Deciphering the signal of external processes on active hydrothermal dynamics represents a critical challenge in understanding volcanic unrest. At the Pisciarelli hydrothermal site, located within the densely populated Campi Flegrei caldera (Italy), we investigate the role of meteoric water influx in modulating shallow hydrothermal fluid discharge. Our analysis reveals a persistent seismic tremor generated at shallow depths, whose amplitudes fluctuate noticeably and closely follow rainfall patterns. Hydrothermal activity also emits distinct acoustic signals related to boiling and steam venting. Importantly, we observe an inverse relationship between tremor and acoustic amplitudes: as tremor increases, acoustic emissions decrease. The influx of cold water is also linked to a rise in hydrothermal water level, indicating increased conduit pressure, which is supported by ground tilt data showing conduit inflation. We propose a conceptual model in which pressurization cycles in the shallow hydrothermal system result from the interplay between steam condensation, meteoric water influx, and steam outgassing. These pressurization cycles may promote seismicity, as evidenced by the seasonal patterns observed in earthquake rates. However, these seasonal effects are secondary to the dominant, accelerating multi-year trend of increasing earthquake rates due to volcanic unrest. Our findings demonstrate that seismo-acoustic monitoring provides a sensitive tool for detecting transient changes in hydrothermal dynamics, offering valuable insights into the interpretation of volcanic unrest signals. This, in turn, contributes to more refined volcanic hazard assessments. Although developed at Campi Flegrei, this approach holds potential for application to other active hydrothermal systems worldwide.

Seismic activity in volcanic regions such as Campi Flegrei (Italy) provides essential insights into subsurface dynamics and potential hazards. However, high background noise and continuous data volume challenge event detection and classification. Here, we apply a Self-Organizing Map (SOM) approach, combined with Linear Predictive Coding (LPC), STA/LTA ratios, and Multiscale Entropy (MSE), to analyze single-station seismic data. The method successfully identifies uncatalogued events and anomalies associated with fumarolic tremor, and reveals temporal relationships between clustering variation, [Formula: see text] emissions, and rainfall, suggesting environmental modulation. To assess the real-time applicability, the trained SOM was used on independent data from early 2025, confirming its ability to detect tremor intensification and anticipate a major local earthquake (Md 4.4). These results highlight the potential of entropy-based unsupervised learning for rapid seismic characterization and continuous volcano monitoring.

The Campi Flegrei caldera, located in Southern Italy, is one of the most hazardous volcanoes in the world. Seismic activity in the caldera has increased in the last year due to a new unrest phase of the volcano. This has driven the attention of the scientific community and both local and national media. The Campi Flegrei area is densely populated, with a very low signal-to-noise ratio, making the detection of small earthquakes or the recognition of non-seismic events more challenging. This study introduces a novel approach to the detecting and clustering of seismic signals. Unlike conventional methods that primarily focus on the regularity of seismic patterns, our research pivots towards a complexity-centric perspective using the Multiscale Entropy (MSE) algorithm, in conjunction with the Self-Organized Map (SOM) algorithm for effective data clustering. This methodological shift allows for a more nuanced exploration of the intricate dynamics inherent in seismic activities. Seismic signals are not random or chaotic but rather complex structures that vary across different scales. By employing the MSE algorithm, we unravel these complex patterns, offering insights into the seismic behaviours that traditional methods may overlook. Our findings indicate a significant correlation between the complexity of seismic signals and key geophysical events related to the dynamic of the volcano, suggesting that complexity analysis could be a significant tool in seismic monitoring and prediction. The analysed dataset is a six-month long continuous signals recorded in period at the V0102 temporary seismic station installed by the INGV-Osservatorio Vesuviano close to the Pisciarelli area, located in the Campi Flegrei caldera. MSE has been applied to one-minute-long traces of signal and clusterized through the application of the SOM analysis, revealing hidden layers of complexity across multiple scales. Moreover, the study explores the potential of integrating MSE analysis with other seismic analysis techniques to enhance the accuracy of seismic interpretations. By combining complexity analysis with traditional approaches, we aim to develop a more deep and comprehensive understanding of seismic signals, potentially leading to an improvement of the seismic risk assessment.

