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SUMMARY Infrasound monitoring plays an important role in the framework of the surveillance of Mt. Etna, Europe’s largest active volcano. Compared to seismic monitoring, which is particularly effective for buried sources, infrasound signals mirror the activity of shallow sources like Strombolian explosions or degassing. The interpretation of infrasound signals is difficult to the untrained eye, as we have to account for volcanic and non-volcanic sources. The problem of handling large and complex data sets can be tackled with machine learning, namely pattern recognition techniques. Here, we focus on so-called ‘Unsupervised Learning’, where we identify groups of patterns being similar to each other. The degree of similarity is based on a metric measuring the distance among the features of the patterns. This work aims at the identification of typical regimes of infrasound radiation and their relation to the state of volcanic activity at Mt. Etna. For this goal, we defined features describing any infrasound pattern. These features were obtained using wavelet transform. We applied ‘Self-Organizing Maps’ (SOM) to the features projecting them to a 2-D representation space—the ‘map’. An intriguing aspect of SOM resides in the fact that the position of the patterns on the map can be expressed by a colour code, in a manner that similar patterns are assigned a similar colour code. This simplified representation of multivariate patterns allows to follow the development of their characteristics with time efficiently. During a training phase we considered a reference data set, which encompassed a large variety of scenarios. We identified typical groups of patterns which correspond to a specific regime of activity, being representative of the state of the volcano or noise conditions. These groups form areas on the 2-D maps. In a second step, we considered a test data set, which was not used during the training phase. Applying the same pre-processing as for the training data, we blindly assigned the test patterns to the regimes found before, identifying the one whose colour code is most similar to the one calculated to the test pattern. We are thus able to assess the validity of the prediction. The classification scheme presented provides a reliable assessment of the state of activity and adds useful and supplementary details to the results of the real-time automatic system in operation at Istituto Nazionale di Geofisica e Vulcanologia—Osservarorio Etneo. This is of particular importance when no visible information of the volcanic activity is available either for unfavourable meteorological conditions or during night time.

Over the last 20 years, infrasound signals have been used to investigate and monitor active volcanoes during eruptive and degassing activity. In particular, infrasound amplitude information has been used to estimate eruptive parameters such as plume height, magma discharge rate and lava fountain height. Active volcanoes are characterized by pronounced topography and, during eruptive activity, the topography can change rapidly, affecting the observed infrasound amplitudes. While the interaction of infrasonic signals with topography has been investigated by several authors over the past decade, the impact of changing topography on the infrasonic amplitudes has not yet been explored. In this work, the infrasonic signals accompanying 57 lava fountain paroxysms at Mount Etna (Italy) during 2021 were analyzed. In particular, the temporal and spatial variations of the infrasound amplitudes were investigated. During 2021, significant changes in the topography around the most active crater (the South-East Crater) took place and were reconstructed in detail through unoccupied aerial system surveys. Through analysis of the observed infrasound signals and numerical simulations of the acoustic wavefield, we demonstrate that the observed spatial and temporal variation in the infrasound signals can be explained by the combined effects of changes in the location of the acoustic source and changes in the near-vent topography. This work demonstrates the importance of accurate source locations and high-resolution topographic information, particularly in the near-vent region where the topography is most likely to change rapidly. Changing topography should be considered when interpreting local infrasound observations over long time-scales.

Rotational sensors have been deployed on several volcanoes worldwide including Kilauea, Stromboli, Etna and a few volcanoes in Iceland. Within this presentation we focus on our first experiment using a rotational sensor on Etna in Italy. We recorded the volcanic activity including degassing and vigorous strombolian activity in August to September 2019. We compare our results using a rotational sensor with a normal Trillium Compact seismometer and the seismic network maintained by the INGV. We detect LP events, VT events and volcanic tremor, study the wavefield and back azimuths of the events and derive phase velocities of the ground. Luckily, we were able to assess the quality of our results using the detailed earthquake catalogues and locations derived at the INGV. We can easily detect changes in the wavefield e. g. when strong strombolian activity kicks in and are looking forward to applications on other volcanoes where details of the volcanic activity or changes might go unnoticed if no rotational sensor is present.

