic caldera using tools and methods taken from deformation and fluid‐flow modeling in reservoirs? If the target is Campi Flegrei (southern Italy) the application must certainly include parameter such as the recently characterized "caprock," which partially mitigates the stress induced by dyke intrusions and fluid injections during deformation unrests. Through the coupling of mechanical and fluid‐flow simulators, we targeted the seismic response of the caldera to hot‐water injections in order to reproduce the strongest seismic unrest in the last 35 years (2019–2020). The results show that hot‐water injections can reproduce the depth–magnitude trend at the start of the unrest in isothermal conditions, especially if the temperature of the injections increases. Including rock‐sealing formations in the modeling of seismic unrest at this and other volcanoes is vital to understand seismic precursors and hazard. In order to model the strongest seismicity, the effect of either magma or magmatic fluids must likely be included in the modeling. Key Points: Injection‐induced caprock deformation is a major controlling parameter of seismicity at Campi Flegrei calderaThe seismo‐volcanic unrests are thermally driven but hydromechanical in natureHot‐water injections modulated by injection rate and temperature, caprock and fault permeabilities, reproduce the swarm at the onset of the unrest [ABSTRACT FROM AUTHOR]

We present 4 years of continuous seafloor deformation measurements carried out in the Campi Flegrei caldera (Southern Italy), one of the most hazardous and populated volcanic areas in the world. The seafloor sector of the caldera has been monitored since early 2016 by the MEDUSA marine research infrastructure, consisting of four instrumented buoys installed where sea depth is less than 100 m. Each MEDUSA buoy is equipped with a cabled, seafloor module with geophysical and oceanographic sensors and a subaerial GPS station providing seafloor deformation and other environmental measures. Since April 2016, the GPS vertical displacements at the four buoys show a continuous uplift of the seafloor with cumulative measured uplift ranging between 8 and 20 cm. Despite the data being affected by environmental noise associated with sea and meteorological conditions, the horizontal GPS displacements on the buoys show a trend coherent with a radial deformation pattern. We use jointly the GPS horizontal and vertical velocities of seafloor and on-land deformations for modeling the volcanic source, finding that a spherical source fits best the GPS data. The geodetic data produced by MEDUSA has now been integrated with the data flow of other monitoring networks deployed on land at Campi Flegrei.

The eruptions of Campi Flegrei (Southern Italy), one of the most studied and dangerous active volcanic areas of the world, are fed by mildly potassic alkaline magmas, from shoshonite to trachyte and phonotrachyte. Petrological investigations carried out in past decades on Campi Flegrei rocks provide crucial information for understanding differentiation processes in its magmatic system. However, the compositional features of rocks are a palimpsest of many processes acting over timescales of 100–104 years, including crystal entrapment from multiple reservoirs with different magmatic histories. In this work, olivine, clinopyroxene and feldspar crystals from volcanic rocks related to the entire period of Campi Flegrei’s volcanic activity are checked for equilibrium with combined and possibly more rigorous tests than those commonly used in previous works (e.g., Fe–Mg exchange between either olivine or clinopyroxene and melt), with the aim of obtaining more robust geothermobarometric estimations for the magmas these products represent. We applied several combinations of equilibrium tests and geothermometric and geobarometric methods to a suite of rocks and related minerals spanning the period from ~59 ka to 1538 A.D. and compared the obtained results with the inferred magma storage conditions estimated in previous works through different methods. This mineral-chemistry investigation suggests that two prevalent sets of T–P (temperature–pressure) conditions, here referred to as “magmatic environments”, characterized the magma storage over the entire period of Campi Flegrei activity investigated here. These magmatic environments are ascribable to either mafic or differentiated magmas, stationing in deep and shallow reservoirs, respectively, which interacted frequently, mostly during the last 12 ka of activity. In fact, open-system magmatic processes (mixing/mingling, crustal contamination, CO2 flushing) hypothesized to have occurred before several Campi Flegrei eruptions could have removed earlier-grown crystals from their equilibrium melts. Moreover, our new results indicate that, in the case of complex systems such as Campi Flegrei’s, in which different pre-eruptive processes can modify the equilibrium composition of the crystals, one single geothermobarometric method offers little chance to constrain the magma storage conditions. Conversely, combined methods yield more robust results in agreement with estimates obtained in previous independent studies based on both petrological and geophysical methods.

