ALBERT

Description: On July 18, 2001, two main eruptive vents opened on the southern flank of Mount Etna volcano (Italy) at ~2100 m and ~2550 m a.s.l., respectively. The former vent fed mild strombolian activity and lava flows, while the latter represented the main explosive vent, producing strong phreato-magmatic explosions. Explosions at this latter vent, however, shifted to a strombolian style in the following days, before switching back to phreato-magmatic activity towards the end of the eruption, which ended on August 9, 2001. On August 3, a small seismoacoustic array was deployed close to the eruptive vents. The array was composed of three stations, which recorded seismic and infrasonic waves coming from both of the eruptive vents. A further seismoacoustic station, equipped with a thermal-infrared sensor, was also installed several kilometers north of the first array. Seismic signals relating to the strombolian activity at the 2100-m vent were characterized by a strong decompression at the source. Analysis of the time delays between seismic, infrasonic and infrared event onsets also revealed that ejection velocities during explosions from both vents were subsonic. Time delays between the onset of explosive events apparent in the infrared and infrasound data indicated that the explosion source at the 2550-m vent was located 220–250 m below the crater rim. In comparison, the depth of the seismic source was estimated to be between 230 and 335 m below the rim. This converts to 120–150 and 130–235 m below the preexisting ground surface. In addition, time delays between seismic and infrasonic signals recorded for the lower (2100 m) vent also revealed a seismic source that was no more than a few tens of meters deeper than the fragmentation surface.

Description: Published

Description: 219-230

Description: partially_open

Description: Soil radon emissions have been proved as a useful tool for predicting earthquakes and volcanic eruptions and furthermore aided in determining the location of active faults. Continuous radon monitoring was carried out near Southeast Crater of Mt. Etna in September–November 1998, during a period of frequent eruptive episodes at that crater. Radon anomalies were detected when eruptive episodes and the accompanying volcanic tremor became increasingly intense: no anomalies in radon activity were observed during the first five, and weaker, eruptive episodes, whereas significant spikes in radon activity preceded the latter five episodes by ≥46 hours. This probably reflects increased gas leakage through fractures intersecting the shallow plumbing system, as gas pressure in the Southeast Crater conduit became higher with time. Radon monitoring thus might serve to better understand eruptive mechanisms and possible precursors, making further studies in this field a promising perspective.

Description: Published

Description: 1-4

Description: partially_open

Description: Continuous soil radon monitoring was carried out near the Southeast Crater (SEC) of Mt. Etna during the 10-day July 2006 Strombolian-effusive eruption. This signal was compared with simultaneously acquired volcanic tremor and thermal radiance data. The onset of explosive activity and a lava fountaining episode were preceded by some hours with increases in radon soil emission by 4–5 orders of magnitude, which we interpret as precursors. Minor changes in eruptive behavior did not produce significant variations in the monitored parameters. The remarkably high radon concentrations we observed are unprecedented in the literature. We interpret peaks in radon activity as due primarily to microfracturing of uranium-bearing rock. These observations suggest that radon measurements in the summit area of Etna are strongly controlled by the state of stress within the volcano and demonstrate the usefulness of radon data acquisition before and during eruptions.

Description: Published

Description: L24316

Description: reserved

Description: The pattern of volcanic tremor accompanyingthe 1989 September eruption at the south-east summit crater of Mount Etna is studied. In specific, sixteen episodes of lava fountaining, which occurred in the first phase of the eruption, are analysed. Their periodic behaviour, also evidenced by autocorrelation, allows us to define the related tremor amplitude increases as intermittent volcanic tremor episodes. Focusingon the regular intermittent behaviour found for both lava fountains and intermittent volcanic tremors, we tried an a posteriori forecast using simple statistical methods based on linear regression and the Student’ t-test. We performed the retrospective statistical forecast, and found that several eruptions would have been successfully forecast. In order to focus on the source mechanism of tremor linked to lava fountains, we investigated the relationship between volcanic and seismic parameters. A mechanism based on a shallow magma batch ‘regularly’ refilled from depth is suggested.

