E. De Lauro

Self-sustained oscillations at Volcán de Colima (México) inferred by Independent Component Analysis

We have analyzed Long-Period (LP) seismic events of Volcán de Colima (Mexico) recorded by four three-component broad-band seismometers during January 2006. Frequency-domain analysis shows spectra mainly peaked in the range 0.3–2 Hz and characterized by a monotonic decreasing envelope, as those produced in a cylindrically-symmetric self-oscillating cavity interacting with a confined jet. Independent Component Analysis, a time-series decomposition method, extracts two or three nonlinear oscillation modes depending on the station, with the fundamental one peaked at 0.4–0.5 Hz. These decomposed waves have a well defined spectral content and are self-sustained oscillations with low dimensionality, as shown by dynamic system reconstruction methods. Moreover, they show radial polarization in near field and transverse polarization in far field in North-West South-East direction. Finally, we hypothesize a branched plumbing system with two principal conduits having alignments orthogonal to one another.

ANALYSIS OF WATER LEVEL OSCILLATIONS BY USING METHODS OF NONLINEAR DYNAMICS

Sea level variations collected in several areas in the world have been analyzed trying to infer their non linear characteristics. Analyzed data were acquired in several sites in West and East coasts of the North American continent, in the Hawaii and Bermuda islands, representing oceanic sites, and in Adriatic sea, representing a well known basin type. Data have been analyzed through Independent Component Analysis, False Nearest Neighbours and the estimation of correlation dimension using Grasberger and Procaccia integral. Results show a clear non linear features in all the sites, characterized by second and third order Landau mode.

Limit cycles in nonlinear excitation of clusters of classical oscillators

In this paper we develop a numerical procedure for detecting the existence of limit cycles in nonlinear excitation of clusters of classical harmonic oscillators. Our technique is able to compute also the main parameters of a limit cycle, that is the amplitudes and the period. The numerical method, based on the propagation matrix formalism, is transparent and easy to apply. It may find application in various areas where nonlinear excitations are involved, e.g., sound and mechanic vibrations in musical instruments, ground vibrations in volcanic areas, and sea tides.

Convolutive independent component analysis for processing massive datasets: a case study at Campi Flegrei (Italy)

A novel procedure is proposed to analyse continuous seismic signal on hourly scales to have a prompt discrimination among the different sources. Specifically, this approach is applied to a massive dataset recorded at Campi Flegrei caldera during the year 2006 when a swarm of volcano-tectonic earthquakes occurred. The convolutive independent component analysis is adopted to obtain a clear separation among meteo-marine microseism, anthropogenic noise, hydrothermal tremor in the absence of volcano-tectonic activity, whereas in non-stationary conditions a contribution connected to the corner frequency of the earthquakes emerges. A coarse-grained variable to be monitored continuously is introduced, i.e. the frequency associated with the maximum amplitude of the power spectral density of the deconvolutive independent components. That parameter is sensitive to the variation in the frequency bands of interest (e.g. that corresponding to the corner frequencies of volcano-tectonic events) and can be used as marker of the insurgence of seismic activity.

Identification of soil redistribution using 137Cs for characterizing landslide-prone areas: a case study in Sarno-Quindici, Italy

Debris flows are among the most hazardous and unpredictable processes whose hazard evaluation requires an understanding of the processes that govern sediment supply, sediment bulking, flow volume, and deposition. In line with this challenge, we estimate the net soil redistribution rates in an area characterized by recurrent instability phenomena using 137Cs as a tracer for monitoring soil dynamics. An extension of this method by applying it to the case of major landslides is addressed. Specifically, surveys of soil 137Cs content at sites in the Pizzo d’Alvano massif (Italy), where a set of particularly damaging landslides occurred on 5–6 May 1998, are reported. Being this region prone to debris flows, two zero-order basins are examined, one of which experienced a landslide event. In that basin deposition is observed, whereas the other shows a net loss of soil. These results appear to be relevant, particularly because the rate of hollow filling could be correlated with the magnitude and/or frequency of debris flow occurrence or with some measure of the hazard presented by individual gullies.

Observations of the 18.6-year cycle effects on the sea-level oscillations in the North Atlantic Ocean

We investigate the physical processes that generate the ocean tides, whose understanding has important influence on the marine activities. We analyze historical sea-level oscillations, continuously recorded from six stations in the North Atlantic Ocean spanning a time period of eighty years from 1926. In this paper, an Independent-Component-Analysis?based approach is adopted to obtain a clear identification of the main tidal constituents in term of waveform in time domain from the simultaneously recorded signals. This technique separates at most six nonlinear tidal components which are weakly superimposed. The fundamental objective is to extract information on the degree of complexity of the involved dynamics and on the very long-term tidal constituents. This is particularly significant to understand the response of the ocean to the tidal forcing. We put the emphasis on the near-bidecadal time scale and its influence on the short-periods tides. In details the Moon 18.6 y nodal cycle modulation acts in the ocean in quite an analogous manner to the fortnightly modulation in many shallow seas. Our results give new insights into the evidence for an 18.6 y effect in the climate/ocean variation whose physical mechanism details remain murky.

STROMBOLIAN-LIKE VOLCANO ACTIVITY: A COMMON MACROSCOPIC BEHAVIOUR

We report the results of the analysis of the occurrence times of the strombolian-like explosions at Stromboli (Italy) and Erebus volcano (Antarctica). The distribution of the inter-times between successive explosion-quakes is exponential for both volcanoes, the only difference being the occurrence rate. It suggests that the signature of the standard Strombolian activity can be identified in a Poissonian process that rules the mechanism of generation of the explosions. Finally, we adopt coalescence Chandrasekar-Landau mean-field model for the formation of the exploding gas bubbles. The model provides a size distribution that is in a good agreement with the observed bubble dimensions.

ICA for modelling and generating organ pipes self-sustained tones

Acoustic signals emitted by organ pipes in a variety of experimental frameworks have been recorded and analyzed by using independent component analysis. Starting from this analysis, relevant features of the signals related to single tones of the chords have been extracted. Three Landau modes are extracted with three well defined frequencies. Following the dynamical systems approach, a simple and suitable analogical model, able to reproduce the registered waveform and sound in listening, have been constructed. The conclusion is that, in first approximation, the low dimensional dynamical system representing on average the fluid-dynamical equations modelling organ pipe is constituted by three linearly coupled nonlinear oscillators in limit cycle regime.