Abstract
Abstract. We investigate the physical processes associated with volcanic tremor and explosions. A volcano is a complex system where a fluid source interacts with the solid edifice so generating seismic waves in a regime of low turbulence. Although the complex behavior escapes a simple universal description, the phases of activity generate stable (self-sustained) oscillations that can be described as a non-linear dynamical system of low dimensionality. So, the system requires to be investigated with non-linear methods able to individuate, decompose, and extract the main characteristics of the phenomenon. Independent Component Analysis (ICA), an entropy-based technique is a good candidate for this purpose. Here, we review the results of ICA applied to seismic signals acquired in some volcanic areas. We emphasize analogies and differences among the self-oscillations individuated in three cases: Stromboli (Italy), Erebus (Antarctica) and Volcán de Colima (Mexico). The waveforms of the extracted independent components are specific for each volcano, whereas the similarity can be ascribed to a very general common source mechanism involving the interaction between gas/magma flow and solid structures (the volcanic edifice). Indeed, chocking phenomena or inhomogeneities in the volcanic cavity can play the same role in generating self-oscillations as the languid and the reed do in musical instruments. The understanding of these background oscillations is relevant not only for explaining the volcanic source process and to make a forecast into the future, but sheds light on the physics of complex systems developing low turbulence.
Highlights
The ultimate goal in Volcanology is to forecast eruptions
We review the results achieved by applying Independent Component Analysis (ICA) to seismic signals recorded at different volcanoes, critically individuating in their complex behaviour the basic elements that can lead to a simple universal description of the volcano activity
The characteristic vibration time scales depend on the local geometry of the volcanic edifice and rheological properties
Summary
The ultimate goal in Volcanology is to forecast eruptions. Since at least three decades, the rising number of observed phenomena associated with eruptions led many scientists to believe that it could be possible to obtain a simple set of empirical relationships suitable for this purpose. There are other approaches based on similar ideas as ICA, such as least-dependent component analysis (MILCA algorithm, Stogbauer et al, 2004) or non-Gaussian component analysis (SNICA algorithm, Kawanabe et al, 2007) They use a Mutual Information (MI) estimator instead of nonGaussianity to find the least dependent components under a linear transformation. It is important to underline that ICA with the FastICA algorithm is very efficient to extract self-oscillations and periodic signals from noise or non-linear mixtures up to an amplitude ratio smaller than three order of magnitude, making www.nonlin-processes-geophys.net/18/925/2011/. Chouet et al (2003) investigated the seismic wavefield associated with very long period events (VLP), and retrieved the equivalent elastic source mechanism which includes both moment tensor and single force components These authors estimated for a typical VLP event a volume change on the order of 200 m3 and a single vertical force with a magnitude on the order of 108N. The application of ICA intends to recover the simplest oscillations related to the vibrations of the volcanic edifice strictly connected to the source process
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