This study presents a new method, called the Radial Interpolation Method, to interpolate data characterized by an approximately radial pattern around a relatively constrained central zone, such as the ground deformation patterns shown in many active volcanic areas. The method enables the fast production of short-term deformation maps on the base of spatially sparse ground deformation measurements and can provide uncertainty quantification on the interpolated values, fundamental for hazard assessment purposes and deformation source reconstruction. The presented approach is not dependent on a priori assumptions about the geometry, location and physical properties of the source, except for the requirement of a locally radial pattern, i.e., allowing multiple centers of symmetry. We test the new method on a synthetic point source example, and then, we apply the method to selected time intervals of real geodetic data collected at the Campi Flegrei caldera during the last 39 years, including examples of leveling, Geodetic Precise Traversing measurements and Global Positioning System. The maps of horizontal displacement, calculated inland, show maximum values lying along a semicircular annular region with a radius of about 2–3 km in size. This semi-annular area is marked by mesoscale structures such as faults, sand dikes and fractures. The maps of vertical displacement describe a linear relation between the maximum vertical uplift measured and the volume variation. The multiplicative factor in the linear relation is about 0.3 × 106 m3/cm if we estimate the proportion of the ΔV that is captured by the GPS network onland and we use this to estimate the full ΔV. In this case, the 95% confidence interval on K because of linear regression is ± 5%. Finally, we briefly discuss how the new method could be used for the production of short-term vent opening maps on the base of real-time geodetic measurements of the horizontal and vertical displacements. [ABSTRACT FROM AUTHOR]

The Neapolitan volcanic area includes three active and high-risk volcanoes: Campi Flegrei caldera, Somma–Vesuvius, and Ischia island. The Campi Flegrei volcanic area is a typical example of a resurgent caldera, characterized by intense uplift periods followed by subsidence phases (bradyseism). After about 21 years of subsidence following the 1982–1984 unrest, a new inflation period started in 2005 and, with increasing rates over time, is ongoing. The overall uplift from 2005 to December 2019 is about 65 cm. This paper provides the history of the recent Campi Flegrei caldera unrest and an overview of the ground deformation patterns of the Somma–Vesuvius and Ischia volcanoes from continuous GPS observations. In the 2000–2019 time span, the GPS time series allowed the continuous and accurate tracking of ground and seafloor deformation of the whole volcanic area. With the aim of improving the research on volcano dynamics and hazard assessment, the full dataset of the GPS time series from the Neapolitan volcanic area from January 2000 to December 2019 is presented and made available to the scientific community.

In this study, we analyze the 2009–2013 uplift phenomenon at Campi Flegrei (CF) caldera in terms of temporal and spatial variations in the stress/strain field due to the effect of an inflating source. We start by performing a 3D stationary finite element (FE) modeling of X-band COSMO-SkyMed DInSAR and GPS mean velocities to retrieve the geometry and location of the deformation source. The modeling results suggest that the best-fit source is a three-axis oblate spheroid ~3 km deep, which is mostly elongated in the NE–SW direction. Furthermore, we verify the reliability of model results by calculating the total horizontal derivative (THD) of the modeled vertical velocity component; the findings emphasize that the THD maxima overlap with the projection of source boundaries at the surface. Then, we generate a 3D time-dependent FE model, comparing the spatial and temporal distribution of the shear stress and volumetric strain with the seismic swarms beneath the caldera. We found that low values of shear stress are observed corresponding with the shallow hydrothermal system where low-magnitude earthquakes occur, whereas high values of shear stress are found at depths of about 3 km, where high-magnitude earthquakes nucleate. Finally, the volumetric strain analysis highlights that the seismicity occurs mainly at the border between compression and dilatation modeled regions, and some seismic events occur within compression regions.

