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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.

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) is an Italian research institution with focus on earth sciences. Moreover, the INGV is the operational center for seismic surveillance and earthquake monitoring in Italy and is a part of the civil protection system as a center of expertise on seismic, volcanic, and tsunami risks.INGV operates the Italian National Seismic Network and other networks at national scale and is a primary node of the European Integrated Data Archive for archiving and distributing strong-motion and weak-motion seismic recordings. In the control room in Rome, INGV staff performs seismic surveillance and tsunami warning services; in Catania and Naples, the control rooms are devoted to volcanic surveillance. Volcano monitoring includes locating earthquakes in the regions around the Sicilian (Etna, Eolian Islands, and Pantelleria) and the Campanian (Vesuvius, Campi Fregrei, and Ischia) active volcanoes. The tsunami warning is based on earthquake location and magnitude (M) evaluation for moderate to large events in the Mediterranean region and also around the world. The technologists of the institute tuned the data acquisition system to accomplish, in near real time, automatic earthquake detection, hypocenter and magnitude determination, and evaluation of several seismological products (e.g., moment tensors and ShakeMaps). Database archiving of all parametric results is closely linked to the existing procedures of the INGV seismic surveillance environment and surveillance procedures. Earthquake information is routinely revised by the analysts of the Italian seismic bulletin. INGV provides earthquake information to the Department of Civil Protection (Dipartimento di Protezione Civile) to the scientific community and to the public through the web and social media. We aim at illustrating different aspects of earthquake monitoring at INGV: (1) network operations; (2) organizational structure and the hardware and software used; and (3) communication, including recent developments and planned improvements.

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.

Two paroxysmal explosions occurred on Stromboli in the summer of 2019 (July 3 and August 28). The first of these explosions resulted in the death of one person. Furthermore, an effusive phase began on July 3 and lasted until August 30, 2019. This dangerous eruptive phase of Stromboli was not preceded by evident variations in the geophysical parameters routinely monitored, therefore the volcano was considered to be in a state of normal activity.To investigate the precursors of the 2019 eruptive crisis and explain the absence of variations in the parameters routinely monitored, we analyzed the seismo-acoustic signals with an unsupervised neural network capable of discovering hidden structures of the data. We clustered about 14,200 seismo-acoustic events recorded in 10 months (November 15, 2018 - September 15, 2019) using a Self-Organizing Map (SOM). Then we compared the clustering result with the images of visible and thermal monitoring cameras, that were installed and managed by the Istituto Nazionale di Geofisica e Vulcanologia, Italy, and with the Ground-Based Interferometric Synthetic Aperture Radar displacement measurements of the summit area of the volcano recorded by BGInSAR devices, which were installed and managed by Università Degli Studi di Firenze, Italy.The SOM analysis of the seismo-acoustic features associated with the selected dataset of explosions allowed us to recognize three main clusters in the period November 15, 2018 - September 15, 2019. We named these three clusters Red, Blue, and Green. The analysis of a subset of the selected explosions (approximately 180 events) through the videos of the visible and thermal monitoring cameras allowed us to associate distinct explosive types to the three main seismo-acoustic clusters. In particular, the cluster Red was associated with explosions characterized by well collimated oriented jets of ~ 200 m height, which eject incandescent ballistics and produce a significant infrasonic transient. The cluster Blue was associated with gas explosions with a height of 10 - 20 m and with little or no ash and pyroclastic fragment ejection. These types of explosions may not be detected by the camera recordings and infrasonic sensors. On the contrary, they are well recorded in the VLP seismic signals (filtered in the 0.05 - 0.5 Hz frequency band). The cluster Green includes explosions characterized by the emission of incandescent spatter-like fragments, with a wide range of ejection angles and hemispherical shape. The explosions of the cluster Red are mainly generated in the NE vent region, whereas the explosions of clusters Blue and Green are generally located in the central and SW vent regions.Comparing these results with the temporal evolution of the displacement of the summit area measured by the GBInSAR device, we discovered that the variations of the eruptive style that were highlighted by the SOM clustering of the seismic-acoustic features are recognizable in the ground deformation temporal pattern. Our findings are relevant for the improvement of monitoring of volcanoes with persistent activity and volcano early warning.