Description: Published

Description: open

Description: In the last thirty years, numerous eruptions and associated deformation episodes have occurred at Mt. Etna volcano. Datasets recorded by continuous monitoring of these episodes provide a unique opportunity to study the relationships between volcanism, flank instability and faulting activity. We have investigated the stress triggering mechanism between magmatic reservoir inflation, intrusive episodes and flank dynamics. Using three-dimensional numerical Boundary Elements Models we simulated volcano-tectonic events and calculated Coulomb stress changes. Using this modeling approach, we analyzed four realistic scenarios that are representative of recent kinematics occurring at Mt. Etna. The main results obtained highlight how (1) the inflation of a deep spherical magma source transfers elastic stress to a sliding plane and faults (2) the opening of the NE Rift and S Rift (to a less efficient extent) favor movements of the instable sector and may encourage seismicity on the eastern flank faults, and (3) flank instability may trigger the uprising of magma. Defining the effects of the elastic stress transfer and relationships among the main forces acting on volcano, may help to forecast possible eruption scenarios during future episodes of unrest at Mount Etna and provide an important tool for decision makers during volcanic emergencies involving the highly populated areas of the volcano.

Description: Published

Description: 149-158

Description: 3.6. Fisica del vulcanismo

Description: JCR Journal

Description: open

Description: Active volcanoes characterized by open conduit conditions generate sonic and infrasonic signals, whose investigation provides useful information for both monitoring purposes and studying the dynamics of explosive processes. In this work, we discuss the automatic procedures implemented for a real-time application to the data acquired by a permanent network of five infrasound stations running at Mt. Etna volcano. The infrasound signals at Mt. Etna consist in amplitude transients, called infrasound events. The adopted procedure uses a multi-algorithm approach for event detection, counting, characterization and location. It is designed for an efficient and accurate processing of infrasound records provided by single-site and array stations. Moreover, the source mechanism of these events can be investigated off-line or in near real-time by using three different models: (1) Strombolian bubble; (2) resonating conduit and (3) Helmholtz resonator. The infrasound waveforms allow us to choose the most suitable model, to get quantitative information about the source and to follow the time evolution of the source parameters.

Description: Published

Description: 1215–1231

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: The seismic events caused by human engineering activities are commonly termed as “triggered” and “induced”. This class of earthquakes, though characterized by low-to-moderate magnitude, have significant social and eco- nomical implications since they occur close to the engineering activity responsible for triggering/inducing them and can be felt by the inhabitants living nearby, and may even produce damage. One of the first well-documented examples of induced seismicity was observed in 1932 in Algeria, when a shallow magnitude 3.0 earthquake occurred close to the Oued Fodda Dam. By the continuous global improvement of seismic monitoring networks, numerous other examples of human-induced earthquakes have been identified. Induced earthquakes occur at shallow depths and are related to a number of human activities, such as fluid injection under high pressure (e.g. waste-water disposal in deep wells, hydrofracturing activities in enhanced geothermal systems and oil recovery, shale-gas fracking, natural and CO2 gas storage), hydrocarbon exploitation, groundwater extraction, deep underground mining, large water impoundments and underground nuclear tests. In Italy, induced/triggered seismicity is suspected to have contributed to the disaster of the Vajont dam in 1963. Despite this suspected case and the presence in the Italian territory of a large amount of engineering activities “capable” of inducing seismicity, no extensive researches on this topic have been conducted to date. Hence, in order to improve knowledge and correctly assess the potential hazard at a specific location in the future, here we started a preliminary study on the entire range of engineering activities currently located in Sicily (Southern Italy) which may “potentially” induce seismicity. To this end, we performed: • a preliminary census of all engineering activities located in the study area by collecting all the useful information coming from available on-line catalogues; • a detailed compilation of instrumental and historical seismicity, focal mechanisms solutions, multidisciplinary stress indicators, GPS-based ground deformation field, mapped faults, etc by merging data from on-line catalogues with those reported in literature. Finally, for each individual site, we analysed: i) long-term statistic behaviour of instrumental seismicity (mag- nitude of completeness, seismic release above a threshold magnitude, depth distribution, focal plane solutions); ii) long-term statistic behaviour of historical seismicity (maximum magnitude estimation, recurrence time inter- val, etc); iii) properties and orientation of faults (length, estimated geological slip, kinematics, etc); iv) regional stress (from borehole, seismological and geological observations) and strain (from GPS-based observations) fields.