Imaging methods able to discriminate type and content of fluids in volcanic areas and to track their migration with time are fundamental to get a picture of the volcanic structure and to constrain its shallow dynamics. In this study we provide an image of the Solfatara crater, located in the Campi Flegrei caldera, a volcanic area of Southern Italy, through a statistical description of the scatterers. We analyze active seismic data recorded in a 3D geometry during the first Repeated Induced Earthquake and Noise (RICEN) experiment, held in September 2013. After extraction of seismic sections, we evaluate the coherent and incoherent intensities, averaging over sources, and receivers sharing the same source-station distance. We thus compute the ratio between the two intensities for the direct surface wave, which is sensitive to the scattering mean free path in the area. We find that the intensity ratio in all the analyzed frequency bands exponentially decreases with distance, allowing for the computation of the scattering mean free path as a function of the frequency. We report that the scattering mean free path exhibits a small increase between 7.5 and 10 Hz from 50 to 60 m, and then it decreases down to about 10 m, at a frequency of 21.5 Hz. We thus model the scattering mean free path computing the backscattered field from a single scatterer of cylindrical shape in a homogeneous medium and we determine the model parameters through a fit with the mean free path inferred from data. We find that the best fit model is obtained for a size of the scatterer of about 10 m, with a small increase of the Rayleigh wave velocity inside the scatterer. The scatterers are here interpreted as regions richer in water with respect to the background and eventually due to the condensed steam running below the investigated area. The connection between the scattering mean free path and the type, and content of fluids retrieved here is of fundamental importance to image the volcanic structure. In addition, monitoring of this quantity with time can track fluid migration eventually related to magma injection and the unrest state of a volcano toward an eruption.

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.

A probabilistic hazard analysis of tsunami generated by subaqueous volcanic explosion is applied to the Campi Flegrei caldera (Campania, Italy). An event tree is developed to quantify the tsunami hazard due to the submarine explosions by: i) defining potential size classes of explosion magnitude on the basis of past volcanic activity in the Campi Flegrei caldera and sites in the underwater part of the caldera; ii) simulating the generation and propagation of the consequent tsunami waves able to reach the coasts of the Campania region for all combinations of tsunami-generating vents and sizes; and iii) quantifying the tsunami probability and relative uncertainty, conditional upon the occurrence of an underwater eruption at Campi Flegrei. Tsunami hazard generated by subaqueous volcanic explosions is considered crucial because of its potential high impact on the densely populated coastal areas of the Pozzuoli Bay and Gulf of Naples even if the probability for eruptions in the submarine part of the caldera is certainly low. The tsunami hazard analysis is presented using conditional hazard curves and maps, that is calculating the probability (and relative uncertainties) of exceeding given tsunami intensity thresholds (wave amplitudes at the coast), given the occurrence of a subaqueous eruption. The results indicate that a significant tsunami hazard exists in many areas of the Bay of Naples. • Subaqueous volcanic explosions can generate tsunamis. • The Campi Flegrei caldera is used as a case-study for applying a probabilistic hazard analysis of tsunami generated by subaqueous. • The results show that this kind of scenario represents a significant hazard in the Pozzuoli Bay and Gulf of Naples. [ABSTRACT FROM AUTHOR]

The Campi Flegrei caldera is a large volcanic complex lying in the Campanian Plain, Southern Italy. During its history the caldera experienced episodes of bradyseism and intense swarm seismicity. The mechanism leading to unrest episodes is still debated, and great efforts are ongoing to improve the knowledge of this structure and its evolution due to the high volcanic risk in such a densely populated area. Here we present a resistivity model from a two‐dimensional inversion of audiomagnetotelluric data acquired along an approximately 5.6‐km long profile crosscutting the Solfatara‐Pisciarelli district and the Agnano plain. The resistivity model shows (1) very low resistivity values confined in the first 500 m of depth both in correspondence of the Solfatara‐Pisciarelli districts and the Agnano depression; (2) a resistive plume that extends underneath the Solfatara crater down to 2,000‐ to 3,000‐m depth, and (3) an adjoining relative conductive unit eastward. We discuss the resistivity structures in a multidisciplinary framework integrating inedited geochemical and seismological observations with existing surface geology and subsurface information. The Solfatara‐Pisciarelli district and the Agnano plain, both being expression of intense hydrothermal activity, show different characteristics. Below the Solfatara‐Pisciarelli area, the shallow conductive zone is interpreted as a faulted clay cap that overlies a highly active vapor‐dominated reservoir characterized by a convective mechanism. Conversely, below the Agnano plain, a liquid phase seems to prevail in the reservoir. The spatiotemporal variations of seismicity imply a combined action of preexisting tectonic lineaments and fluid interaction between the gas/steam reservoir and the outflow zone. Key Points: We present new audiomagnetotelluric, geochemical, and seismological observations data collected in the central‐eastern sector of Campi Flegrei calderaThe multidisciplinary analysis provides information on the structure and properties of the hydrothermal reservoir and of the fluid pathwaysSeismic events seem linked to the combined action of tectonic lineaments and fluid interactions between gas/steam reservoir and the outflow zone [ABSTRACT FROM AUTHOR]

