Campi Flegrei Caldera Research Articles

3D structure of the Campi Flegrei caldera central sector reconstructed through short-period magnetotelluric imaging

The Campi Flegrei caldera experienced an unrest phase dating to 2005, which primary expression is the impressive ground uplift, accompanied by increasing degassing and seismic activities. Such last two phenomena developed mainly in the caldera central sector, including the Solfatara–Pisciarelli complex. However, the inner structure of such an area is still not defined, and this originates a poor understanding of the ongoing unrest. This paper describes the results of a new magnetotelluric survey performed in the Campi Flegrei caldera central sector. Through the inversion of data collected in 47 independent soundings, a 3D model of the electrical resistivity has been retrieved, which evidenced a partition of the investigated structure. The Agnano–Astroni area seems to be associated with a liquid-dominated geothermal reservoir, whereas the Solfatara–Pisciarelli area seems to be characterized by a single mixed liquid and gasses-dominated geothermal reservoir, which supplies the main caldera fumaroles. The proposed reconstruction of the geometrical characteristics of the hydrothermal system and the primary fluid rising pathways gives substantial clues about the significance of the detected structures in the evolution of the caldera unrest.

Structure and Present State of the Astroni Volcano in the Campi Flegrei Caldera in Italy Based on Multidisciplinary Investigations

AbstractDespite 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.

Thirty years of volcano geodesy from space at Campi Flegrei caldera (Italy)

This work provides the mean ground deformation rates and ground displacement time series of the Campi Flegrei caldera (Italy) retrieved by satellite remote sensing data analysis from 1992 to 2021. Synthetic Aperture Radar (SAR) images acquired by ERS 1–2 (1992–2002), Envisat (2003–2011) and Cosmo-SkyMed (2011–2021) are processed by multi-temporal SAR Interferometry (InSAR) approach using the same technique, parameters and reference system, to obtain for the first time a homogeneous and time-continuous dataset. The validation of the InSAR products is carried out by comparison with the measurements provided by precise levelling lines and cGNSS stations. The produced outcomes offer an overview on the temporal behaviour of ground deformation at Campi Flegrei along an unprecedented time window of about 30 years and can be exploited by the scientific community for supporting and improving the knowledge of the dynamics of the caldera.

Field demonstration of an optical fiber hydrophone for seismic monitoring at Campi-Flegrei caldera

In this paper, we report on the development and field test of a fiber optic seismic hydrophone. The seismic hydrophone was designed to offer performance suitable to operate with resolution down to the sea state zero in underwater environments. The sensing configuration exploits a fiber coil interferometer in Michelson configuration, wrapped around a sensitive composite cylinder. The materials and the size of the hydrophone were selected by numerical analysis in order to guarantee the desired responsivity. The sensing system was integrated in the seismologic monitoring system of the Campi-Flegrei caldera for demonstrating the operative capability of the developed system. To this aim, the hydrophone was installed in a submarine module onto the seabed in the Gulf of Pozzuoli and the optoelectronic read-out system was placed on an instrumented geodetic buoy. During the field trials, we detected several earthquakes occurred in the area and compared the results with a reference piezoelectric hydrophone. The seismic sequence was used to retrieve the sensor responsivity in the frequency range 1–80 Hz. The sensing system reaches a responsivity of about −300 nm/Pa and exhibits an average noise floor level down to 100µPa/√Hz. The comparison of the traces recorded by the optical and reference hydrophones reveals a high level of similarities with autocorrelation higher than 85 %. The reported field trial at the Campi-Flegrei caldera demonstrates the capability of optical fiber hydrophones to operate in relevant environments for seismological monitoring.

Clues of Ongoing Deep Magma Inflation at Campi Flegrei Caldera (Italy) from Empirical Orthogonal Function Analysis of SAR Data

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.