The study concerns the analysis of 220Rn (thoron) recorded in the surface soil in two sites of the Campi Flegrei caldera (Naples, Southern Italy) characterized by phases of volcanic unrest in the seven-year period 1 July 2011–31 December 2017. Thoron comes only from the most surface layer, so the characteristics of its time series are strictly connected to the shallow phenomena, which can also act at a distance from the measuring point in these particular areas. Since we measured 220Rn in parallel with 222Rn (radon), we found that by using the same analysis applied to radon, we obtained interesting information. While knowing the limits of this radioisotope well, we highlight only the particular characteristics of the emissions of thoron in the surface soil. Here, we show that it also shows some clear features found in the radon signal, such as anomalies and signal trends. Consequently, we provide good evidence that, in spite of the very short life of 220Rn compared to 222Rn, both are related to the carrier effect of CO2, which has significantly increased in the last few years within the caldera. The hydrothermal alterations, induced by the increase in temperature and pressure of the caldera system, occur in the surface soils and significantly influence thoron’s power of exhalation from the surface layer. The effects on the surface thoron are reflected in both sites, but with less intensity, the same behavior of 222Rn following the increasing movements and fluctuations of the geophysical and geochemical parameters (CO2 flux, fumarolic tremor, background seismicity, soil deformation). An overall linear correlation was found between the 222−220Rn signals, indicating the effect of the CO2 vector. The overall results represent a significant step forward in the use and interpretation of the thoron signal.

Two paroxysmal explosions occurred at Stromboli volcano in the Summer 2019, the first of which, on July 3, caused one fatality and some injuries. Within the 56 days between the two paroxysmal explosions, effusive activity from vents located in the summit area of the volcano occurred. No significant changes in routinely monitored parameters were detected before the paroxysmal explosions. However, we have calculated the polarization and the fractal dimension time series of the seismic signals from November 15, 2018 to September 15, 2019 and we have recognized variations that preceded the paroxysmal activity. In addition, we have defined a new parameter, based on RSAM estimation, related to the Very Long Period events, called VLP size, by means of which we have noticed significant variations through the whole month preceding the paroxysm of July 3. In the short term, we have analyzed the signals of a borehole strainmeter installed on the island, obtaining automatic triggers 10 minutes and 7.5 minutes before the July 3 and the August 28 paroxysms, respectively. The results of this study highlight mid-term seismic precursors of paroxysmal activity and provide valuable evidence for the development of an early warning system for paroxysmal explosions based on strainmeter measurements.

In summer 2019, two paroxysmal explosions occurred in Stromboli. The first one occurred on July 3, when the Strombolian ordinary eruptive activity did not show a significant intensification. The explosion formed an eruptive column more than 3 km high. A pyroclastic flow ran down the “Sciara del Fuoco” slope causing a victim and some injuries. Moreover, the pyroclastic flow spread over the sea surface for about one kilometer. On August 28, a second paroxysmal explosion occurred, similar to the previous one. Also in this case the eruption formed an eruptive column of more than 3 km and a pyroclastic flow that expanded along the “Sciara del Fuoco” slope and traveled about 1 km on the sea surface. In the period between the two paroxysms, effusive activity occurred from the summit crater area. The eruptive phase of summer 2019, which began with the paroxysm of 3 July, was not preceded by significant changes in the routinely monitored parameters, such as the hourly frequency (daily average) of the VLP events (typical of Stromboli) and the amplitude of the seismic signal (RSAM). For this reason, we have analyzed the seismic and dilatometric data, which were recorded by the INGV geophysical network in the period November 2018 - September 2019, focusing our attention on other parameters that can give indications on the activity state of the volcano. In particular, we analyzed the data of the broadband seismic stations, equipped with the Guralp CMG40T sensors, and the data of one Sacks-Evertson borehole strainmeter. We defined the "VLP size", which takes into account the waveform of the VLP events, in terms of both amplitude and duration. We also applied time varying Fractal Dimension (FD) analysis to the seismograms of a seismic station close to the crater area and we analyzed the polarization of the same signal. We carried out the polarization analysis both without applying a filter and by filtering the seismic signal in the typical frequency bands of the Stromboli volcanic tremor (1-3 Hz) and of the VLPs (0.5-0.05 Hz). We found that the "VLP size", the FD and the polarization parameters showed significant changes about one month before the paroxysm of July 3. In the short term, we applied an appropriately tuned STA/LTA algorithm to the data of the borehole strainmeter, which is installed on the island at about 2km from the craters, and we obtained an automatic detection of the paroxysmal events 10 and 7.5 minutes before the explosion of July 3 and August 28, respectively.

