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In the past two decades, the central portion of Campi Flegrei caldera has experienced ground uplift up to 15 mm/month, with an increase of rate, magnitude and extent of the seismicity. In this work, we perform multi‐scale precise earthquake relocation of the 2014–2024 seismicity, mapping in detail activated fault zones. We relate the geometry, extent, and depth of these zones with up‐to‐date structural reconstructions of the caldera. The current seismicity is mainly driven by ground‐uplift‐induced stress concentration on pre‐existing, weaker fault zones, some of which identified for the first time. These structures are not only related to the inner caldera and dome resurgence but also to volcano‐tectonic events of the last 10 ka. The extent of imaged fault segments suggests they can accommodate ruptures up to a moment magnitude 5.1, significantly increasing seismic hazard in the area. Plain Language Summary During the past 2 yrs, there has been a marked increase in ground uplift, number and size of earthquakes at Campi Flegrei caldera. This increase in activity has raised concerns among the population and public authorities about the impact of seismic activity on buildings and infrastructure in the area and about the best actions to undertake during the seismic emergency to reduce the risk. Additionally, the possibility of a future volcanic eruption is being considered, although currently geochemical and geophysical monitoring shows no unequivocal signs of precursory phenomena. In this work, we map the last decade of seismicity with a new technique providing high‐precision earthquake locations that allow us to unveil the currently activated fault zones of the inner caldera. The results show a near‐elliptical distribution of seismicity at the global scale of the caldera, which at a smaller scale delineates complex seismogenic structures. The size of imaged faults suggests that earthquakes up to a moment magnitude 5.1 can occur, significantly increasing seismic hazard in the area. Key Points High‐precision location of 2014–2024 seismicity in Campi Flegrei depicts active fault zones with unprecedented detail Seismicity occurs along different volcano‐tectonic structures including the caldera inner ring fault zone and faults bounding the Solfatara crater A new structure has been recognized in the eastern sector of the caldera, hosting the largest magnitude event in the analyzed period

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 1540–1582 During 1540–1582, a central sill‐like source (∼3.5 km depth) transfers magma below Monte Nuovo, representing an aborted eruption From 1538 to 1650 a deeper magmatic layer (∼8 km depth) experienced continuous magma supply, also during caldera subsidence

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.

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.

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 Volcano We indicate the locations where we first detected hydrothermal emissions and temperature anomalies Resistivity anomalies and identified structural lineaments suggest the presence of active structures within the Astroni Volcano

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 × 10 6  m 3 /cm if we estimate the proportion of the Δ V that is captured by the GPS network onland and we use this to estimate the full Δ V . 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.

In volcanic regions, the analysis of Thermal InfraRed (TIR) satellite imagery for Land Surface Temperature (LST) retrieval is a valid technique to detect ground thermal anomalies. This allows us to achieve rapid characterization of the shallow thermal field, supporting ground surveillance networks in monitoring volcanic activity. However, surface temperature can be influenced by processes of different natures, which interact and mutually interfere, making it challenging to interpret the spatio-temporal variations in the LST parameter. In this paper, we use a workflow to detect the main thermal patterns in active volcanic areas by analyzing the Independent Component Analysis (ICA) results applied to satellite nighttime TIR imagery time series. We employed the proposed approach to study the surface temperature distribution at the Campi Flegrei caldera volcanic site (Southern Italy, Naples) during the 2013–2022 time interval. The results revealed the contribution of four main distinctive thermal patterns, which reflect the endogenous processes occurring at the Solfatara crater, the environmental processes affecting the Agnano plain, the unique microclimate of the Astroni crater, and the morphoclimatic aspects of the entire volcanic area.

After the subsidence phase that followed the 1982–1984 bradyseismic crisis, a gradual ground uplift at Campi Flegrei caldera resumed in 2005, while volcanic-tectonic earthquakes have steadily increased in frequency and intensity since 2018, with a significant intensification observed since 2023. This rise in seismic activity enabled a new tomographic study using data collected from 2020 to June 2024. In this work, 4161 local earthquakes (41,272 P-phases and 14,683 S-phases) were processed with the tomoDDPS code, considering 388,166 P and 107,281 S differential times to improve earthquake locations and velocity models. Compared to previous tomographic studies, the 3D velocity models provided higher-resolution images of the central caldera’s structure down to ~4 km depth. Additionally, separate inversions of the two 2020–2022 (moderate seismicity) and 2023–2024 (intense seismicity) datasets identified velocity variations ranging from 5% to 10% between these periods. These changes observed in 2023–2024 support the existence of two pressurized sources at different depths. The first, located at 3.0–4.0 km depth beneath Pozzuoli and offshore, may represent either a magma intrusion enriched in supercritical fluids or an accumulation of pressurized, high-density fluids—a finding that aligns with recent ground deformation studies and modeled source depths. Additionally, the upward migration of magmatic fluids interacting with the geothermal system generated a secondary, shallower pressurized source at approximately 2.0 km depth beneath the Solfatara-Pisciarelli area. Overall, these processes are responsible for the recent acceleration in uplift, increased seismicity and gases from the fumarolic field, and changes in crustal elastic properties through stress variations and fluid/gas migration.

The Campi Flegrei caldera is one of the highest risk volcanic areas on the Earth. Our research documents a 150 year‐long period of intense volcanism following less than 200 years of repose after the Agnano‐Monte Spina Plinian eruption (4.1 ka). The new data show that the renewal of volcanism was preceded by an uplift of a few tens of meters, triggered by mafic refilling of reservoirs at depths of 3 km or less. Our studies also indicate for the first time the occurrence of contemporaneous eruptions from at locations in different sectors of the caldera. These results suggest that a future eruptive crisis will likely be preceded by several meters of caldera‐wide uplift in response to magma movements at depth. The trend of uplift of the caldera since 1969 may thus represent the unrest expected before a renewal of volcanism within an interval of decades to centuries.

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.