Local site conditions can significantly influence the amplitude, duration, and frequency content of seismic recordings, making the characterization of subsoil properties a critical component in seismic hazard assessment. However, despite extensive research, standardized methodologies for assessing site effects are still lacking. This study presents preliminary steps in the site characterization of a small area of Campi Flegrei caldera (Italy), with the aim of enhancing understanding of local lithology and seismic wave propagation. The analysis focuses on the broad-band seismic station CSTH, installed in 2021, and incorporates data from a temporary 2D array of five short-period sensors deployed around the station. These sensors recorded both ambient noise and seismic events associated with caldera dynamics. To improve the robustness of the characterization, data from two additional permanent broad-band stations (CPIS and CSOB) of the Istituto Nazionale di Geofisica e Vulcanologia—Osservatorio Vesuviano’s monitoring network, also located nearby a hydrothermal field, were included. Spectral analyses such as Power Spectral Density (PSD), Horizontal-to-Vertical (H/V) spectral ratios, and f-k array technique were performed to evaluate the frequency-dependent response of the site and to support the development of a comprehensive seismic site model.

The Campi Flegrei caldera, Italy, is considered one of the most high-risk volcanic areas on the planet due to its location within the metropolitan area of Naples. Campi Flegrei caldera is currently undergoing unrest. The unrest is linked to an uplift phase that started in 2005, which is accompanied by gas emissions and volcano-tectonic seismicity. Owing to the limited knowledge of the plumbing system and the pre-eruptive processes, tracking the evolution of a volcanic unrest is often based on probabilities obtained by experts’ elicitation. In this work, we present the daily variation of the probability that the unrest is driven by a shallow magma movement and the monthly probability of eruption based on the Bayesian Event Tree for Eruption Forecasting (BET_EF) model calibrated for Campi Flegrei by means of the outcomes of the VI experts’ elicitation carried out in 2015. The results show that according to the interpretive framework provided by experts, the probability that the mechanism behind the current unrest is shallow magma movements is not negligible, but the monthly probability of eruption remained overall constant during the entire period.

The real-time monitoring of densely populated areas with high seismic and volcanic risk is of crucial importance for the safety of people and infrastructures. When an earthquake occurs, the Earth surface experiences both translational and rotational motions. The latter are usually not monitored, but their measurement and characterization are essential for a full description of the ground motion. Here we present preliminary observational data of a highsensitivity rotational sensor based on a 2 -km -long fiber-optic Sagnac gyroscope, presently under construction in the middle of the Campi Flegrei Volcanic Area (Pozzuoli, Italy). We have evaluated its performance by analyzing data continuously recorded during an acquisition campaign of five months. The experimental setup was composed of a digital nine -component seismic station equipped with both a rotational sensor and conventional seismic sensors (seismometers, accelerometers, and tiltmeters). During this experiment we detected seismic noise and ground rotations wavefield induced by small to medium local earthquakes (M-D < 3). The prototype gyroscope shows a very promising sensitivity in the range of 5 x 10 (- 7) - 8 x 10( - 9) rad / s /root Hz over the frequency bandwidth 5 mHz-50 Hz. Future upgrades and perspectives are discussed. (c) 2024 Optica Publishing Group

The present work describes preliminary results of the analisys carried out on the signals provided by a rotational seismometer installed in the Campi Flegrei volcanic area. Starting from January 2021, a rotational seismometer (Lunitek Tellus R2) was installed in the area. The analyzed data consists of 20 local earthquakes occurred in Campi Flegrei from February 2021. The local seismicity is composed by low magnitude volcano tectonic earthquakes ( M D max = 3.6) . 6) located at depth between 0.3 and 4 km b.s.l. For the strongest earthquake, we observed maximum rotational velocity equal to 6 mrad/s. The joint use of the rotational sensor and the accelerometer confirms the possibility of estimating the azimuth and the apparent velocity of the incoming wavefront and highlighted some more correlated phases in the coda of seismograms that could provide useful indications about the diffuse wave field recorded in the investigated area.

