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Historic volcanic activity in Tenerife was concentrated within two of the island's three dorsals and on the Teide-Pico Viejo complex located inside Las Cañadas caldera. Eruptions on the island are primarily characterised by basaltic and trachybasaltic fissural eruptions. However, the Teide-Pico Viejo complex also hosted explosive and effusive phonolitic eruptions. Our study focused on the analysis of the 2004-2005 unrest in Tenerife, which was characterised by an onset of a ground deformation pattern, heightened on-land seismic activity, changes in the chemical composition of fumaroles of the Teide composite volcano, an increase in diffusive emissions of carbon dioxide along the NW rift, and, significant gravity changes. We used the Envisat-ASAR satellite images from 2003 to 2010 to generate the Line-Of-Sight SBAS- DInSAR deformation time series to investigate the source responsible for ground deformation. We applied the Independent Component Analysis (ICA) to separate distinct ground deformation patterns. Specifically, we selected four components for the Independent Component Analysis decomposition: the first one mainly affects the stratovolcano's summit region of Teide and shows a circular symmetry; the second and third components are possibly related to the topography and atmospheric artifacts, while the fourth contains only a noisy signal. We employed a non-linear optimisation approach in a Finite Element modelling environment to determine the source geometry responsible for the first identified ICA pattern of ground deformation within Las Cañadas. Our results revealed the existence of an ellipsoidal ground deformation source oriented along the E-W axis, located beneath the Teide and Montaña Blanca volcanoes at 1,600 m a.s.l. This source became active during the seismic crisis of 2004-2005, which was associated with the degassing of a magma batch that intruded into the northwest rift of Tenerife island. We propose that the ground deformation observed at Teide volcano during the 2004-2005 crisis was related to hydrothermal activity within the volcano. [ABSTRACT FROM AUTHOR]

El Hierro island is one of the most active islands in the Canary Islands from a volcanological point of view. This is the reason why the imaging of the internal crustal structure is of huge importance. The geophysical exploration methods employed on El Hierro Island, such as gravimetry and seismic tomography, allowed obtaining the high-resolution characterization of the crust's deep part. However, these methods did not yield significant information about the surface and the shallower part of the crust. To gain a deeper insight into the shallow geological structure of El Hierro island, we employed Ambient Noise Tomography to construct a 3D S-wave velocity model. Our investigation revealed the presence of seven significant seismic velocity anomalies, partly identified by previous studies. We identified two high-velocity anomalies located in the eastern and western parts of the island at a depth between 0 and 3 km below sea level (b.s.l.). We interpreted these anomalies as dense intrusive complexes of dikes, possibly linked to the Tanganasoga volcano and the formation of the Tinor edifice. Additionally, we observed two high-velocity anomalies in the northern and southern parts of the island at a depth between 3 and 4 km b.s.l., which we related to the accumulation of solidified igneous rocks. On the other hand, a low-velocity anomaly was observed in the Golfo valley, between 0 and 0.5 km b.s.l., and we interpreted itas megalandslide deposits. This anomaly was evidenced for the first time in the present study. Finally, two low-velocity anomalies were observed in the southern part of the island at different depths, between 0-0.5 km b.s.l. and 0-2 km b.s.l. These were interpreted as fractures generated by Quaternary volcanism along the SSE Rift. Also, one of them was evidenced for the first time in this study, corresponding to the zone of the fractures produced during the Quaternary volcanism. This study has allowed us to gain a more detailed understanding of the shallow geological structure of the island. Even if most of the anomalies had been evidenced previously, we could observe the existence of two low-velocity zones in the shallow crust that have not been observed before. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

