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At active volcanoes, distinct eruptions are preceded by complex and different precursory patterns; in addition, there are precursory signals that do not necessarily lead to an eruption. The main purpose of this paper is to present an unprecedented application of the recently developed code named BET_EF (Bayesian Event Tree_Eruption Forecasting) to the quantitative estimate of the eruptive hazard at Mt. Etna volcano. We tested the model for the case history of the July-August 2001 flank eruption. Anomalies in geophysical, geochemical and volcanological monitoring parameters were observed more than a month in advance of the effective onset of the eruption. As a consequence, eruption probabilities larger than 90% were estimated. An important feature of the application of BET_EF to Mt. Etna was the probabilistic estimate of opening vent locations. The methodology allowed a clear identification of assumptions and the monitoring of parameter thresholds and provided rational means for their revision if new data or information are incoming. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

The beginning of the 1991–1993 large eruption at Mt. Etna was marked by a seismic swarm and a sudden increase of volcanic tremor amplitude. Several earthquakes were recognized as clusters of shallow events with nearly identical waveforms (multiplets). We have analyzed these signals with a cross-spectral technique that allows us to locate the relative position of the events within a multiplet, obtaining a precision higher than with traditional routines. The relocated shocks occurred some kilometers away from the propagating eruptive fracture, east of both the summit craters and the modeled eruptive dykes. They clustered in a relatively small volume, elongated in the NNW–SSE direction, the depth of which ranged from about 1100 to 2000 m below sea level. These earthquakes are interpreted as the passive brittle response of the volcanic basement to the intrusion of the eruptive dyke, rather than being directly linked to the magma transport. [Copyright &y& Elsevier]

Abstract: Combined GPS measurements and radar interferometry (InSAR) have been applied at Mt. Etna to study the ground deformation affecting the volcano both over the long- (1993–2000) and short-term (1997–1998 and 1998–2000). The aim was to better understand the dynamics of the volcano during the magma-recharging phase following the 1991–93 eruption. Since 1993, InSAR and GPS data indicate that Mt. Etna has undergone an inflation. A deep intrusion was detected by InSAR, on the western flank of the volcano, between March and May 1997. In the following months, this intrusion rose up leading to a seismic swarm occurring in January 1998 in the western sector. This now shallow intrusion is confirmed by GPS data. From 1998 to 2000, a general deflation affecting the upper part of the volcano was detected. Over the whole study period, a continuous eastward to south-eastward motion of the eastern sector of the volcano was also evidenced. The analytical inversions of GPS data inferred a plane dipping about 12°ESE, located beneath the eastern flank of the volcano at a depth of 1.4 km b.s.l. The movement along this plane is able to reproduce the observed south-eastward motion of a sector bounded northward by the Pernicana fault, westward by the North–East Rift and the South Rift, and southward by the Mascalucia–Tremestieri–Trecastagni fault system. InSAR data have validated this model. [Copyright &y& Elsevier]

Abstract: We have studied the volcanic tremor recorded at Mt. Etna during January–June 1999, by using three-component seismic stations. This time period was characterised by both explosive and effusive eruptions occurring at the summit craters. We found significant time variations in the trend of the overall spectral amplitude of tremor, as well as in the dominant spectral peaks. Moreover, the tremor wavefield features (polarization, particle motion and ratios between the amplitude of the three components of the ground velocity) have been studied too, confirming significant time variations. This leads us to suggest the existence of at least two tremor sources: a shallow one, mainly characterised by relatively high frequencies (3.5–7 Hz), is linked to the upper portions of the active conduits, and directly related to the observed eruptive activity. The latter deeper source was active for only a few weeks, and characterised by frequencies lower than about 2.5 Hz. It had no relationship with the observed eruptive activity, and was speculatively related to the gas exsolution from fresh magma refilling a small batch. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: During 1999, the volcanic activity at Mt. Etna was both explosive and effusive at the summit craters: Strombolian activity, lava fountains and lava flows affected different areas of the volcano, involving three of the four summit craters. Results from analysis of the 1999 volcanic tremor features are shown at two different time scales. First, the long-term time variation of the features of the volcanic tremor (including spectral and polarization parameters), during the entire year, was compared with the evolution of the eruptive activity. This approach demonstrated the good agreement between tremor data and observed eruptive activity; the activation of different tremor sources was suggested. Then, a more refined analysis of the volcanic tremor, recorded during 14 lava fountain eruptions, was performed. In particular, a shift of the dominant frequencies towards lower values was noted which corresponds with increasing explosive activity. Similar behaviour in the frequency content has already been observed in other explosive eruptions at Mt. Etna as well as on other volcanoes. This behaviour has been explained in terms of either an increase in the tremor source dimension or a decrease in the sound speed in the magma within the conduit. These results confirm that the volcanic tremor is a powerful tool for better understanding the physical processes controlling explosive eruptions at Mt. Etna volcano. [Copyright &y& Elsevier]