Since September 2021, signs of unrest at Vulcano Island have been noticed after four years of quiescence, along with CO2 degassing and the occurrence of long-period and very long-period events. With the intention of improving the monitoring activities, a submarine fiber optic telecommunications cable linking Vulcano Island to Sicily was interrogated from 15 January to 14 February 2022. Of particular interest has been the recording of 1488 events with wide range of waveforms made up of two main frequency bands (from 3 to 5 Hz and from 0.1 to 0.2 Hz).With the aim of the automatic detection of seismic-volcanic events, different approaches were explored, particularly investigating whether the application of machine learning could provide the same performance as conventional techniques. Unlike many traditional algorithms, deep learning manages to guarantee a generalized approach by automatically and hierarchically extracting the relevant features from the raw data. Due to their spatio-temporal density, the data acquired by the DAS can be assimilated to a sequence of images; this property has been exploited by re-designing deep learning techniques for image processing, specifically employing Convolutional Neural Networks.The results demonstrate that deep learning not only achives good performance but that it even outperforms classical algorithms. Despite providing a generalized approach, Convolutional Neural Networks have been shown to be more effective than traditional tecniques in expoiting the high spatial and temporal sampling of the acquired data.

The Hunga Tonga-Hunga Ha’apai volcano (Pacific Ocean) generated a cataclysmic explosion on 15 January 2022, triggering several atmospheric disturbances at a global scale, as a huge increase in the total electron content (TEC) in the ionosphere, and a pressure wave travelling in the troposphere. We collected and analysed data over the Mediterranean to study these disturbances, and in particular, (i) data from the barometric and infrasonic stations installed on Italian active volcanoes by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) for investigating the tropospheric pressure waves; (ii) barometric data from the INGV-TROPOMAG and SIAS (Sicilian Agro-meteorological Information System) networks, for investigating the interaction between the orography and pressure waves; (iii) ionograms from the Advanced Ionospheric Sounder-INGV ionosonde at Gibilmanna (Sicily, Italy); (iv) data from the RING (Rete Italiana Integrata GNSS) network, to retrieve the ionospheric TEC; (v) soil CO2 flux data from the INGV surveillance network of Vulcano Island. The analysis of the ground-level barometric data highlights that pressure waves were reflected and diffracted by the topographic surface, creating a complex space–time dynamic of the atmospheric disturbances travelling over Sicily, driven by the interference among the different wavefronts. The ionograms show that a medium-scale travelling ionospheric disturbance (MSTID), with a horizontal wavelength of about 220 km and a period of about 35 min, propagated through the ionospheric plasma in the correspondence of the first barometric variations. Moreover, comparing detrended TEC and barometric data, we further confirmed the presence of the aforementioned MSTID together with its close relation to the tropospheric disturbance.

Etna volcano in Italy is one of the most active volcanoes in Europe. We recorded the volcanic activity including degassing and vigorous strombolian activity using a seismometer and a rotational sensor in August to September 2019. We test the newly developed rotational sensor in the field in comparison to the broadband seismometer and seismic-network-based locations using the INGV network. We demonstrate that a single rotational sensor co-located with a seismometer can be used to identify specific seismic wave types, to estimate the back azimuth of wave arrivals and the local seismic phase velocities.Using the rotational sensor, we easily detected the dominant SH-type waves composing volcanic tremor during weak volcanic activity and the recorded VLP/ LP events. Changes in the composition of the tremor wavefield caused by the onset of vigorous volcanic activity are obvious and can be detected in near real-time if data is streamed. We discuss the changes in the wavefield composition from SH-type waves to a mixed wavefield in the context of the volcanic activity, the back azimuth of the signals and associated phase velocities. Our findings are consistent with observations by INGV and hence the rotational sensor reliably enlarges our sensor portfolio in volcanic environments. In fact, wavefield and ground properties can be derived using just one sensor instead of a sensor network, which makes experiments in remote areas cheaper and easier to maintain. In addition, you can observe phenomena that otherwise go unnoticed, like near vent block rotation.