Data analysis of the unsteadily accelerating GPS and seismic records at Campi Flegrei caldera from 2000 to 2020

Ongoing resurgence affects Campi Flegrei caldera (Italy) via bradyseism, i.e. a series of ground deformation episodes accompanied by increases in shallow seismicity. In this study, we perform a mathematical analysis of the GPS and seismic data in the instrumental catalogs from 2000 to 2020, and a comparison of them to the preceding data from 1983 to 1999. We clearly identify and characterize two overlying trends, i.e. a decennial-like acceleration and cyclic oscillations with various periods. In particular, we show that all the signals have been accelerating since 2005, and 90–97% of their increase has occurred since 2011, 40–80% since 2018. Nevertheless, the seismic and ground deformation signals evolved differently—the seismic count increased faster than the GPS data since 2011, and even more so since 2015, growing faster than an exponential function The ground deformation has a linearized rate slope, i.e. acceleration, of 0.6 cm/yr2 and 0.3 cm/yr2 from 2000 to 2020, respectively for the vertical (RITE GPS) and the horizontal (ACAE GPS) components. In addition, all annual rates show alternating speed-ups and slow-downs, consistent between the signals. We find seven major rate maxima since 2000, one every 2.8–3.5 years, with secondary maxima at fractions of the intervals. A cycle with longer period of 6.5–9 years is also identified. Finally, we apply the probabilistic failure forecast method, a nonlinear regression that calculates the theoretical time limit of the signals going to infinity (interpreted here as a critical state potentially reached by the volcano), conditional on the continuation of the observed nonlinear accelerations. Since 2000, we perform a retrospective analysis of the temporal evolution of these forecasts which highlight the periods of more intense acceleration. The failure forecast method applied on the seismic count from 2001 to 2020 produces upper time limits of [0, 3, 11] years (corresponding to the 5th, 50th and 95th percentiles, respectively), significantly shorter than those based on the GPS data, e.g. [0, 6, 21] years. Such estimates, only valid under the model assumption of continuation of the ongoing decennial-like acceleration, warn to keep the guard up on the future evolution of Campi Flegrei caldera.

Fault systems in the offshore sector of the Campi Flegrei caldera (southern Italy): Implications for nested caldera structure, resurgent dome, and volcano-tectonic evolution

The structure of a caldera may influence its activity, making its understanding crucial for hazard assessment. Here, we analysed high-resolution seismic profiles in the Campi Flegrei (southern Italy) offshore sector. We recognised two main fault systems, including those associated with the formation of the caldera and those affecting the resurgent dome. The former system comprises three broadly concentric fault zones (inner, medial and outer ring fault zones) depicting a nested caldera geometry. Considering the relations between faults and seismic units that represent the marine and volcaniclastic successions filling the caldera, all ring faults were formed during the Campanian Ignimbrite eruption (40 ka) and subsequently reactivated during the Neapolitan Yellow Tuff eruption (15 ka). In this last caldera-forming event, the inner and medial fault zones accommodated most of the collapse and were episodically reactivated during the younger volcano-tectonic activity. The second fault system occurs in the apical zone of the resurgent dome and comprises dominantly high-angle normal faults that are mainly related to the volcano-tectonic collapse that followed the Agnano-Monte Spina Plinian eruption (4.55 ka). Finally, we provide a volcano-tectonic evolutionary model of the last 40 kyr, considering the interplay among ring and dome faults activity, volcaniclastic sedimentation, ground deformation and sea-level changes.

A Model Based on Fractional Brownian Motion for Temperature Fluctuation in the Campi Flegrei Caldera

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.

Identifying the Fingerprint of a Volcano in the Background Seismic Noise from Machine Learning-Based Approach

This work is devoted to the analysis of the background seismic noise acquired at the volcanoes (Campi Flegrei caldera, Ischia island, and Vesuvius) belonging to the Neapolitan volcanic district (Italy), and at the Colima volcano (Mexico). Continuous seismic acquisition is a complex mixture of volcanic transients and persistent volcanic and/or hydrothermal tremor, anthropogenic/ambient noise, oceanic loading, and meteo-marine contributions. The analysis of the background noise in a stationary volcanic phase could facilitate the identification of relevant waveforms often masked by microseisms and ambient noise. To address this issue, our approach proposes a machine learning (ML) modeling to recognize the “fingerprint” of a specific volcano by analyzing the background seismic noise from the continuous seismic acquisition. Specifically, two ML models, namely multi-layer perceptrons and convolutional neural network were trained to recognize one volcano from another based on the acquisition noise. Experimental results demonstrate the effectiveness of the two models in recognizing the noisy background signal, with promising performance in terms of accuracy, precision, recall, and F1 score. These results suggest that persistent volcanic signals share the same source information, as well as transient events, revealing a common generation mechanism but in different regimes. Moreover, assessing the dynamic state of a volcano through its background noise and promptly identifying any anomalies, which may indicate a change in its dynamics, can be a practical tool for real-time monitoring.

