Temporal Changes of Seismic Velocity Caused by Volcanic Activity at Mt. Etna Revealed by the Autocorrelation of Ambient Seismic Noise

Raphael S M De Plaen,Andrea Cannata,Flavio Cannavo',Thomas Lecocq,Olivier Francis,Corentin Caudron

doi:10.3389/feart.2018.00251

Abstract

On active volcanoes, ambient noise-based seismic interferometry, able to detect very slight variations in seismic velocity associated with magma transport towards the surface, can be a very useful monitoring tool. In this work, we performed the autocorrelation of ambient seismic noise recorded at Mt. Etna volcano, by three stations located close to the active summit craters, during April 2013 - October 2014. Such an interval was chosen because of the number and variety of eruptions. The method implemented to perform autocorrelation was the phase cross-correlation, which does not require normalization of the signals. The detected seismic velocity variations were very consistent for all three stations throughout the study period, mainly ranging between 0.3 and -0.2%, and were time-related to both sequences of paroxysmal eruptions and more effusive activities. In particular, we observed seismic velocity decreases accompanying paroxysmal eruptions, suggesting an intense pressurization within the plumbing system, which created an area of extensional strain with crack openings. It is worth noting that classical cross-station approach failed to detect seismic velocity changes related to volcano activity. In addition, seismic velocity variations over time were integrated with ground deformation data recorded by GPS stations and volcanic tremor centroid locations. Finally, we showed that, although the investigated frequency band (1-2 Hz) contains most of the volcanic tremor energy, our results did not indicate a particular contamination of seismic velocity variation measurements by variations of tremor sources.

Highlights

Magma intrusion and pressurization of the plumbing system can change the seismic velocity structure of a volcano (e.g., Brenguier et al, 2008, 2016)
The ambient noise interferometry, based on the fact that the Earth is not static but permanently vibrating, has many advantages (e.g., Brenguier et al, 2016): (i) it has a very high precision, much higher than classic tomography techniques; (ii) unlike coda wave interferometry applied on repeating earthquakes (e.g., Cannata, 2012; Hotovec-Ellis et al, 2014), ambient noise interferometry allows a continuous monitoring of the medium velocity changes; (iii) it is a non-invasive or destructive monitoring method
The mainly effusive eruption (E47) initiated at a cluster of vents at the eastern base of the NSEC cone in late January 2014 was preceded by a consistent increase of seismic velocity that coincides with a change in style of activity from paroxysmal to mainly effusive

Summary

Introduction

Magma intrusion and pressurization of the plumbing system can change the seismic velocity structure of a volcano (e.g., Brenguier et al, 2008, 2016). The ambient noise interferometry, based on the fact that the Earth is not static but permanently vibrating, has many advantages (e.g., Brenguier et al, 2016): (i) it has a very high precision, much higher than classic tomography techniques (it is able to detect velocity variations much lower than 1%; e.g., Brenguier et al, 2016; Donaldson et al, 2017); (ii) unlike coda wave interferometry applied on repeating earthquakes (e.g., Cannata, 2012; Hotovec-Ellis et al, 2014), ambient noise interferometry allows a continuous monitoring of the medium velocity changes; (iii) it is a non-invasive or destructive monitoring method For these reasons, measurement of seismic velocity by ambient noise interferometry is a promising tool to monitor volcanoes (e.g., Brenguier et al, 2008; Duputel et al, 2009). Named the New South East Crater (NSEC, Figure 1), the cone was built up of material accumulated from lava fountains

Results

Discussion

Conclusion