The volcanic island of Ischia has shown to have an important seismogenic potential, being the location of several destructive earthquakes, e.g. 1881, 1883 and 2017. The damage caused by these earthquakes was more connected to the proximity of the source to the surface than to their magnitude (M w < 5.2). The need to monitor and model this seismicity required the installation of a dense and modern seismic network. The first modern seismic station on the island was installed in 1993, and the network was successively increased with time. A meaningful improvement to the network was made after the earthquake that occurred on the 21 of August 2017. The network currently has 11 sites with velocimeters and some of them with accelerometers installed too. We analysed the seismic network configuration in comparison with the seismicity that characterizes the area to mark a starting point for future seismological analysis. The network is currently able to locate shallow earthquakes with duration magnitude greater or equal to 0 in the whole island.

Seismicity at Mt. Vesuvius has been relatively weak in the last decades. While the occurrence of shallow volcanotectonic (VT) events at Mt. Vesuvius is well known, the occurrence of deeper low frequency events (LF) was only recently recognized. Previous source studies only targeted VT events, which were found to have quite heterogeneous focal mechanisms. In this paper, we perform for the first time the source inversion of LF seismicity at Mt. Vesuvius, analysing 27 LF events recorded from 2012 to 2021 with the aim to investigate their source processes. Given the challenges of analysing weak LF earthquakes, we implement a specific moment tensor (MT) inversion approach that combines the fit of displacement seismograms in the time domain and amplitude spectra in the frequency domain. The inversion is simultaneously performed for the source depth and moment tensor components in the 2-7 and 2-5 Hz frequency band, assuming either a full or deviatoric MT representation. Source parameter uncertainties are estimated by using a Bayesian bootstrapping scheme. Our results confirm a larger depth of LF events compared to VTs and show a strong heterogeneity of the LF seismic sources, which present various rupture types, different orientations and heterogeneous, whilst poorly resolved, non-double-couple components. The MT variability is qualitatively confirmed by significant differences among the recorded waveforms. The heterogeneity of both VT and LF source processes is attributed to complex source processes in a highly fractured seismogenic volume submitted to a heterogeneous stress field.

Mt. Vesuvius is a high-hazard active volcano surrounded by a densely populated area. Since human activities generate high levels of seismic noise, recognizing low-amplitude seismic events in the signals recorded by the local seismic monitoring network operating at Vesuvius is very difficult. Here, we describe an automatic procedure applied to continuous data with the aim of finding low-amplitude–low-frequency events hidden in the recorded signals. The methodology is based on the computation of two spectral parameters, central frequency Ω and shape factor ẟ, at selected sites, and the coherence of the seismic signal among different sites. The proposed procedure is applied to 28 months of recordings from 2019 to 2021, tuning the search parameters in order to find low-frequency signals similar to those occasionally observed in the past at the same volcano. The results allowed us to identify 80 seismic events that have the spectral features of low-frequency earthquakes or tremor. Among these, 12 events characterized by sufficiently high signal-to-noise ratio have been classified as deep low-frequency earthquakes, most of which are not reported in the catalog. The remaining events (more than 60) are characterized by similar spectral features but with an extremely low amplitude that prevents any reliable location of the source and definitive classification. The results of this work demonstrate that the low-frequency endogenous activity at Mt. Vesuvius volcano is more frequent that previously thought.