A gravitational origin for volcano-tectonic faults in the Campi Flegrei caldera (southern Italy) inferred from detailed field observations

Faults in volcanic areas can have different origins and can form due to several volcano-tectonic processes. In this work, by means of detailed field observations, we analysed a fault array cutting through a recent pyroclastic succession in the Campi Flegrei caldera of southern Italy to investigate its origin. Geometric and kinematic data indicate that the fault array consists of steeply dipping faults with both normal and reverse senses of movement. Displacement data suggest that these faults with opposed kinematics acted together , in agreement with what was previously described from analogue and numerical experiments for faults formed in response to gravitational collapses in volcanic areas. By integrating these results with considerations of the recent volcano-tectonic evolution of the Campi Flegrei caldera, we propose a model in which the studied fault array developed to accommodate a gravitational collapse triggered by an underground magma migration predating the 1538 CE historical Monte Nuovo eruption. This work is, to our knowledge, the first thorough field description of faults developed to accommodate gravitational collapses in volcanic areas.

Evidence of Seismic-Related Liquefaction Processes within the Volcanic Record of the Campi Flegrei Caldera (Italy)

We report the occurrence of several sand liquefaction structures, such as sand dikes, in the stratigraphic record of the Campi Flegrei volcano, located both inside and outside the caldera. Five sites were analyzed within the caldera and two outside. The grain size analysis of the sand fillings indicates that these deposits are very fine-to-coarse sands generally poorly sorted. All of the granulometry curves fall within the field of the liquefiable, loose sediments. Frequently, dikes are characterized by two fillings: a rim showing poorly sorted finer sands and a core with extremely poorly sorted coarser sediments. We suggest that seismic-related liquefaction processes triggered the injection of these sand dikes during unrest episodes in the last 15 kyr. In particular, the sand dikes located outside the caldera, characterized by larger thicknesses and lengths, mark an important extensional episode, probably associated with the caldera formation during the Neapolitan Yellow Tuff eruption at 15 ka. Furthermore, liquefaction structures within the caldera are related to the seismic activity, probably occurring during the Agnano–Monte Spina caldera formation and the volcano-tectonic ground deformation, predating Epoch 3b (4.3 ka) and the Monte Nuovo (1538 CE) eruptions. This study highlights that these seismic-related liquefaction structures are common within the volcanic record of the Campi Flegrei, suggesting that the sand source can be both the widespread marine succession underlying the Epoch 3 deposits in the caldera central sector and the primary ash layers extensively present in the volcanic record.

Moment magnitude (Mw) from hydrophone records of low energy volcanic quakes

Earthquake magnitude calibration using hydrophone records has been carried out at Campi Flegrei caldera, an active area close to the highly populated area of Naples city, partly undersea. Definite integrals of the hydrophone records amplitude spectra, between the limits of 1 and 20 Hz, were calculated on a set of small volcano-tectonic earthquakes with moment magnitudes ranging from 1 to 3.3. The coefficients of a linear relationship between the logarithm of these integrals and the magnitude were obtained by linear optimization, thus defining a useful equation to calculate the moment magnitude from the hydrophone record spectra. This method could be easily exported to other volcanic areas, where submerged volcanoes are monitored by networks of hydrophones and seismic sensors on land. The proposed approach allows indeed magnitude measurements of small magnitude earthquakes occurring at sea, thus adding useful information to the seismicity of these volcanoes.

Insights into the temporal evolution of magma plumbing systems from compositional zoning in clinopyroxene crystals from the Agnano-Monte Spina Plinian eruption (Campi Flegrei, Italy)

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.