One of the strategies to detect the precursors of an eruption is to define the background dynamical state of a volcano for a prompt recognition of deviations from the basic condition. Mt. Vesuvius (Italy), currently in a quiescent state, is one of the most monitored volcanoes in the world, inciting multidisciplinary advanced studies. Hence an understanding of the links among the different monitored parameters is mandatory. In recent decades the joint analyses of ground tilt and seismicity have added to the understanding of the volcano's activity. In this paper, we outline the first steps towards a comprehension of the link between Mt. Vesuvius earthquakes and co-seismic ground tilt, after excluding the contribution of other external forces acting on the ground, such as tides, landslides or exceptional meteorological phenomena. We used the seismicity with a duration magnitude ≥ 2.0 recorded at Mt. Vesuvius in the period 2018–2020 to estimate the source parameters and to calculate the associated static displacement. Then, we compared the ground inclination retrieved from the estimated seismic deformation with the long-term ground motion trend measured by tiltmeters. We found that in most cases the two vectors have a comparable size and direction. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We analyse tiltmeter time series recorded from April 2015 to March 2019 at three borehole instruments installed at Campi Flegrei caldera (Italy). We evaluate the crustal response in terms of ground tilting to external excitations of medium/long-period tidal constituents by applying a polarization analysis. The azimuths of the tilt vectors show well-defined polarization directions and the ground tilting planes oscillate with the periodicity of the corresponding tidal constituents. For two of the three tiltmeters, the average ground oscillation pattern related to the monthly Mm and fortnightly Mf constituents show seasonal variations, which can be ascribed to rainfall-induced tilting. In addition, for the same two instruments, a clear seasonal amplitude modulation of the diurnal S1 constituent appears in the time series, revealing the occurrence of site thermoelastic effects. The results indicate that the tidal tilting is mainly controlled by the local stress field distribution and rheology; in addition, seasonal exogenous factors like rainfalls play a role in modulating the ground deformation. [ABSTRACT FROM AUTHOR]

We study oscillations recorded at Stromboli and Campi Flegrei by different sensors: seismometers, strainmeters and tiltmeters. We examine both the high-frequency (>0.5 Hz) portion of the spectrum and very long period signals up to tidal scales. In this context, seismicity and deformation are investigated on different time scales (from minutes to days/years) in order to identify the basic elements of their interaction, whose understanding should provide new insights on the predictive models. In this work, the strict relation of tides and volcanic processes is shown. At Stromboli, indeed the transition from the stationary phase to the non-stationary phase seems to have a tidal precursor that is related to the duration of the crisis. The subsequent volcanic activity is interpreted as the response of the volcano to restore the equilibrium condition. The moveout from equilibrium produces, first, variations in the standard statistics of explosions, then leads to effusive stage and to a pressure drop in the shallow feeding system. That process induces the nucleation of a gas bubble and the excitation of low frequencies. Campi Flegrei seismicity shows a correlation between the diurnal solar solid tide and the energy released by the long period signals, indicating that the whole mechanism is modulated on a tidal scale. In other words, in the case of Stromboli, a departure from the equilibrium state is marked by solid tide variations in a certain frequency band. On the other hand, at Campi Flegrei diurnal to annual solid tides modulate an increase of volcanic activity. [ABSTRACT FROM AUTHOR]

Abstract: We present a detailed analysis of the source properties of Long-Period (LP) signals recorded at Campi Flegrei Caldera (Italy) during the last (2005–2006) mini-uplift episode. Moment Tensor inversion via full-waveform modelling of broad-band seismograms indicates a crack-like source with a significant volumetric component. From auto-regressive modelling of the signal''s tail we evaluate the dominant frequency and the attenuation factor of the oscillating source. Considering the acoustic properties of a fluid-filled crack, these values are consistent with the resonant oscillations of a crack filled by a water–gas mixture at variable gas–volume fraction. For these fluids, the crack size would be on the order of 40–420 m, a size range which is consistent with the spatial spreading of LP hypocenters. Analysis of temporally-correlated time series of seismological and geochemical data indicates that climaxing of LP activity was preceded by swarms of volcano-tectonic (VT) events and rapidly followed by a consistent increase of both thermal emissions and gas fluxes recorded at the surface (1 month — 2/3 days, respectively). Following these observations, we propose a conceptual model where VT activity increases permeability of the medium, thus favouring fluid mobility. As a consequence, the hydrothermal system experiences pressure perturbations able to trigger its resonant, LP oscillations. [Copyright &y& Elsevier]