We report, for the first time, evidence of seismically induced soft‐sediment deformations in the central area of the active Campi Flegrei caldera (southern Italy). We analyzed the marine‐transitional and continental sequences located along the coastal La Starza cliffs and several stratigraphic logs exposed during the excavation of a 1‐km‐long tunnel in the Pozzuoli area. The successions host several soft‐sediment structures including sand dikes and sand volcanoes, which are largely dated within the 4.55‐ to 4.28‐kyr BP interval. The volcano‐sedimentary sequence, deposited within the Campi Flegrei caldera in the last 15 kyr, is schematically formed by the superposition of three layers with different rheological behaviors; from the base progressing upward we recognize (1) a massive tuff, (2) marine‐transitional sands of the La Starza unit, and (3) a dominance of continental volcanoclastics. We envisage that during unrest episodes of the volcano, which included ground deformation and seismic activity, the whole volcano‐sedimentary pile was deformed through brittle mechanisms with the formation of normal faults. However, the intermediate layer, when subject to seismic shaking, behaved locally as a viscous material facilitating liquefaction processes and lateral spreading deformation. Furthermore, new geophysical, stratigraphic, and structural surveys allowed us to model the deformation evolution of this area over the last 15 kyr. The evidence of seismically induced soft‐sediment deformation within the volcano‐sedimentary record suggests that moderate earthquakes could occur during future volcano‐seismic unrests. Consequently, liquefaction and related gravitational mass movements must be considered as a hazard during these unrest and volcanic crises. Key Points: Evidence of seismically induced soft‐sediment deformation is reported within the volcano‐sedimentary record at Campi Flegrei calderaGeological observations, well logs, and ERT profiles allowed us to reconstruct the tectonostratigraphic architecture of the central sectorSeismically induced sand dykes and sand volcanoes occurred during the unrest stage in the 4.55‐ to 4.28‐kyr BP interval [ABSTRACT FROM AUTHOR]

Abstract We analyze the crustal rheology beneath the active resurgent Campi Flegrei caldera (CFc) in Southern Italy by modelling the 3D brittle-ductile (B/D) transition, based on available thermal, geological and geophysical data. Firstly, the thermal field in the conductive physical regime is modeled using a finite element method; based on an optimization tool, this method is applied to evaluate the location and dimensions of the deep thermal source beneath the caldera. A horizontally-extended thermal anomaly located at about 5000 m depth below sea level is identified beneath Pozzuoli Bay, a part of the CFc. The same isotherm is located at a depth of 20,000 m beyond the caldera. This indicates a higher horizontal temperature gradient in the caldera with respect to the surrounding area. Next, we utilize this thermal model to image the 3D rheological stratification of the shallow crust below the caldera with two different values of strain rates. Within the caldera, the B/D transitions with ɛ equal to 10−12 s−1 and 10−8 s−1 are located at 3000 m and 5000 m depths, respectively. Outside the caldera, the transition is very deep (15,000–20,000 m), seemingly uninfluenced by the thermal state of the CFc volcanism. Finally, we compare these results with the spatial distribution of earthquake hypocenters, Benioff strain release and b-value distribution to investigate the relationship between crustal rheology and seismicity characteristics. Our analysis reveals that the image of the B/D transition is in agreement with the distribution of earthquake hypocenters, constraining the potential seismogenic volume of the region. Our study demonstrates that knowledge of the rheological state of a volcanic system is an important element to interpret its dynamic, forecast future activity and improve evaluation of the associated seismic hazard. Graphical abstract Highlights • We develop a 3D brittle/ductile (B/D) transition model for the Campi Flegrei caldera. • The 3D B/D model is based on available thermal, geological and geophysical data. • The 3D rheological stratification of the shallow crust is imaged with two strain rates. • We evaluate the Benioff strain release, energy release and b-value from seismicity data. • We compare 3D rheological stratification and seismicity distribution (1982-2016). [ABSTRACT FROM AUTHOR]