The ability to image the underground structures of volcanoes is limited by the precision, resolution and pene-tration depth of each single geophysical method. In order to improve the knowledge of specific volcanic edifices and to better understand the general behavior of structures, the use of a combination of methods is strongly recommended to exploit and maximize their complementary capabilities of resolution and penetration depths. In this work a large dataset of seismic and electromagnetic measurements has been used to provide a more detailed and improved geophysical image of the shallower portion of the northern sector of Ischia Island (Campania region, Italy), severely hit by the August 21, 2017 earthquake (Mw 3.9). We analysed data by using different methodologies: Horizontal-to-Vertical Spectral Ratio (HVSR), seismic array technique (f-k), polarization analysis and Time Domain ElectroMagnetic (TDEM) survey. These methods are sensitive in a different way to tectonic features, lithologies, layer geometry and fluid distribution. Thus, their combination is useful for studying sites with complex crustal structures such as Ischia island, which is characterized by a well-developed geothermal system linked to the presence of a shallow magmatic body. Results of our study provides detailed information of the physical properties of the subsoil through: 1) the spatial distribution of the amplification parameters of ground motion, showing frequency peaks below 1 Hz and/or between 1 Hz and 5 Hz; 2) the definition of the velocity models up to 600 m depth, with shear wave velocities ranging from 150 m/s for the shallower layers to 2500 m/s for the half space; 3) the recognition of the correlation between the principal fault structures and polarization directions of the noise wavefield, mostly oriented along EW and NE-SW directions; 4) the resistivity models of the first 80 m depth with high resistivity values of the shallow layers in the range 50-100 omega.m and low resistivity values of the bottom layers in the range 1-10 omega.m.

ABSTRACT Mefite d’Ansanto (Italy) is a nonvolcanic field characterized by persistent strong degassing activity. A seismic field monitoring carried out during the Summer 2021 reveals a persistent, extended, and complex source of seismic tremor characterized by a spectrum with a frequency content from about 1 Hz to more than 35 Hz. While at frequency smaller than 3 Hz the signal amplitude is stationary, in the intermediate frequency band (3–20 Hz) sudden changes of amplitude are often observed, suggesting the existence of an intermittent source (every few minutes to tens of minutes). Furthermore, very short bursts of high-frequency energy are recognized in the tremor signal. Results of array analysis and seismological observation indicate that the sources of the analyzed tremor are located in a small area centered on the main vent of the degassing area. The persistent low-frequency tremor and the intermediate frequency signals propagate as surface waves to the seismic stations installed around the source and indicate a very shallow source. On the contrary, impulsive signals at frequencies greater than 20 Hz propagate as body waves, revealing a deeper source likely located between 50 and 100 m depth.

We compiled a database for the Campi Flegrei seismic events that occurred from 2011 to 2018 at all stations available (merging permanent and temporary networks). Then we computed the two observables of the crustal anisotropy: time delay between fast and slow S-wave’s arrivals, and polarization direction of the fast S-wave. These results provide useful information about the amount of crustal anisotropy and the main direction, respectively, with this latter representing a proxy for the local stress field. We could thus obtain a picture of their spatial and temporal distributions to be compared with other geophysical and geochemical observations. In particular we could identify common features, such as change points, to several time series. This helps us in building a more complete interpretation of the volcanic system changes that were occurring during the recent ongoing unrest phase, which started in 2005.

A passive seismic experiment is carried out at the non-volcanic highly degassing site of Mefite d’Ansanto located at the northern tip of the Irpinia region (southern Italy), where the 1980 MS 6.9 destructive earthquake occurred. Between 2020 and 2021, background seismic noise was recorded by deploying a broadband seismic station and a seismic array composed of seven 1 Hz three-component sensors. Using two different array configurations, we were allowed to explore in detail the 1–20 Hz frequency band of the seismic noise wavefield as well as Rayleigh wave phase velocities in the 400–800 m/s range. Spectral analyses and array techniques were applied to one year of data showing that the frequency content of the signal is very stable in time. High frequency peaks are likely linked to the emission source, whereas at low frequencies seismic noise is clearly correlated to meteorological parameters. The results of this study show that small aperture seismic arrays probe the subsurface of tectonic CO2-rich emission areas and contribute to the understanding of the link between fluid circulation and seismogenesis in seismically active regions.