Quantitative reconstruction of Holocene ground displacements in the offshore part of the Campi Flegrei caldera (southern Italy): Perspectives from seismo-stratigraphic and archaeological data

Analysis of high-resolution seismic profiles integrated with archaeological data along the coast has allowed quantifying the last ∼10 ka BP displacements in the offshore part of the Campi Flegrei resurgent caldera, one of the world's highest-risk volcanic areas. Previous onland studies revealed that, following the Neapolitan Yellow Tuff (NYT) eruption at 15 ka, post-caldera evolution was associated with rapid ground uplift and subsidence cycles. In this paper, we use the stacking pattern of Prograding Wedges (PWs) and Aggrading Fills (AF) to reconstruct relative sea-level changes and estimate the amount of vertical deformation in the Pozzuoli Bay, the submerged part of the caldera. We document five generations of prograding wedges (PW1 to PW5), associated with as many periods of relative ground stability between uplift and subsidence. Instead, deposition of aggrading fills above prograding wedges underpins the subsidence that typically follows volcanic unrest and uplift. The older wedges PW1 and PW2 are larger and likely indicate whole-caldera uplift events. In particular, the development of PW2 starting from ∼5.2 ka is related to the rapid growth of a resurgent dome in the central part of the caldera. After ∼3.7 ka, subsidence prevailed in the caldera leading to deposition of an aggrading fill (AF2). However, subsidence was interrupted by short-term uplift episodes in historical times at (100 BCE-100 CE, 600–700 CE, 1430–1538 CE) that led to the growth of comparatively minor-sized wedges PW1 to PW3, respectively. Displacement of coastal infrastructures of the Roman age (∼2 ka BP) is consistent with the vertical motion retrieved by the seismostratigraphic analysis, and further indicates the existence of two deformation signals. A short-wavelength signal confined to the Pozzuoli Bay reflects the contribution of an intra-caldera source. This signal is superposed to a long-wavelength regional subsidence increasing between Naples and Procida Island from 1.5 to 2 mm/a, respectively. The east-to-west enhancing subsidence likely reflects the transition between the uplifting Apennines and the foundered Tyrrhenian back-arc basin.

Reconstructing Hydrothermal Fluid Pathways and Storage at the Solfatara Crater (Campi Flegrei, Italy) Using Seismic Scattering and Absorption

Imaging of fluid pathways is crucial to characterize processes taking place in hydrothermal systems, a primary cause of volcanic unrest and associated hazards. The joint imaging of seismic absorption and scattering is an efficient instrument to map fluid flow at crustal scale, and specifically in volcanoes; however, this technique has so far been applied to image volcanoes and hydrothermal systems at the kilometre scale. Here, we use data from a meter-scale, active seismic survey inside the shallow structure of the Solfatara crater to obtain the first frequency-dependent near-surface scattering and absorption model of a hydrothermal system. The Solfatara crater is the place used to monitor historic unrest at Campi Flegrei caldera (Italy), a high-risk volcano under continuous surveillance due to its closeness to a densely populated area. Improving the imaging of the shallow part of this system is crucial to broaden the understanding of unrest processes that are progressively characterizing other portions of the eastern caldera. The scattering contrasts highlight the primary structural feature, a fault separating the hydrothermal plume from zones of CO2saturation nearing fumaroles. While high-absorption anomalies mark zones of high soil temperatures and CO2fluxes, low-absorption anomalies indicate zones of very shallow upflow and are caused by contrasts between liquid-rich and vapour-rich fluids coming from mud pools and fumaroles, respectively. All maps show a SW-NE trend in anomalies consistent with fluid-migration pathways towards the eastern fumaroles. The results provide structural constraints that clarify mechanisms of fluid migration inside the crater. The techniques offer complementary geophysical images to the interpretation of hydrothermal processes and prove that seismic attenuation measurements are suitable to map fluid pathways in heterogeneous media at a detailed scale.

New Insights Into the Recent Magma Dynamics Under Campi Flegrei Caldera (Italy) From Petrological and Geochemical Evidence

AbstractThe 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.

River network and alluvial events in the urban area of Naples (Italy)

The town of Naples experienced alluvial events even more frequently than volcanic ones. These events are testified by alluvial deposits preserved in the subsurface of the town. Unfortunately, the intense urbanization has diffusely masked the presence of river valleys, whose recognition and correct location plays a crucial role in urban planning strategies. To unravel this issue, a detail scale geomorphological and morphometrical analysis of the study area has been carried out by means of detail scale topographic maps and DTMs. The reconstructed river network features are strongly influenced by the topographic and geomorphological setting, with centripetal (on inner flank) and centrifugal (on outer flank) radial patterns in correspondence of the volcanic edifices of the Campi Flegrei caldera and a dendritic/radial pattern where the volcanic apron of the caldera is present. To the east of Naples, an isolated river is present, which is named the Sebeto River and that is also known as the hidden river. Even if rivers are masked by urbanization, their presence is also testified by terms arena, cupa, cavone and vallone, which are commonly used to indicate roads, streets and quarters placed in correspondence of the river valleys.