Deriving eruption source parameters from geophysical data is critical for volcano hazard mitigation, yet remains a challenging task in most volcanoes worldwide. In this work, we explored the temporal relationship between geophysical signals and eruptive parameters measured during six explosive episodes from the New South-East Crater of Mt. Etna (Italy). The quadratic reduced seismic velocity and pressure were calculated to track the temporal variation of volcanic elastic radiation, and the lava fountain height was estimated by thermal camera image processing. The temporal relationships between these geophysical and eruptive time series were studied. In particular, the first considered lava fountain exhibited a “clockwise hysteresis” pattern: higher seismic amplitude with respect to the fountain height during the waxing phase as compared to the waning phase. We also calculated the regression parameters for both linear and power laws, linking seismo-acoustic and eruptive time series. For the linear regressions, we found fairly constant values of the scaling factors in five out of six eruptive episodes, which can be considered as a promising step to derive eruption source parameters from geophysical data in real-time. Regarding power law regressions, a clear relationship was observed between the exponents determined for the power law linking quadratic reduced velocity and lava fountain height, and the time interval duration from the previous eruption. These results suggest that the condition of the uppermost part of the plumbing system (e.g. viscosity of residing magma and conduit conditions) play a key role in the seismic energy generation during the eruptions.

Mount Etna, Europe’s largest and most active volcano is situated close to the Metropolitan area of Catania with about 1 Million inhabitants. Continuous monitoring has therefore been carried out for decades. Among the various disciplines infrasound recordings play an important role in this context. Explosive activity near or above ground as well as shallow tremor processes are easier to identify with airborne sound waves than with seismic waves that are significantly scattered and refracted in the volcanic edifice. However, infrasound signals are often affected by noise, especially by wind noise in the summit area.At Mount Etna five summit craters are currently known with fluctuating levels of activity. This leads to a wide variety of infrasound signal patterns interfered by changing noise levels. Manual distinction of noisy data from real volcanogenic signals brings along a considerable effort and requires expert knowledge. We therefore apply unsupervised pattern recognition techniques for this task. Extracting features from the amplitude spectrum we are able to distinguish different infrasound regimes with Self-Organizing maps (SOMs). SOMs allow to color-code the results for an intuitive interpretation and evidence the presence of transitional activity regimes. We define a reference data set from multiple months of infrasound waveforms to include as many activity regimes as possible to train the SOM. This enables a straight forward interpretation of new data.

The frequent activity taking place at Mount Etna may pose risks to tourists and specialists visiting the summit craters as well as to the nearby population and thus requires constant monitoring. In...

During the COVID-19 pandemic, most countries put in place social interventions, restricting the mobility of citizens, to slow the spread of the epidemic. Italy, the first European country severely impacted by the COVID-19 outbreak, applied a sequence of progressive restrictions to reduce human mobility from the end of February to mid-March 2020. Here, we analysed the seismic signatures of these lockdown measures in densely populated eastern Sicily, characterized by the presence of a permanent seismic network used for earthquake and volcanic monitoring. We emphasize how the anthropogenic seismic noise decrease is visible even at stations located in remote areas (Etna and Aeolian Islands) and that the amount of this reduction (reaching ∼ 50 %–60 %), its temporal pattern and spectral content are strongly station-dependent. Concerning the latter, we showed that on average the frequencies above 10 Hz are the most influenced by the anthropogenic seismic noise. We found similarities between the temporal patterns of anthropogenic seismic noise and human mobility, as quantified by the mobile-phone-derived data shared by Google, Facebook and Apple, as well as by ship traffic data. These results further confirm how seismic data, routinely acquired worldwide for seismic and volcanic surveillance, can be used to monitor human mobility too.