After a period of mild eruptive activity, Stromboli showed between 2017 and 2018 a reawakening phase, with an increase in the eruptive activity starting in May 2017. The alert level of the volcano was raised from “green” (base) to “yellow” (attention) on 7 December 2017, and a small lava overflowed the crater rim on 15 December 2017. Between July 2017 and August 2018 the monitoring networks recorded nine major explosions, which are a serious hazard for Stromboli because they affect the summit area, crowded by tourists. We studied the 2017–2018 eruptive phase through the analysis of multidisciplinary data comprising thermal video-camera images, seismic, geodetic and geochemical data. We focused on the major explosion mechanism analyzing the well-recorded 1 December 2017 major explosion as a case study. We found that the 2017–2018 eruptive phase is consistent with a greater gas-rich magma supply in the shallow system. Furthermore, through the analysis of the case study major explosion, we identified precursory phases in the strainmeter and seismic data occurring 77 and 38 s before the explosive jet reached the eruptive vent, respectively. On the basis of these short-term precursors, we propose an automatic timely alarm system for major explosions at Stromboli volcano.

Mt. Vesuvius (southern Italy) is one of the volcanoes that poses the greatest risk in the world because of its highly explosive eruptive style and its proximity to densely populated areas. The urbanization around Mt. Vesuvius began in ancient times, and the impact of eruptions on human activities has been severe. This is testified to by the ruins of Pompeii, which are covered by the products of the plinian eruption that took place in A.D. 79 (Sigurdsson et al. 1985), and more recently by the published reports of the eruptions that occurred from 1631 to 1944. For these reasons, Mt. Vesuvius was also one of the first volcanoes to be equipped with monitoring instruments. Pioneering instrumental observations began just before the second half of the 1800s, when the Vesuvius Observatory was founded in 1841 (Imbo 1949). At that time, Vesuvius was very active (Ricciardi 2009), and its effusive and explosive eruptions often caused damage to the surrounding areas. At the same time, it was a famous tourist attraction that drew travelers from all over the world (Gasparini and Musella 1991). Since the middle of the 1800s, at least 12 eruptions have occurred that have been superimposed on persistent intra-crater activity that has been characterized by Strombolian explosions and by the formation of small lava lakes. The last eruption occurred on 18 March 1944 and marked a change in the status of Mt. Vesuvius, as it entered a closed-conduit phase that persists today. Following this last eruption, a change occurred in the 1960s, as documented by an increase in the occurrence rate of earthquakes. Since 1972, the monitoring of Mt. Vesuvius has improved over time and become more systematic, so that there is a remarkable dataset relating to the current phase of quiescence. Over more than a century and a half …

Stromboli is a volcanic island that is part of the Aeolian arch in the Mediterranean Sea (Italy). It is one of the most active volcanoes in Europe. Its moderate, but persistent, explosive activity makes it an ideal site for studies into the seismogenic processes in volcanic areas (Auger et al. 2006; Chouet et al. 2003; Chouet et al. 2008; D'Auria and Martini 2008; Del Pezzo et al. 1992; Esposito et al. 2008; Jaupart and Vergniolle 1989; Martini et al. 2007); it also attracts a lot of tourists. In the past, this combination of tourism and volcanic activity was not considered to be dangerous, but over the past few decades, Stromboli has produced stronger explosions that have in some cases injured people visiting the summit area. Moreover, in the recent history of Stromboli, two effusive eruptions have occurred that were accompanied by dangerous phenomena such as tsunami and vulcanian explosions. The first of these effusive eruptions (on 28 December 2002) produced a lava flow on the Sciara del Fuoco side, which is the northwest flank of the island. Two days later, a landslide occurred on this flank, which resulted in the propagation of a 10-m tsunami wave around the coasts of the island. These events demonstrate that Stromboli can be dangerous, even if its activity is not very energetic. Indeed, the Sciara del Fuoco structure is a weakness zone of the volcanic edifice that fractures when the explosive activity increases, giving rise to this effusive activity (Martini et al. 2007). Moreover, during the past two effusive eruptions, vulcanian explosions were associated with the end stages of the effusive phases. These damaged the village of Ginostra and caused fires in the vegetation. For these reasons, in January 2003, the Istituto Nazionale di Geofisica e Vulcanologia (INGV; …

A very compact 32-channel digital delay module has been designed to fit in a single VME 6 U unit. It is equipped with both VME and Fast Intercrate Readout (FAIR) interfaces. Furthermore, a local manual programmability was implemented to use the module also in a stand-alone mode. A variety of NIM and ECL output have been implemented. A special ECL input allows the selection of different output regimes. The thermal stabilization is controlled via a PLL based circuit. The main features of the module are the wide range for the delay and the width of the output pulse: from 25 ns up to 6400 ns, in 25 ns steps; and the low jitter: values of the order of 27 ps (11 ppm) were measured in several bench tests.