Integrated on‐land‐offshore stratigraphy of the Campi Flegrei caldera: New insights into the volcano‐tectonic evolution in the last 15 kyr

AbstractSilicic calderas are volcanic systems whose unrest evolution is more unpredictable than other volcano types because they often do not culminate in an eruption. Their complex structure strongly influences the post‐collapse volcano‐tectonic evolution, usually coupling volcanism and ground deformation. Among such volcanoes, the Campi Flegrei caldera (southern Italy) is one of the most studied. Significant long‐ and short‐term ground deformations characterise this restless volcano. Several studies performed on the marine‐continental succession exposed in the central sector of the Campi Flegrei caldera provided a reconstruction of ground deformation during the last 15 kyr. However, considering that over one‐third of the caldera is presently submerged beneath the Pozzuoli Gulf, a comprehensive stratigraphic on‐land‐offshore framework is still lacking. This study aims at reconstructing the offshore succession through analysis of high‐resolution single and multichannel reflection seismic profiles and correlates the resulting seismic stratigraphic framework with the stratigraphy reconstructed on‐land. Results provide new clues on the causative relations between the intra‐caldera marine and volcaniclastic sedimentation and the alternating phases of marine transgressions and regressions originated by the interplay between ground deformation and sea‐level rise. The volcano‐tectonic reconstruction, provided in this work, connects the major caldera floor movements to the large Plinian eruptions of Pomici Principali (12 ka) and Agnano Monte Spina (4.55 ka), with the onset of the first post‐caldera doming at ca. 10.5 ka. We emphasise that ground deformation is usually coupled with volcanic activity, which shows a self‐similar pattern, regardless of its scale. Thus, characterising the long‐term deformation history becomes of particular interest and relevance for hazard assessment and definition of future unrest scenarios.

Deformation-controlled long-period seismicity in low-cohesion volcanic sediments

Volcano seismicity is an important tool in monitoring and forecasting activity at volcanoes globally. Volcanic earthquakes show diverse spectral characteristics, of which shallow long-period (low-frequency) seismicity and long-duration tremor are generally interpreted as indicators of fluid migration, and as potential eruption precursors. Here we show that a common low-cohesion volcanic sediment from the Campi Flegrei caldera (Italy) produces low-frequency and long-duration seismicity as it undergoes deformation in dry conditions. We employed acoustic-emission rock-deformation experiments at a range of strain rates to produce events that, when normalized for scale, were spectrally indistinguishable from the long-period and tremor seismicity observed in natural volcanic settings. The generation of these signals was enhanced at lower laboratory strain rates. Correlated X-ray tomography of the samples before and after deformation constrained the source as distributed damage. Long-period seismicity at volcanoes may be generated by deformation of weak material in the subsurface, not solely by fluid movement, according to rock deformation experiments.

Fluid migrations and volcanic earthquakes from depolarized ambient noise

Ambient noise polarizes inside fault zones, yet the spatial and temporal resolution of polarized noise on gas-bearing fluids migrating through stressed volcanic systems is unknown. Here we show that high polarization marks a transfer structure connecting the deforming centre of the caldera to open hydrothermal vents and extensional caldera-bounding faults during periods of low seismic release at Campi Flegrei caldera (Southern Italy). Fluids pressurize the Campi Flegrei hydrothermal system, migrate, and increase stress before earthquakes. The loss of polarization (depolarization) of the transfer and extensional structures maps pressurized fluids, detecting fluid migrations after seismic sequences. After recent intense seismicity (December 2019-April 2020), the transfer structure appears sealed while fluids stored in the east caldera have moved further east. Our findings show that depolarized noise has the potential to monitor fluid migrations and earthquakes at stressed volcanoes quasi-instantaneously and with minimum processing.