During the COVID-19 pandemic, most countries worldwide put in place social interventions, consisting of restricting the mobility of citizens, aimed at slowing and mitigating the spread of the epidemic. In particular, Italy, as the first European country violently struck by the COVID-19 outbreak, applied a sequence of progressive restrictions to reduce both human mobility and human-to-human contacts from the end of February to mid-March 2020. Here, we analysed the seismic signatures of these lockdown measures in the densely populated Eastern Sicily, characterised by the presence of a permanent seismic network used for both seismic and volcanic monitoring. We specifically emphasize how the amount of the amplitude reduction of anthropogenic seismic noise (reaching ~50–60%), its temporal pattern and spectral content are strongly station-dependent. As for the latter, we exhibited that on average the frequencies above 10 Hz are the most influenced by the anthropogenic seismic noise. Finally, we found an impressive similarity between the temporal patterns of anthropogenic seismic noise and human mobility, as quantified by the mobile phone-derived data shared by Google, Facebook and Apple. These results further confirm how seismic data, routinely acquired worldwide for seismic and volcanic surveillance, can be used to monitor human mobility too.

In July and August 2019, Stromboli volcano underwent two dangerous paroxysms previously considered “unexpected” because of the absence of significant changes in usually monitored parameters. We applied a multidisciplinary approach to search for signals able to indicate the possibility of larger explosive activity and to devise a model to explain the observed variations. We analysed geodetic data, satellite thermal data, images from remote cameras and seismic data in a timespan crossing the eruptive period of 2019 to identify precursors of the two paroxysms on a medium-term time span (months) and to perform an in-depth analysis of the signals recorded on a short time scale (hours, minutes) before the paroxysm. We developed a model that explains the observations. We call the model “push and go” where the uppermost feeding system of Stromboli is made up of a lower section occupied by a low viscosity, low density magma that is largely composed of gases and a shallower section occupied by the accumulated melt. We hypothesize that the paroxysms are triggered when an overpressure in the lower section is built up; the explosion will occur at the very moment such overpressure overcomes the confining pressure of the highly viscous magma above it.

The evolution of volcanic activity observed at the New South East Crater (NSEC) and Voragine (VOR) between July 2019 and January 2020 has been deciphered by taking into account the changes of textures and chemical zoning of plagioclase and olivine crystals from the erupted lavas and tephra. The petrological observations have been integrated with analyses of the amplitude and source location of volcanic tremor and infrasound data. Characteristics of crystals erupted on July 2019 at the NSEC reflect protracted intrusions of magma into the mid-upper section of the plumbing system, approximately within 290–120 MPa, which acted as the main zone of magma accumulation and crystallization before the beginning of the eruptive activity. Textures and compositions of crystals erupted at VOR emphasize the beginning of volcanic activity driven by recharge/discharge phases that mostly affect the shallowest portion of the Mt. Etna plumbing system (

We report on a systematic record of SO2 flux emissions from individual vents of Etna volcano (Sicily), which we obtained using a permanent UV camera network. Observations were carried out in summer 2014, a period encompassing two eruptive episodes of the New South East Crater (NSEC) and a fissure-fed eruption in the upper Valle del Bove. We demonstrate that our vent-resolved SO2 flux time series allow capturing shifts in activity from one vent to another and contribute to our understanding of Etna's shallow plumbing system structure. We find that the fissure eruption contributed ~50,000 t of SO2 or ~30% of the SO2 emitted by the volcano during the 5 July to 10 August eruptive interval. Activity from this eruptive vent gradually vanished on 10 August, marking a switch of degassing toward the NSEC. Onset of degassing at the NSEC was a precursory to explosive paroxysmal activity on 11-15 August.