The detection of low energy seismic events and tremor related to volcanic activity in areas characterized by high background noise represents a crucial challenge for monitoring and surveillance purposes. In the last three years, the seismicity of the Mt. Vesuvius (southern Italy) has been characterized by low-magnitude volcano tectonic earthquakes, the most of which are located at depth shallower than 3 km b.s.l., while very few low-frequency earthquakes and tremor episodes are located at about 6-7 km depth. It is well known that magmatic and hydrothermal systems can play an important role in the generation of low-frequency seismic events, which could be important precursors for assessing the reawakening of a volcano. Therefore, our main objective is to develop a methodology for detecting the presence of low frequency (LF) events hidden in the background noise and not identifiable by classical detection procedures. In particular, we suggest a frequency domain approach based on a joint application of coherence analysis among signals from local network seismic stations and parameterization of the amplitude spectra according to the statistical moments. The proposed methodology has been applied to the analysis of continuous seismic signals recorded over three years at Mt. Vesuvius. Spectral parameters, such as central frequency W, shape factor d and coherence c, were evaluated on 30-s windows signals in the frequency range between 2 and 40 Hz. The selection of the signal windows that could potentially contain low-frequency events or tremor signals was performed according to the following criteria: a) 0.45 < δ < 0.65; b) 3 Hz < W < 6 Hz and c) c greater than 0.5, which are based on the results of preliminary analyses of the seismicity observed at Mt. Vesuvius. The detected signal windows were visually inspected and compared with the seismic catalogues to eliminate those corresponding to earthquakes occurred outside the area of interest. For the three-years of analyzed data, more than 200 episodes of low frequency signals were identified, 120 of which are not present in the seismic catalog. Most of them appear as low-amplitude tremor episodes, with no clear evidence of P and S phases, hidden in the noisy raw signals but visible at the entire seismic network after proper signal filtering. Compared to the few LF events detected and analysed in the past, our findings suggest that the proposed methodology can be an efficient tool for detecting low-amplitude signals not easily identifiable in the background noise and could represent an improvement for the monitoring system of the Mt. Vesuvius volcanic area.

An accurate survey of old and new datasets allowed us to probe the nature and role of fluids in the seismogenic processes of the Apennines mountain range in Italy. New datasets include the 1985–2021 instrumented seismicity catalog, the computed seismogenic thickness, and geodetic velocities and strains, whereas data from the literature comprise focal mechanism solutions, CO2 release, Moho depth, tomographic seismic velocities, heat flow and Bouguer gravity anomalies. Most of the inspected datasets highlight differences between the western and eastern domains of the Apennines, while the transition zone is marked by high geodetic strain, prevailing uplift at the surface and high seismic release, and spatially corresponds with the overlapping Tyrrhenian and Adriatic Mohos. Published tomographic models suggest the presence of a large hot asthenospheric mantle wedge which intrudes beneath the western side of the Apennines and disappears at the southern tip of the southern Apennines. This wedge modulates the thermal structure and rheology of the overlying crust as well as the melting of carbonate-rich sediments of the subducting Adriatic lithosphere. As a result, CO2-rich fluids of mantle-origin have been recognized in association with the occurrence of destructive seismic sequences in the Apennines. The stretched western domain of the Apennines is characterized by a broad pattern of emissions from CO2-rich fluids that vanishes beneath the axial belt of the chain, where fluids are instead trapped within crustal overpressurized reservoirs, favoring their involvement in the evolution of destructive seismic sequences in that region. In the Apennines, areas with high mantle He are associated with different degrees of metasomatism of the mantle wedge from north to south. Beneath the chain, the thickness and permeability of the crust control the formation of overpressurized fluid zones at depth and the seismicity is favored by extensional faults that act as high permeability pathways. This multidisciplinary study aims to contribute to our understanding of the fluid-related mechanisms of earthquake preparation, nucleation and evolution encouraging a multiparametric monitoring system of different geophysical and geochemical observables that could lead the creation of a data-constrained and reliable conceptual model of the role of fluids in the preparatory phase of earthquakes in the Apennines.