Ground deformation in volcanic areas is linked to various, often interconnected processes such as magma intrusion, pressurized fluid migration, and thermal expansion effects. The presence of active and extended hydrothermal systems plays a key role and affects the deformation phenomenon in complex ways. In this study, we propose a generalized conceptual and mathematical model, which allows retrieving the flow rate of fluid injection in a volcanic hydrothermal system, assuming a ground deformation data set as input. The basic assumption is that short‐term ground uplift episodes (with characteristic periods of less than 5 years) depend on the injection of volcanic fluids into the hydrothermal system. Then, assuming a deformation field shape independent of time and a linear time‐invariant relation between the amount of injected fluid and the resulting ground deformation, we define a Green's function as the product of spatial and temporal components. The case study is a 3‐D elastic model with permeability and porosity for the Campi Flegrei caldera, Italy. By Green's function, a 2‐km‐long source at 2.4‐km depth, which matches the interferometric synthetic aperture radar deformation, is localized and the amount of injected volcanic fluid in the last 20 years of high‐frequency deformation episodes estimated. In conclusion, we find a good agreement between the measured and estimated temporal deformation patterns and, principally, that fluid injection rates can be retrieved from the deformation field at volcanoes. Key Points: What kind of physical process could produce ground short‐term bradyseismic oscillation in calderas unrest?Linear time invariant approximation applied to multiphase thermo‐fluid dynamical evolution of the Campi Flegrei hydrothermal systemShort‐term deformation source parameters estimated by Green's function‐based inversion of the ground deformation at Campi Flegrei caldera [ABSTRACT FROM AUTHOR]

Abstract We analyse tiltmetric time series recorded at borehole instruments recently installed at Campi Flegrei caldera (Italy), a volcanic area subjected to the phenomenon of bradyseism, which consists in fast ground uplift phases alternated to slow subsidence. For the first time, we evaluate the crustal response in terms of ground tilting of the entire caldera to external excitations such as long/medium-period tidal constituents, by adopting Independent Component Analysis. Indeed, we recognize diurnal (solar) and long-period (fortnightly and monthly) components, superimposed to the normal deformation trend of the area. They show well defined polarization directions and are associated with an oscillatory deformation pattern with the same periodicity of the corresponding tidal constituents. The comparison with the local geology evidences that the tidal tilting is controlled by the local stress field distribution and the rheology, thus inducing structural and thermoelastic site effects. Highlights • Interaction between earth tides and crustal dynamics to better model volcanic unrest. • ICA of tiltmetric signals at Campi Flegrei reveals the crustal response to tides. • Knowledge of the crustal response allows focusing on endogenous volcanic sources. [ABSTRACT FROM AUTHOR]

Abstract: To study the dynamics of active volcanic environments, minerals involved in fresh lava flows are a nice probe and Raman spectroscopy is an excellent tool for their characterization. Because of the highly hostile environment linked to volcanic activity, studying lava cooling and weathering needs remote setups and specific procedures dedicated to medium‐high temperature targets. In this paper, we present the remote Raman characterization of sulfur produce by the fumarolic activity at Solfatara crater (Phlegraean Fields, Italy). Two medium temperature sites (280–380 K) were probed with two different Raman setups. For both, β‐sulfur is the prominent type in the entire temperature range and the absolute local temperature of the sulfur was remotely estimated by mean of a Stokes/anti‐Stokes procedure with ±10 K uncertainty. These results represent a good starting point for the development of new setups and protocols aimed to an increase of both measurement distance and temperature range. [ABSTRACT FROM AUTHOR]

In the present paper, we analyse ground tilt and seismicity at Campi Flegrei caldera and Ischia Island, two volcanic areas located in the south of Italy. These areas have been well studied for many years from a petrological, volcanological and geophysical view point. Moreover, due to the high seismic and volcanic risk for the populations living there, they are continuously monitored by networks of geophysical and geochemical sensors. We summarize the most important results that we obtained so far, concerning the observations of relationships between seismic activity and ground tilt anomalies, focusing on the time interval 2015–2018. First, we present a detailed description of the tiltmeter and seismic networks in both the investigated areas, as well as their development and improvement over time that has enabled high quality data collection. From the joint analysis of the seismic and borehole tiltmeter signals, we often notice concurrence between tilt pattern variations and the occurrence of seismicity. Moreover, the major tilt anomalies appear to be linked with the rate and energy of volcano-tectonic earthquakes, as well as with exogenous phenomena like solid Earth tides and hydrological cycles. The analysis that we present has potential applicability to other volcanic systems. Our findings show how the joint use tilt and seismic data can contribute to better understanding of the dynamics of volcanoes.