Between 2011 and 2013, there were 43 lava fountain episodes from Mount Etna's New South-East summit crater (NSEC). In 2014, this intense activity was supplanted by sporadic Strombolian explosions and the opening of an eruptive fissure between July and August. The only lava fountaining episode of the year occurred on 28 December; this was characterized by the emplacement of a shallow dike that, at the surface, fed two distinct lava flows from an ENE-WSW trending eruptive fissure. Here we provide a detailed picture of the onset of the dike emplacement, as well as the mechanism driving its migration, using a multidisciplinary data set based on seismic, geodetic, geochemical, and volcanological observations. The dike emplacement was preceded by a pressurization of the magmatic plumbing system recorded from August 2014 on. This pressurization has been modeled as a vertically elongated magmatic source located beneath the summit craters at ~4.5 km below sea level. From September to October, magma rising was also detected by seismic and geochemical data that highlighted pressurization of the shallower portion of the plumbing system. We suggest that the 28 December 2014 dike emplacement resulted from a modification of the preexisting NSEC shallow plumbing system, largely due to drainage of the main shallow conduit during the July–August 2014 eruptive fissure activity. Such a structural modification might have created the conditions for magma emplacement as a dike-like structure.

On 5 July 2014, an eruptive fissure opened on the eastern flank of Etna volcano (Italy) at ~3.000 m a.s.l. Strombolian activity and lava effusion occurred simultaneously at two neighbouring vents. In the following weeks, eruptive activity led to the build-up of two cones, tens of meters high, here named Crater N and Crater S. To characterize the short-term (days) dynamics of this multi-vent system, we performed a multi-parametric investigation by means of a dense instrumental network. The experimental setup, deployed on July 15-16th at ca. 300 m from the eruption site, comprised two broadband seismometers and three microphones as well as high speed video and thermal cameras. Thermal analyses enabled us to characterize the style of eruptive activity at each vent. In particular, explosive activity at Crater N featured higher thermal amplitudes and a lower explosion frequency than at Crater S. Several episodes of switching between puffing and Strombolian activity were noted at Crater S through both visual observation and thermal data; oppositely, Crater N exhibited a quasi-periodic activity. The quantification of the eruptive style of each vent enabled us to infer the geometry of the eruptive system: a branched conduit, prone to rapid changes of gas flux accommodated at the most inclined conduit (i.e. Crater S). Accordingly, we were able to correctly interpret acoustic data and thereby extend the characterization of this twovent system., 10 figures

Programma Nazionale Ricerche . Grant Number: PNRA14_00011 University of Catania FIR 2014 . Grant Numbers: 2F119B , PRA 2016‐18 , 22722132120

European FP7 MED-SUV (MEditerranean SUpersite Volcanoes). Grant Number: 308665 European Research Council European FP7 (FP/2007-2013)/ERC. Grant Number: 279802 SIGMA (Sistema Integrato di sensori in ambiente cloud per la Gestione Multirischio Avanzata)

We monitored geyser activity in the Lower Geyser Basin (LGB) of Yellowstone National Park with dual four-element microphone arrays separated by ~ 600 m. The arrays were independently used to identify incident coherent plane wave energy, then conjoint cross beam back-azimuths from the two arrays were used to precisely locate signal sources. During a week in August 2011 we located repeating infrasound events, peaked in energy between 1 and 10 Hz, originating from at least five independent geothermal features, including the episodically erupting Great Fountain, Fountain and Kaleidoscope Geysers, as well as periodic infrasound from nearby Botryoidal and persistent sound from Firehole Spring. Although activity from nearby cone-type geysers was not detected in the infrasound band up through 50 Hz, the major fountain-type geysers (i.e., with columns greater than 10 m) could be detected at several kilometers, and two minor geysers (i.e., a few meters in eruption height) could be tracked at distances up to a few hundred meters. Detection of geyser activity was especially comprehensive at night when ambient noise was low. We conclude that infrasound monitoring of fountain-type geysers permits convenient tracking of geyser activity, episodicity, signal duration, energy content, and spectral content. These parameters enable objective statistical quantification of geyser behavior and changes over time that may be due to external forcing. Infrasonic study of geyser activity in an individual basin has great monitoring utility and can be reasonably accomplished with two or more distributed sensor arrays.