The Mt. Pollino area has been affected by a 4-year long seismic sequence, occurred between 2010 and 2014 and characterized by low-to-moderate seismicity and two moderate events (ML 4.3 and ML 5.0). The sequence developed as a combination of swarm-like and aftershocks. The two main earthquakes occurred late in the sequence, with a slow-slip event starting 3-4 months before the largest earthquake and lasting for a year. Despite the lack of historical and instrumental recordings of strong earthquakes (M>6), paleo-seismological investigations confirm the occurrence in the last 10,000 years of at least two M 6.5-7 earthquakes on the Pollino and Castrovillari faults, located in the SE sector of the Mt. Pollino area. Thus, the area has been marked as the widest high seismic hazard gap in Italy. In this study we present the most recent advancements in the comprehension of the main peculiarities of the last seismic sequence and of its space and time evolution. New local 3D P- and S-wave tomographic images offered a detailed picture of the main lithological units involved in the sequence and more reliable earthquake hypocenter locations. The inferred velocity contrasts have been compared with 2D scattering and absorption maps computed for the area, along with total direct wave attenuation. Clusters of events of similar waveforms (cross-correlation higher than 0.8) have been selected and located applying the master-slave relative location technique. New fault mechanisms have been computed. These mechanisms allowed modeling the local stress field and performing a Focal Mechanism Tomography. Its result was an evaluation of the excess of pore fluid pressure in the volume interested by the sequence. A 1D diffusivity analysis suggests a pore fluid pressure diffusion which, in addition to the Coulomb static stress transfer, can explain the delayed triggering of the two larger events. This work has been supported by the CORE (“sCience and human factor for Resilient sociEty”) project, funded from the European Union’s Horizon 2020 - research and innovation program under grant agreement No 101021746 and by PRIN-MATISSE (20177EPPN2) project funded by Italian Ministry of Education and Research.

SUMMARY Following the Mw 3.9 earthquake that occurred in the Ischia island (Naples, southern Italy) on 21 August 2017, the local monitoring seismic network was significantly improved in terms of both number of stations and instrumentation performance. Due to the huge amount of collected seismic ambient noise data, in this paper we present a first 3-D shear wave velocity model of the island retrieved from the inversion of horizontal-to-vertical spectral ratio curves by fixing the shear wave velocities (Vs) and modifying the thicknesses to get the corresponding 1-D Vs models. We are confident about the robustness of the attained models since the inversion process provided a good convergence towards the best-fitting solutions. Then, a first 3-D velocity model was obtained by contouring all the 1-D models obtained for the selected seismic stations to highlight possible lateral variations of the layer thicknesses and to reconstruct the morphology of the deeper interface characterized by a high-impedance contrast. A good correspondence between the 3-D Vs model and the geological features of the island was observed, especially in the northern sector where most of the stations are installed. In particular, the top of the high-impedance contrast interface appears deeper in the northern coastal areas and shallower in the central sector. This result agrees with the structural settings of the island likely due to the resurgence of Mount Epomeo.

This article presents findings from two episodes of seismicity and gas emission that occurred on 7 October 2015 and 6 December 2019 in Campi Flegrei caldera. This caldera has been affected by long-term unrest since 2004. The 6 December 2019 episode, consisting of a swarm of 38 earthquakes (maximum duration magnitude 3.1, the largest between 1984 and March 2020), occurred at the end of a one month period characterized by an increase in the ground uplift rate from 0.19 +/- 0.01 to 0.72 +/- 0.05 mm/day. A sudden increase in the fumarolic tremor amplitude, which is a proxy of gas emission related parameters recorded at Solfatara?Pisciarelli hydrothermal area (e.g., CO2 air concentration), was observed during the seismicity episode. The uplift rate decreased immediately after the swarm (0.10 +/- 0.01 mm/day), whereas the fumarolic tremor amplitude remained higher than that observed prior to the swarm. Through analyzing the time series of uplift recorded in Pozzuoli (central area of the caldera) from differential measurements on tide gauges, we were able to identify the 2015 episode. This episode was characterized by increasing uplift rates that culminated in a seismic swarm of 33 earthquakes on 7 October, which was followed by decreasing uplift rates. We computed double-difference locations of earthquakes from the two swarms and found that they located along a conduit-like path, coinciding with a high-resistivity contrast zone, previously identified by audiomagnetotelluric measurements. The focal mechanisms of the major earthquakes of both swarms indicate fault planes radial with respect to the maximum uplift area. These phenomena can be interpreted as episodes of the volcanic and (or) hydrothermal system pressurization that culminate in an injection of fluids along the conduit-like path, which behaves as a valve that allows fluid discharge and the temporary depressurization of the source region.