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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: In the last 30years, 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) favors 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. [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]

The pattern of volcanic tremor accompanying the 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. Focusing on 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. [Copyright &y& Elsevier]

Abstract: Almost 50,000 long period (LP) events were recorded at Mt. Etna from November 2003 to May 2006. We analysed these events, as well as very long period (VLP) events which were associated with some of them. During some intervals the spectral and wavefield features of LP events remained steady, with significant changes occurring between the intervals. Based on the times of the changes, we distinguish five different sub-periods. In particular, during sub-period III (June–November 2005) the wavefield at the stations nearest to the summit area was composed of P waves. Locations for 150 LP events occurring in sub-period III, determined using radial semblance, changed, at the same time as the events'' spectral features changed. It was during this sub-period that many of the LP events were associated with VLP events. Based on similarity of the waveforms, we distinguished two families of VLP events, with gradually evolving waveforms. The two families are located in slightly different places, but near the sources of the LP events. The change between the families occurred at the same time as the spectra of the LP events changed. Finally the source of the VLP events was investigated by performing complete waveform moment tensor inversion of stacks of the two families. Synthetic Green''s functions for the full moment tensor were calculated for a homogeneous half-space. For both families, the source region with the highest variance reduction lies approximately beneath the active craters, at 500m below the altitudes of the stations. The solutions for both families are very similar with sources that are between 60 and 70% isotropic. Attempts to determine deviatoric moment tensors produced consistently poorer fits. The remaining energy is poorly constrained and is likely to be noise. In conclusion, these results highlight changes in the LP and VLP events at Mt. Etna over time, and the causal relationship between them. [Copyright &y& Elsevier]

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]