Monitoring volcanic phenomena is a key question, for both volcanological research and for civil protection purposes. This is particularly true in densely populated volcanic areas, like the Campi Flegrei caldera, which includes part of the large city of Naples (Italy). Borehole monitoring of volcanoes is the most promising way to improve classical methods of surface monitoring, although not commonly applied yet. Fiber optics technology is the most practical and suitable way to operate in such high temperature and aggressive environmental conditions. In this paper, we describe a fiber optics Distributed Temperature Sensing (DTS) sensor, which has been designed to continuously measure temperature all along a 500 m. deep well drilled in the west side of Naples (Bagnoli area), lying in the Campi Flegrei volcanic area. It has then been installed as part of the international ‘Campi Flegrei Deep Drilling Project’, and is continuously operating, giving insight on the time variation of temperature along the whole borehole depth. Such continuous monitoring of temperature can in turn indicate volcanic processes linked to magma dynamics and/or to changes in the hydrothermal system. The developed monitoring system, working at bottom temperatures higher than 100 °C, demonstrates the feasibility and effectiveness of using DTS for borehole volcanic monitoring.

The Solfatara volcano is the main degassing area of the Campi Flegrei caldera, characterized by 60 years of unrest. Assessing such renewal activity is a challenging task because hydrothermal interactions with magmatic gases remain poorly understood. In this study, we decipher the complex structure of the shallow Solfatara hydrothermal system by performing the first 3-D, high-resolution, electrical resistivity tomography of the volcano. The 3-D resistivity model was obtained from the inversion of 43,432 resistance measurements performed on an area of ~0.68 km2. The proposed interpretation of the multiphase hydrothermal structures is based on the resistivity model, a high-resolution infrared surface temperature image, and 1,136 soil CO2 flux measurements. In addition, we realized 27 soil cation exchange capacity and pH measurements demonstrating a negligible contribution of surface conductivity to the shallow bulk electrical conductivity. Hence, we show that the resistivity changes are mainly controlled by fluid content and temperature. The high-resolution tomograms identify for the first time the structure of the gas-dominated reservoir at 60 m depth that feeds the Bocca Grande fumarole through a ~10 m thick channel. In addition, the resistivity model reveals a channel-like conductive structure where the liquid produced by steam condensation around the main fumaroles flows down to the Fangaia area within a buried fault. The model delineates the emplacement of the main geological structures: Mount Olibano, Solfatara cryptodome, and tephra deposits. It also reveals the anatomy of the hydrothermal system, especially two liquid-dominated plumes, the Fangaia mud pool and the Pisciarelli fumarole, respectively. [ABSTRACT FROM AUTHOR]

Barium diffusion chronometry applied to sanidine phenocrysts from the trachytic Agnano-Monte Spina eruption (∼4.7 ka) constrains the time between reactivation and eruption of magma batches in the Campi Flegrei caldera. Backscattered electron imaging and quantitative electron microprobe measurements on 50 sanidine phenocrysts from representative pumice samples document core-to-rim compositional zoning. We focus on compositional breaks near the crystal rims that record magma mixing processes just prior to eruption. Diffusion times were modeled at a magmatic temperature of 930 °C using profiles based on quantitative BaO point analyses, X-ray scans, and grayscale swath profiles, yielding times ≤60 years between mixing and eruption. Such short timescales are consistent with volcanological and geochronological data that indicate that at least six eruptions occurred in the Agnano-San Vito area during few centuries before the Agnano-Monte Spina eruption. Thus, the short diffusion timescales are similar to time intervals between eruptions. Therefore, the rejuvenation time of magma residing in a shallow reservoir after influx of a new magma batch that triggered the eruption, and thus pre-eruption warning times, may be as short as years to a few decades at Campi Flegrei caldera. [ABSTRACT FROM AUTHOR]