On 13 May 2008, an eruption began at Mt. Etna from an eruptive fissure that opened on the upper eastern flank of the volcano. During 12–13 May, 157 infrasonic events, together with the related seismic transients, were collected. We carried out several analyses to obtain dominant frequencies, pseudospectrograms, peak-to-peak amplitudes, source locations and time lags between infrasonic and seismic events. Spectra of the infrasonic events show two main spectral peaks in the frequency bands ~ 0.4–0.7 Hz and 1.5–2.0 Hz, respectively. Both infrasonic and seismic events were separately located below the North-East crater, where no eruptive activity was observed. Moreover, significant changes in infrasound spectral content, as well as in the infrasonic–seismic lags, were found a few hours before the beginning of the eruption. On the basis of the collected information the infrasound source mechanism was modeled as a superposition of pipe and Helmholtz resonance, also leading to outline the geometry of the shallower portion of the North-East crater plumbing system. The occurrence of these seismo-infrasonic events together with other geological and geophysical evidences, led us to inferring a direct link between North-East crater activity and the eruptive fissure. Further, based on variations over time of both spectral features and seismic–infrasonic time lag, shallowing phenomena of the free magma column inside North-East crater conduit were hypothesized. Such an uprise of magma was likely caused by a pressure increase inside the plumbing system occurring before the beginning of the 2008–2009 eruption.

[1] Mt Etna volcano (Italy) typically generates lava fountains and Strombolian eruptions from the summit craters, producing significant emissions of tephra which may cause moderate to high impact both in the atmosphere and on the ground. However, we have also witnessed a number of minor ash emissions that, although far less powerful, may pose considerable volcanic hazards. In this paper, we deal with three ash emissions occurring in 2010 (8 April at Southeast Crater, 25 August at Bocca Nuova, and 14–15 November at Northeast Crater), correlating their volcanological features with the associated seismo-acoustic signals. We provide details regarding the chronology, eruption column, dispersal of the deposit, and texture (grain size, componentry and morphology) of the emitted ash. Each eruptive episode has also been characterized by means of seismo-acoustic analyses evaluating the volcanic acoustic-seismic ratio (VASR). Furthermore, the source of volcanic tremor recorded from March to December 2010 was localized. The joint volcanological and seismo-acoustic analyses allowed distinguishing two main kinds of ash emissions: types a (8 April and 25 August) and b (14–15 November). Regarding the former, the accumulation of gas below a dense cap rock obstructing the conduit vent, giving rise, with the uncorking, to impulsive explosivity, was hypothesized. The latter type instead is characterized by a longer-lasting and less explosivity, likely due to the existence of open conduit conditions. Therefore, type b ash emissions are less hazardous than type a. This simple model, regarding minor explosive activity, may constitute a starting point to assess the volcanic risk from unexpected explosions.

Here we report the first measurements of gas masses released during a rare period of strombolian activity at the Bocca Nuova crater, Mt. Etna, Sicily. UV camera data acquired for 195 events over an ≈ 27 minute period (27th July 2012) indicate erupted SO2 masses ranging from ≈ 0.1 to ≈ 14 kg per event, with corresponding total gas masses of ≈ 0.1 to 74 kg. Thus, the activity was characterised by more frequent and smaller events than typically associated with strombolian activity on volcanoes such as Stromboli. Events releasing larger measured gas masses were followed by relatively long repose periods before the following burst, a feature not previously reported on from gas measurement data. If we assume that gas transport within the magma can be represented by a train of rising gas pockets or slugs, then the high frequency of events indicates that these slugs must have been in close proximity. In this case the longer repose durations associated with the larger slugs would be consistent with interactions between adjacent slugs leading to coalescence, a process expedited close to the surface by rapid slug expansion. We apply basic modelling considerations to the measured gas masses in order to investigate potential slug characteristics governing the observed activity. We also cross correlated the acquired gas fluxes with contemporaneously obtained seismic data but found no relationship between the series in line with the mild form of manifest explosivity.

