Characterization and location of infrasonic sources in active volcanoes: Mount Etna, September–November 2007

Abstract

[1] The period September-November 2007 was characterized at Mount Etna by explosive activity and intense degassing. During this time interval, infrasonic signals were recorded by an infrasonic network. By a triggering procedure, about 1000 infrasonic events were found, characterized by very high signal-to-noise ratio and grouped into nine families. Successively, the spectral analysis allowed subdividing these nine families into three clusters based on the peak frequency and the quality factor of the events. Finally, by the location analysis a cluster (cluster 1) was related to the degassing activity of the northeast crater (NEC), while the other two (clusters 2 and 3) to the explosive activity of the southeast crater (SEC). The comparison between the stacked infrasonic waveforms, interpreted as generated by the vibration of large gas bubbles, and the synthetic ones, permitted to calculate radius, length of the bubble, and initial overpressure, by a genetic algorithm method. The higher overpressure values of cluster 3 compared to the cluster 2 values were in good agreement with the stronger intensity of the explosions accompanying the infrasonic events of cluster 3. The variation of both intensities and waveforms is tentatively attributed to the occasional accumulation of lithic clasts (due to moderate landslides?) on the explosive vent. Indeed, events belonging to cluster 3 were no longer observed once the landslides had ended. Finally, the daily emitted gas volume, related to the active degassing, was estimated for NEC and SEC by using the infrasonic data during the studied period.

Decadal evolution of a degassing magma reservoir unravelled from fire fountains produced at Etna volcano (Italy) between 1989 and 2001

“…As for the 1995 eruption, the episodes with a duration exceeding 1 day are probably the result of using a premature initiation based on mild surface activity of the open vent. The last episode on February 4, 1999 is marked by a brief and violent fire fountain while changing to a flank eruption (La Delfa et al 2001;Behncke et al 2006) the gas volume at the vent Vergniolle and Ripepe 2008;Cannata et al 2009). This technique, based on a best fit between a recorded explosion and its synthetic waveform, also provides an estimate for both the average and the maximum gas volume for a given duration (Vergniolle and Ripepe 2008).…”

Seismic and infrasound signals at Mt. Etna: Modeling the North-East crater conduit and its relation with the 2008–2009 eruption feeding system

“…Infrasound radiation from these vents can be classified into continuous and sporadic activity. The former consists in short-duration releases of pressure bursts constantly generated by NEC and related to degassing activity (Cannata et al, 2009b), while the latter occurs solely during explosive activity (Strombolian explosions, lava fountains, ash emissions). Indeed, over the last few years, SEC and BN have generated infrasonic tremor and amplitude transients during eruptive episodes (e.g.…”

Towards an Automatic Monitoring System of Infrasonic Events at Mt. Etna: Strategies for Source Location and Modeling

“…Thereafter, the gas volume expelled in the atmosphere can be deduced from the bubble volume at the vent by using the overpressure values and the perfect gas law (VER- GNIOLLE and RIPEPE, 2008). By summing the gas volume obtained for all the infrasonic events, the degassing rate of the volcano can also be estimated (CANNATA et al, 2009b). However, since gas at volcanoes is also emitted without detectable infrasound radiation (VERGNIOLLE and RIPEPE, 2008), the estimated gas flux is to be considered an underestimation.…”

“…On this basis and according to CANNATA et al (2009b), a simple spectral analysis of the infrasonic events recorded at a single station, together with the amplitude estimation, can give preliminary information on the ongoing volcanic activity and active craters. In particular, spectral and amplitude variations over the time of such infrasound signals can be a good indicator of changes in the volcanic activity.…”

“…3). As shown in CANNATA et al (2009b), the Sompi method (KUMAGAI, 2006 and reference therein) is a useful algorithm to calculate the dominant frequency and the quality factor of the events. Similar to the detecting step, the infrasound characterization is carried out on the signal recorded by EBEL, considered as reference station.…”