New duration-based local (ML) and moment (Mw) magnitude scales are obtained for the Campi Flegrei area through analysis of a dataset of local volcano-tectonic earthquakes. First, the S-wave quality factor for the investigated area was experimentally calculated, and then the distance-correction curve, log A0(r), to be used in the Richter formula ML = log Amax - log A0(r), was numerically estimated by measuring the attenuation properties and, hence, propagating a synthetic S-wave packet in the earth medium. The local magnitude scale was normalized to fit the Richter formula that was valid for Southern California at a distance of 10 km. ML was estimated by synthesizing Wood--Anderson seismograms and measuring the maximum amplitude. For the same dataset, the moment magnitude was obtained from S-wave distance-corrected and site-corrected displacement spectra. Comparisons between local and moment magnitudes determined, along with the old duration magnitude (MD) routinely used at the Istituto Nazionale di Geofisica e Vulcanologia--Osservatorio Vesuviano, are presented and discussed. Moreover, the relationships between ML and Mw calculated for two reference sites are also derived. [ABSTRACT FROM AUTHOR]

We study the time changes of (1) the b-value of the Gutenberg-Richter distribution, (2) the inverse coda Q(Qc-1), and (3) the shear-wave splitting parameters (i.e., the time delay Td between qS1 and qS2 phases and the polarization direction of the qS1 wave) for small-magnitude volcano-tectonic earthquakes of Mt. Vesuvius, Italy. We used for (1) the seismic catalog of Mt. Vesuvius seismicity starting from January 1994, for (2) a selected (on the basis of the best signal-to-noise ratio) set of data with hypocentral distances smaller than 4 km recorded at station BKE (analogical) with a 1-Hz vertical seismometer during the period from January 1994 until the present, and for (3) a set of data recorded at two digital, high dynamical range, portable short-period seismic stations. These stations (BKE and BKN) were in operation in two periods, BKE (digital) from January 1999 to the middle of 2000 and BKN from January 1999 to the end of 1999; the hypocentral distances were not greater than 4 km. We found evidence of time changes of Qc-1 measured at high frequency (6, 12, and 18 Hz). The changes seem to be correlated with the occurrence of two swarms with largest magnitudes of 3.4 and 3.6, respectively in April 1996 and October 1999. The earthquake with the largest magnitude in the second swarm appears to be the largest event since the latest eruption in 1944. An increase in Qc-1starts after the occurrence of both swarms, reaching a maximum after more than 1 yr for the first swarm and after 6 months for the second swarm. These two changes were not accompained by any corresponding variation of the b-value, which shows an almost constant (inside the statistical uncertainty) pattern. The last swarm (M 3.6) was precedeed by an increase of Td at both stations, indicating a possible change of the stress state before the M 3.6 earthquake. The qS1... [ABSTRACT FROM AUTHOR]

In the last decades, thermal infrared ground-based cameras have become effective tools to detect significant spatio-temporal anomalies in the hydrothermal/volcanic environment, possibly linked to impending eruptions. In this paper, we analyzed the temperature time-series recorded by the ground-based Thermal Infrared Radiometer permanent network of INGV-OV, installed inside the Solfatara-Pisciarelli area, the most active fluid emission zones of the Campi Flegrei caldera (Italy). We investigated the temperatures' behavior in the interval 25 June 2016–29 May 2020, with the aim of tracking possible endogenous hydrothermal/volcanic sources. We performed the Independent Component Analysis, the time evolution estimation of the spectral power, the cross-correlation and the Changing Points' detection. We compared the obtained patterns with the behavior of atmospheric temperature and pressure, of the time-series recorded by the thermal camera of Mt. Vesuvius, of the local seismicity moment rate and of the CO2 emission flux. We found an overall influence of exogenous, large scale atmospheric effect, which dominated in 2016–2017. Starting from 2018, a clear endogenous forcing overcame the atmospheric factor, and dominated strongly soil temperature variations until the end of the observations. This paper highlights the importance of monitoring and investigating the soil temperature in volcanic environments, as well as the atmospheric parameters. [ABSTRACT FROM AUTHOR]