Previous studies performed on Mt. Etna on short and discontinuous time intervals indicate the North East Crater (NEC) as the most active source of infrasound. The source mechanism of NEC infrasound events was modeled as a double resonance. This lead to infer the connection between the NEC and both the southeast crater (SEC) and the eruptive fissure (EF), that opened at the beginning of the 2008–2009 eruption. Nevertheless, there are still several open questions that need to be addressed. For instance, the steadiness of NEC event features should be studied, as well as the orderliness of spectral changes of NEC events time-related to eruptive activity of other vents. The investigation of such topics is strongly enhanced by the possibility of analysing infrasound signals during year-long time periods. With this aim about 40,000 infrasound events, recorded at Mt. Etna from August 2007 to December 2009 were analysed by using spectral and location techniques. It was noted in particular that the NEC events featured periods with very steady waveforms and spectral characteristics lasting from days to months with slow or sudden variations. The most important eruptive episodes occurring at the SEC or the EF were accompanied by significant spectral changes in NEC events. In light of such systematic behaviour the connection between the NEC and the SEC/EF plumbing systems was not considered temporary but rather stable even during a relatively long time interval (2006–2009). Moreover, study of NEC event spectral features and their changes over multiple years supports the double resonance source model. Such a model, together with the inferred connections between NEC and SEC/EF feeding systems, implies that level fluctuations of a magma column inside the NEC conduit correspond to magmastatic pressure decrease/increase inside the main plumbing system. These findings open up new and interesting possibilities for monitoring magma pressure changes inside the Mt. Etna plumbing system.

[1] On 16 November 2006, a 1 day long paroxysmal eruption occurred at the summit craters of Mt. Etna volcano. A multiparametric approach, consisting of analyzing infrasonic, seismic, and video camera recordings, was carried out to follow its evolution. Volcanological and geophysical observations identified three eruptive phases. In the first phase, infrasonic and seismic characteristics reflected the highly explosive nature of the activity. Waveform characterization of infrasound events confirmed the activity of the several explosive vents at the summit of Southeast Crater (SEC). During the second phase, results highlighted the decoupling between seismic and infrasonic sources, which was due to the decrease in explosive activity and the reactivation of effusive vents located south of Bocca Nuova and on the saddle between Bocca Nuova and SEC. The third phase was the most intense and was characterized by various volcanic phenomena (pyroclastic flows, jets of dark ash, and white steam). The very high radiated infrasonic energy, together with infrasound event features, led us to infer a gas enrichment of the shallow magma column, preceding by a few minutes and likely related to the pyroclastic flows in the SEC area. After the eruption at SEC, variations in infrasound events related to the activity of Northeast Crater (NEC) were found. The observed spectral changes and the source mechanism modeling of the NEC infrasound events suggest the existence of a link in the plumbing system feeding the two craters.

On 13 May 2008 an eruptive fissure opened on Mount Etna's eastern flank feeding both explosive activity and lava effusion from multiple vents for about 14 months. During the investigated May-September 2008 eruptive period, infrasound recordings from a 4 station-sparse network allowed tracking of the explosive activity in terms of location and dynamics. In order to focus on activity from the eruptive fissure, the infrasonic events generated by the summit craters were selected by using both spectral features and time delays between pairs of stations and excluded from our analysis. Then, to accurately locate events from the fissure, we used a composite method, based on the semblance and brightness functions. This enabled the study of the co-existence of more than one infrasound source and/or its migration along the eruptive fissure. Hence, results permitted us to discriminate the number of active vents and their location along the fissure even when, due to poor weather conditions, it was not possible to access the vents or carry out direct observations. The eruptive activity was characterised by variations in the number of active vents according to the overall intensity of the eruptive event. Variability of the infrasound waveforms highlighted either that distinct vents produced signals with different waveforms, or that single vents generated different events during distinct periods of time, or finally both the previous phenomena. We applied the strombolian bubble vibration model to model waveform differences and attributed the signal variations to bubble radius changes.