We have analyzed the recent seismicity of Mt. Vesuvius, with particular emphasis on the period 1999-2012. Since 1972 continuous observations with electromagnetic seismometers allowed the compilation of a detailed earthquake catalogue for the station OVO. Furthermore since 1999 another, more complete, catalogue for the station BKE, closer to the crater, is available. The Gutenberg-Richter distribution of magnitudes shows a temporal decrease of the b-value since 1985, with current values close to 1.0. The temporal pattern of the strain release shows a non-stationary behavior with periods of increased release rates (as in 1989-1990, 1995-1996 and 1999-2000). The spatial distribution of the seismicity consists in two main seismogenic volumes, one with hypocenters clustered below the Mt. Vesuvius crater at depths mostly between 1-6 km, and another with hypocenters clustered within the Gran Cono volcanic edifice, with depths above the sea level. We compare the statistical properties of the seismicity occurring within these two volumes and their spatial and temporal patterns. Moreover we analyze the statistical distribution of focal mechanisms for each volume. Our results point to gravity-induced stresses as the source of the shallow seismicity and of a combined effect of crustal heterogeneities, regional stress and hydrothermal dynamics for the deeper seismicity. Finally we discuss possible future developments of the seismic monitoring system in the light of the past and current seismicity.

Optimization of in soil air radon monitoring using thoron and radon and thoron parents

Stromboli island, located in the southern Tyrrhenian sea, is the emerged part (about 900 meters above sea level) of an approximately 3 kilometer-high stratovolcano. Its persistent Strombolian activity, documented for more than 2000 years, is sometimes interrupted by lava effusions or major explosions. Despite the number of recently published geophysical studies aimed at clarifying the volcano's eruption dynamics, the spatial extent and geometrical characteristics of its plumbing system remain poorly understood. In fact, knowledge of the inner structure and the zones of magma storage is limited to the upper few hundred meters of the volcanic edifice (Chouet et al., 2003; Mattia et al., 2004) and P and S wave velocity models are available only in restricted areas (Petrosino et al., 2002). Over the past few years, Stromboli has experienced an increase in its activity, which started with the eruptive crisis of December 2002 to July 2003. The observed variations in Stromboli's eruptive behavior and the occurrence in December 2002 of a tsunami and in April 2003 of a paroxysmal vulcanian-type event with strong explosions and lava flows (Bonaccorso et al., 2003), have focused attention on the need to determine more suitable internal structure and velocity models of the volcano. To meet this need, a seismic tomography experiment through active seismics using air gun sources was done. A preliminary two-dimensional (2-D) tomographic study is presented here.

The Stromboli geophysical experiment, performed to acquire onshore and offshore seismic data through a combined on-land and marine network, was finalized to reconstruct the seismic tomography of the volcano and to investigate the deep structures and the location of magma chambers. A detailed swath bathymetry around the volcano has also been acquired by the R/V Urania Multibeam. In particular, high resolution bathymetry of the 'Sciara del Fuoco' area allows to image the present-day seafloor setting of the area involved by the submarine slide of 2002-12-30. During the experiment wide angle refraction seismics was performed all around the Stromboli volcano by a 4 GI-GUN tuned array. The data were recorded by the permanent seismic network of the INGV and 20 temporary stations and 10 OBS deployed on the SE, SW and NE submerged flanks of the volcano after detailed morpho-bathymetric analysis.

Campi Flegrei and Mount Vesuvius are active Italian volcanoes though presently in aquiescent stage. The last eruption of Mount Vesuvius occurred during the spring of 1944. Campi Flegrei last erupted in 1538 but experienced a subsidence trend from the early 1900s to 1970, which was followed by episodes of ground uplift accompanied by seismic swarms [see, e.g., Aster et al., 1992]. Over the past 37 years at Campi Flegrei, about 4 meters of maximum ground uplift has occurred followed by about 1 meter of subsidence, as measured near the center of the caldera in two episodes during 1970–1972 and 1982–1984. This deformation has occurred close to the harbor of the town of Pozzuoli, putting about 400,000 people at heightened risk. From 1985 to 2004, Campi Flegrei's caldera showed gradual subsidencethat was interrupted by five mini-uplift episodesin 1976, 1989, 1995, 2000, and, most recently, 2004–2006, each amounting to a few centimeters of uplift [Pingue et al., 2006]. The caldera is generally aseismic during gradual subsidence, but small swarms of microearthquakes are produced during periods of elevated strain rates associated with the mini-uplift episodes [Saccorotti et al., 2001 ].