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

We analyse and compare volcano-tectonic seismicity, rainfall, atmospheric pressure, ground deformation and Earth tide time series at the Campi Flegrei caldera (CFc) in the time interval 2005-2016, looking for possible tidal and hydrological triggers of hydrothermal and seismic activity. By applying statistical analysis, we show that the occurrence of clustered seismicity at the CFc has a cyclic behaviour on several time scales, from semidiurnal and diurnal to longer periods, such as fortnightly, monthly, semiannual and annual. The medium and long-period time scales match those of the main lunar and solar tidal constituents, while the exactly 24-h and 12-h periodicities can be ascribed not only to the Earth tides but also to other effects such as atmospheric pressure and temperature variations. In addition, a clear dependence on rainfall arises from the analysis of the hydrological parameters, indicating a strong seasonal periodicity along the entire observational interval, with the most numerous and energetic seismic swarm occurring in the wet season.We interpreted these observations in terms of a model linking magmatic degassing, ground deformation and seismicity. The proposed model considers the magmatic-hydrothermal fluid circulation system as a cascade hydraulic circuit, where in/out fluid flow and pressure condition of each segment are regulated by those of the contiguous ones. Our results suggest that volcano-tectonic seismicity at CFc is driven by both variations in the deep magmatic feeding system and exogenous phenomena, as rainfall or global inflation/deflation cycles of the Earth' s crust, controlled by the lunisolar interaction. Consequently, the role of exogenous triggers (combined with endogenous processes) in the evolution of the recent dynamics should be properly considered in the elaboration of volcanic risk scenarios. [ABSTRACT FROM AUTHOR]

Long period volcanic tremors (LPTs), typically termed as VLP, have been widely observed in many volcanic systems around the world. LPTs in different volcanic settings are often repetitive, suggesting a nondestructive source and providing critical insights into the fluid dynamic processes operating inside a volcanic system. Recently, a deep triggering source of LPT has also been discovered, located between the LPT source and the magma chamber. These diverse and rich signals not only help monitor the state of shallow volcanic conduit and its link to upcoming eruptions. it also helps understand causal triggering of LPT and potentially intra-crustal transport of magma. Given diverse and rich signals within the Aso volcanic system, we further explore other possible sources in the tidal period. We analysed continuous data recorded at two borehole tiltmeters installed at Aso volcano, during the period of 2011-2016. We first perform a polarisation analysis of the ground tilt, filtering in the frequency bands matching the medium-long period tidal constituents, Mm, Mf and S1. The results evidence a well-defined direction of the tilting plane at both sites, with minor fluctuations occurring in 2012 and 2014. Moreover, we investigated the tilt time series the 1-8 hour band. There is a clear variation in azimuth orientation in at least one of the two tiltmeters and these azimuthal variations occur roughly from September 2013 at least until November 2014. In the meantime, LPTs were strong and more active, ash eruption occurred at the end of August 2014, followed by a major Strombolian eruption episode starting at the end of November 2014. In this time interval between September 2013 and November 2014, we further discovered a sequence of nearly monochromatic signals, with repetitive waveforms, a typical period of about 2 hours and a duration of about 9 hours. We extracted these signals and will compare their timing against the LPT catalog we constructed previously. Moreover, we will perform waveform correlation in order to quantify the degree of similarity among these monochromatic signals. Finally, we will attempt to reconcile all these observations in terms of the volcano dynamics and of its different eruptive styles during the 2011-2016 eruption cycle. [ABSTRACT FROM AUTHOR]

The ability to image the underground structures of volcanoes is limited by the precision, resolution and penetration 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 Ω.m and low resistivity values of the bottom layers in the range 1–10 Ω.m. • Four 1D velocity models of Casamicciola area (Ischia, Italy) were obtained using a joint inversion of dispersion and spectral ratio curves • The polarization analysis, applied to seismic noise, highlights possible correlation with the major faults system direction • HVSR frequency peaks and amplification values were used to map possible site effects of the August 27, 2017 earthquake • 1D resistivity models of the first 100 m depth were obtained applying TDEM geophysical surveys. [ABSTRACT FROM AUTHOR]