Description: Unpublished

Description: Vienna (Austria)

Description: 6T. Sismicità indotta e caratterizzazione sismica dei sistemi naturali

Description: open

Description: Active volcanoes characterized by open conduit conditions effectively generate sonic and infrasonic signals, whose investigation provides useful information for both monitoring purposes and study of the dynamics of explosive phenomena. At Mt. Etna volcano (Italy) a clustering algorithm based on spectral features and amplitude of the infrasonic events was developed. It allows to recognize the active vent with no location algorithm and by using only one station. Moreover, a waveform inversion procedure was coded, based on genetic algorithm, that enables us to quantitatively investigate the infrasound source parameters.

Description: Published

Description: 195-200

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: reserved

Description: 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.

Description: Published

Description: B09301

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: Earthquakes caused by human engineering activities are commonly termed as “triggered” or “induced”. This class of earthquakes, though characterized by low-to-moderate magnitude, have significant social and economical implications since they occur close to the engineering activity responsible for triggering/inducing them and can be felt by the inhabitants living nearby, and may even produce damage. One of the first well-documented examples of induced seismicity was observed in 1932 in Algeria, when a shallow magnitude 3.0 earthquake occurred close to the Oued Fodda Dam (Gupta, 1985). By the continuous global improvement of seismic monitoring networks, numerous other examples of human-induced earthquakes have been identified (see Davies et al., 2013 for an overview). Induced earthquakes occur at shallow depths and are related to a number of human activities, such as fluid injection under high pressure (e.g. waste-water disposal in deep wells, hydrofracturing activities in enhanced geothermal systems and oil recovery, shale-gas fracking, natural and CO2 gas storage), hydrocarbon exploitation, groundwater extraction, deep underground mining, large water impoundments and underground nuclear tests (Davies et al., 2013). Despite the presence in the Sicilian territory of a large amount of engineering activities “potentially capable” of inducing seismicity, no extensive researches on this topic have been conducted to date. Hence, in order to improve our knowledge, and correctly assess the hazard at a specific location in the future, we started a preliminary study on the main engineering activities located on- and off-shore of Sicily (Southern Italy). To this end, in a first step we collected all the useful information coming from available on-line national and regional catalogues. The compiled database includes 46 dams, 598 quarries and 839 oil and gas wells for a total of 1483 engineering activities. Among these, 175 are located along the southern Sicilian coastal off-shore while the remaining 1308 are located inland. As a second step, we performed a detailed compilation of instrumental seismicity striking the investigated area. Continuous seismic monitoring of the whole Italian territory started in the 90s and is currently performed by the National Seismic Network managed by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). In addition, monitoring and systematic analysis of seismic activity in eastern Sicily, by means of a dense local network, is performed also by the “Osservatorio Etneo” (INGV-OE), an INGV-branch located in Catania, close to Mt. Etna. Since 1983, earthquakes occurred in the entire Italian territory have been analysed and archived in the catalogue managed by the INGV headquarters in Rome (INGV-CNT; Castello et al., 2005; ISIDe Working Group - INGV, 2010), while since 1999, earthquakes occurred in eastern Sicily have been analysed and archived in the database of the INGV-OE (Alparone et al., 2009; Gruppo Analisi Dati Sismici, 2016). Because the INGV-OE catalogue covers with great details only the eastern sector of Sicily, while the INGV-CNT catalogues, extends back in time since 1983, in order to identify possible prospective effects of the human activities on the seismicity, here we took into account both catalogues. We considered only the seismicity occurred within the first 10 km of the crust because the induced seismicity should be likely confined in the shallower crust. As a final step, in order to identify prospective effects of the human activities on the seismicity, we investigated the spatio-temporal relationships between engineering activities and earthquakes, by adopting a statistic approach aimed to the detection of anomalous seismicity densities. Finally, we identified 46 engineering activities (2 dams, 16 wells and 28 quarries) characterized by anomalous seismicity density. These activities are mainly located in Western Sicily and on the eastern sector of the Hyblean Plateau, while a few number of activities are locate in northern Sicily and on the Island of Vulcano. Currently, we are performing detailed analyses on the nature of the observed seismicity activity in proximity of these engineering activities.

Description: Published

Description: Napoli (Italy)

Description: 6T. Sismicità indotta e caratterizzazione sismica dei sistemi naturali

Description: restricted