This study investigates the seismic ambient noise within the Campi Flegrei caldera (Naples, Italy) to improve the detection capability and reliability of the earthquake monitoring system managed by the INGV, Osservatorio Vesuviano. It focuses on the spectral characteristics and spatial variability of the ambient noise field, aiming to identify the dominant frequency bands that control signal detectability and to provide information for optimizing the monitoring network. Due to the dense urban environment surrounding the caldera, seismic recordings are often contaminated by anthropogenic noise, which can mask low-magnitude volcanic or seismic signals. Power spectral density (PSD) analysis was applied to evaluate background noise levels at several broadband stations from permanent and temporary seismic networks over the period January 2022 to January 2023. The resulting PSD estimates were compared with the global Peterson noise models to assess station performance. Results show marked variability among stations, related to local human activity, proximity to infrastructure, and different installation settings (buried vs. surface). The study emphasizes the importance of noise monitoring to ensure high-quality seismic data, support optimal station siting, and refine monitoring strategies in densely populated volcanic regions such as Campi Flegrei, where the reliable detection of low-amplitude seismic and volcanic signals is essential.

Pantelleria island is the southernmost active volcanic system in Italy and hosts intense hydrothermal activity associated with significant geothermal potential. To improve seismic monitoring and investigate the shallow crustal structure of the island a temporary seismic network, integrated with the permanent INGV stations, was deployed. In this study, we present the first 1D shear-wave velocity (Vs) model for Pantelleria Island down to ∼1 km depth, based on ambient seismic noise analysis. Seismic array technique was applied to ambient noise recordings collected across the volcanic island. Stratigraphic information from deep wells, together with data from geophysical and geological surveys, was used to support the geological interpretation of the retrieved velocity model. The results provide new constraints on the shallow structure of Pantelleria and contribute to a better understanding of the dynamics of the volcanic-hydrothermal system. In addition the velocity model may improve earthquake localisation and support future studies of local seismicity potentially related to geothermal resources.

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

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.

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.

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.

Following the MD4.0 (Mw3.9) earthquake of August 21 2017 which occurred on the Ischia island (Naples, southern Italy), the local monitoring seismic network was significantly improved in terms of both number of stations and instrumentation performance. Due to the considerable amount of collected data, in particular of seismic noise recorded at broadband stations, some efforts have been addressed in particular to the definition of a 1D average velocity model effective for the whole island. This is an important scientific step because, in complex volcanic areas, the use of reliable velocity models is essential for an accurate localization of local earthquakes. In this work, the main target is to retrieve a pseudo-3D velocity model of the Ischia island. Specifically, we inverted H/V curves and frequency peaks evaluated at about twenty sites to obtain a velocity profile for each of the investigated measurement points. Taking into account that the H/V frequency peak depends on both velocity and thickness of layers, for each site we applied an inversion process fixing the velocities and modifying the thicknesses in order to obtain the corresponding 1D velocity models. We are quite enough confident about the robustness of models, since during the inversion process, we achieved a good convergence towards the best-fit solutions. Then, a pseudo-3D velocity model was obtained by contouring the 1D models of each station site 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 pseudo-3D model and the geological features of the island was observed, especially in the northern sector where most of the stations is 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 is in agreement with the structural setting of the island likely due to the resurgence of Mount Epomeo.

In volcanic areas, accurate localization of earthquakes requires detailed velocity and, possibly, attenuation models, taking into account wide lithological variability and high geothermal gradients. Ischia island (Campania region, Italy) is a seismically-active volcano recently affected by a Mw 3.9 event (Casamicciola, August 21, 2017, 1 km depth). Due to the lack of a specific velocity model, the earthquakes occurred on the island were localized using the one developed for the nearby Campi Flegrei caldera. The aim of this work is the definition of a mean representative ID shear-wave velocity (Vs) and attenuation (Q) model of the shallower crust (up to 2 km depth) of Ischia. Seismic noise array and spectral ratios techniques were applied to broad band seismic signals recorded by temporary and permanent networks updated after the August 2017 earthquake. The values of both shear-wave velocity (Vs) and quality factor (Q) are realistic, with Q values comparable with those obtained for Campi Flegrei and Stromboli volcanic areas. By taking into account stratigraphic information from deep wells and ultrasonic measurements of velocity on granite and trachytic lava samples, a geological interpretation of the resulting velocity model is provided. Such a model can have significant implications for understanding the dynamics of a volcano, mainly those leading to seismic activity. (C) 2020 Elsevier B.V. All rights reserved.

Spectral analysis has been applied to almost thousand seismic events recorded at Vesuvius volcano (Naples, southern Italy) in 2018 with the aim to test a new tool for a fast event classification. We computed two spectral parameters, central frequency and shape factor, from the spectral moments of order 0, 1, and 2, for each event at seven seismic stations taking the mean among the three components of ground motion. The analyzed events consist of volcano-tectonic earthquakes, low frequency events and unclassified events (landslides, rockfall, thunders, quarry blasts, etc.). Most of them are of low magnitude, and/or low maximum signal amplitude, therefore the signal to noise ratio is very different between the low noise summit stations and the higher noise stations installed at low elevation around the volcano. The results of our analysis show that volcano-tectonic earthquakes and low frequency events are easily distinguishable through the spectral moments values, particularly at seismic stations closer to the epicenter. On the contrary, unclassified events show the spectral parameters values distributed in a broad range which overlap both the volcano-tectonic earthquakes and the low frequency events. Since the computation of spectral parameters is extremely easy and fast for a detected event, it may become an effective tool for event classification in observatory practice.

249 PRELIMINARY ESTIMATION OF THE SHEAR WAVE VELOCITY MODEL OF ISCHIA ISLAND USING AMBIENT NOISE BROAD BAND RECORDS D. Galluzzo1, L. Nardone1, R. Manzo3, R. Di Maio3, M. Moretti2, F. Bianco1, M. Orazi1 1 Istituto Nazionale di Geofisica e Vulcanologia, Sez. di Napoli, Osservatorio Vesuviano, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Sez. di Roma Osservatorio Nazionale Terremoti, Italy 3 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università di Napoli Federico II, Italy