We present an improved density model and a new structural map of the Neapolitan Yellow Tuff caldera, the active portion of the nested Campi Flegrei caldera. The model was built using a new 3‐D inversion of the available high‐precision gravity data, and a new digital terrain and marine model. The inversion procedure, based on a variable‐depth lumped assembling of the subsurface gravity distribution via cell aggregation, gives better defined insights into the internal caldera architecture, that well agree with the available geological, geophysical, and geochemical data. The adopted 3‐D gravity method is highly efficient for characterizing the shallow caldera structure (down to 3 km depth) and defining features related to regional or volcano tectonic lineaments and dynamics. In particular, the resulting density distribution highlights a pronounced density low in correspondence of the central portion of the caldera with a detail not available till now. The joint interpretation of the available data suggests a subsurface structural setting that supports a piecemeal collapse of the caldera, and allows the identification of its headwall. Positive gravity anomalies localize dense intrusions (presently covered by late volcanic deposits) along the caldera marginal faults, and the main structural lineaments both bordering the resurgent block and cutting the caldera floor. These results allow us to both refine the current geological‐structural framework and propose a new structural map that highlights the caldera boundary and its internal setting. This map is useful to interpret the phenomena occurring during unrest, and to improve both short‐term and long‐term volcanic hazards assessment.

In this work the time dynamics of GPS time series recorded at the Campi Flegrei caldera from 2000 to 2019 was investigated by using the Fisher Information Measure (FIM) and the Shannon entropy power (SEP), two informational methods that allow the detection of changes in the dynamical behavior of a complex system, like the volcanic one, quantifying respectively the order/organization and the disorder/uncertainty. By jointly analyzing the FIM and the SEP through the Fisher-Shannon Information Plane (FSIP) it was shown that the SEP discriminates quite well the vertical displacement time series, while the FIM discriminates better the horizontal ones, which are featured by a more complex behavior. Globally, the most striking change in the SEP is found in 2012, while at local scale abrupt loss of order (indicated by low values of FIM) well correlate with the occurrence of seismic swarms. A further informational quantity, the complexity C (given by the product between SEP and FIM), has been revealed to be sensitive to the baseline deformation level increases and to the small-amplitude modulation of deformation signals within the caldera. • Informational properties detect time and space transitions on the dynamics of the deformation source • Shannon entropy power well discriminates the vertical displacement time series • Fisher Information Measure well discriminates the horizontal displacement time series • Complexity is sensitive to baseline deformation level increases and to small-amplitude modulation of deformation signals [ABSTRACT FROM AUTHOR]

Solfatara crater, located in the Campi Flegrei caldera, is a volcano with one of the highest degassing rates on Earth, more than 1500 t of CO 2 released by diffusion or through vents. Here, we investigated how this gas release can be disrupted by the passage of seismic waves. We performed continuous soil CO 2 flux measurements during the propagation of seismic vibrations in the range of 5 Hz to 200 Hz induced by a vibroseis truck. The CO 2 flux was continuously recorded using the accumulation chamber method. The data show a temporary and drastic (up to two-fold) increase in CO 2 flux exclusively during the vibrations, before returning to the initial flux values. These transient variations are interpreted as fluidization of the surficial granular layer that releases the stored gas. Similar degassing processes might occur at a larger scale during earthquakes, to cause temporary increases in the total gas outflow in volcanic or tectonic areas. Our findings are useful to better assess and monitor the potential hazard from sudden CO 2 flux release during earthquakes as several cases of intoxication or death have already been related to volcanic degassing. [ABSTRACT FROM AUTHOR]

We present a long-range terrestrial laser scanner application for the geostructural mapping of Coroglio cliff, a tuff rock face exposed along the coastal zone of Campi Flegrei, Napoli. The procedure includes several phases (geomorphological analysis, structural field survey, laser scanner data acquisition and data processing, 3-D model development and analysis, geostructural classification of discontinuity orientation data and 2-D vertical cartographic production). Field data were processed with specific software dedicated to geostructural and geometric analysis. Spatial data were managed with a geographical information system and have been used for the construction of 2-D and 3-D geometric models of the rock cliff surface and geostructural interpretation. The main product of this study is a vertical geostructural map of the Coroglio cliff at 1:500 scale that illustrates the spatial distribution and orientation of the major families of structural discontinuities detected along the exposed surface of the cliff. The cartographic product includes base information useful to identify the main rock failure mechanisms along the cliff and represents a first step for the zonation of areas susceptible to block failures and the planning of monitoring activities. [ABSTRACT FROM PUBLISHER]

This work is devoted to the analysis of seismic signals continuously recorded at Campi Flegrei Caldera (Italy) during the entire year 2006. The radiation pattern associated with the Long-Period energy release is investigated. We adopt an innovative Independent Component Analysis algorithm for convolutive seismic series adapted and improved to give automatic procedures for detecting seismic events often buried in the high-level ambient noise. The extracted waveforms characterized by an improved signal-to-noise ratio allows the recognition of Long-Period precursors, evidencing that the seismic activity accompanying the mini-uplift crisis (in 2006), which climaxed in the three days from 26–28 October, had already started at the beginning of the month of October and lasted until mid of November. Hence, a more complete seismic catalog is then provided which can be used to properly quantify the seismic energy release. To better ground our results, we first check the robustness of the method by comparing it with other blind source separation methods based on higher order statistics; secondly, we reconstruct the radiation patterns of the extracted Long-Period events in order to link the individuated signals directly to the sources. We take advantage from Convolutive Independent Component Analysis that provides basic signals along the three directions of motion so that a direct polarization analysis can be performed with no other filtering procedures. We show that the extracted signals are mainly composed of P waves with radial polarization pointing to the seismic source of the main LP swarm, i.e. a small area in the Solfatara, also in the case of the small-events, that both precede and follow the main activity. From a dynamical point of view, they can be described by two degrees of freedom, indicating a low-level of complexity associated with the vibrations from a superficial hydrothermal system. Our results allow us to move towards a full description of the complexity of the source, which can be used, by means of the small-intensity precursors, for hazard-model development and forecast-model testing, showing an illustrative example of the applicability of the CICA method to regions with low seismicity in high ambient noise. [ABSTRACT FROM AUTHOR]

Sr-isotopic microanalysis has been performed on selected minerals from the Campi Flegrei caldera, together with Sr and Nd isotopic ratio determinations on bulk mineral and glass fractions. The aim was a better characterization of the chemically homogeneous, but isotopically distinct magmatic components which fed volcanic eruptions of the caldera over the past 5 ka, in order to enhance our knowledge about one of the most dangerous volcanoes on Earth. Information on the involved magmatic endmembers, unobtainable by analyzing the isotopic composition of whole rock samples and bulk mineral fractions, has been acquired through high-precision determination of 87 Sr/ 86 Sr on single crystals and microdrilled mineral powders. We focused our investigations on the products emplaced during the Astroni 6 eruption (4.23 cal ka BP), assumed representative of the expected event in case of renewed volcanic activity in the Campi Flegrei caldera. Data on single crystals and microdrilled mineral powders have been compared with Sr and Nd isotopic compositions of bulk mineral fractions from products emplaced during the whole Astroni activity, which included seven distinct eruptions. The 87 Sr/ 86 Sr ratios of single crystals and microdrilled mineral powders are in the 0.7060 to 0.7076 range, much wider than that of bulk mineral fractions, which range from 0.7066 to 0.7076. Moreover, the Sr isotopic ratios are inversely correlated to 143 Nd/ 144 Nd. The new data allow us to better define the magmatic endmembers involved in mingling/mixing processes that occurred prior to/during the Astroni activity. One magmatic endmember, characterized by average 87 Sr/ 86 Sr ratio of ~ 0.70750, was quite common in the past 15 ka activity of the Campi Flegrei caldera; the other, as evidenced by the isotopic composition of single feldspar and clinopyroxene crystals, is less enriched in radiogenic Sr ( 87 Sr/ 86 Sr ~ 0.70724). The latter is interpreted to represent a new magmatic component that entered the Campi Flegrei caldera feeding system in the past 5 ka, the previously recognized Astroni 6 component. However, diopside crystals in Astroni 6 are characterized by even lower 87 Sr/ 86 Sr, in the range of 0.7060–0.7068 and by the highest 143 Nd/ 144 Nd ratios measured in the products of Astroni activity. These diopsides may represent another common magmatic component, as they have been found in most of the Phlegrean Volcanic District products emplaced over the past 75 ka. These diopsides, crystallized in a mantle-derived mafic magma, were entrapped by the Astroni 6 magma during ascent, before it mingled/mixed with the more differentiated and enriched in radiogenic Sr resident magma, thus attaining an intermediate Sr-Nd isotopic fingerprint. These results have an important outcome on the understanding of the volcano behavior, as renewed activity can be triggered by the arrival of fresh magma in the feeding system that would mingle/mix with the resident magma. Such an event may be able to start an unrest phase at the volcano that could last for years or decades, perhaps culminating in a new eruption. [ABSTRACT FROM AUTHOR]