ALBERT

Description: We report here on thefirst record of carbon dioxide gas emission rates from a volcano, captured at≈1 Hz. These data were acquired with a novel technique, based on the integration of UV camera observations (to measure SO2 emission rates) and field portable gas analyser readings of plume CO2/SO2 ratios. Our measurements were performedat the North East crater of Mount Etna, southern Italy, and the data reveal strong variability in CO2 emissions over timescales of tens to hundreds of seconds, spanning two orders of magnitude. This carries importantimplications for attempts to constrain global volcanic CO2 release to the atmosphere, and will lead to an increased insight into short term CO2 degassing trends. A common oscillation in CO2 and SO2 emission rates in addition to the CO2/SO2 ratios was observed at periods of ≈89 s. Our results are furthermore suggestive of an intriguing temporal lag between oscillations in CO2 emissions and seismicity at periods of ≈300–400 s, with peaks and troughs in the former series leading those in the latter by ≈150 s. This work opens the way to the acquisition of further datasets with this methodology across a range of basaltic systems to better our understandingof deep magmatic processes and of degassing links to manifest geophysical signals

Description: Published

Description: 115–121

Description: 2V. Dinamiche di unrest e scenari pre-eruttivi

Description: JCR Journal

Description: restricted

Description: This work describes the automatic picking of the P-phase arrivals of the 3*10^6 seismic registers originated during the TOMO-ETNA experiment. Air-gun shots produced by the vessel “Sarmiento de Gamboa” and contemporary passive seismicity occurring in the island are recorded by a dense network of stations deployed for the experiment. In such scenario, automatic processing is needed given: (i) the enormous amount of data, (ii) the low signal-to-noise ratio of many of the available registers and, (iii) the accuracy needed for the velocity tomography resulting from the experiment. A preliminary processing is performed with the records obtained from all stations. Raw data formats from the different types of stations are unified, eliminating defective records and reducing noise through filtering in the band of interest for the phase picking. The advanced multiband picking algorithm (AMPA) is then used to process the big database obtained and determine the travel times of the seismic phases. The approach of AMPA, based on frequency multiband denoising and enhancement of expected arrivals through optimum detectors, is detailed together with its calibration and quality assessment procedure. Examples of its usage for active and passive seismic events are presented.

Description: Published

Description: S0434

Description: 2V. Dinamiche di unrest e scenari pre-eruttivi

Description: JCR Journal

Description: open

Description: earthquake source parameters in a given region requires a good knowledge of attenuation and properties of the medium. Especially, information on high-frequency seismicwave attenuation in the lithosphere is of particular interest (Yoshimoto et al. 1993). In southeastern Sicily, the attenuation of seismicwaves has been studied by using different data set and methods (e.g. Castro et al. 1993; Giampiccolo et al. 2002, 2003, 2004; de Lorenzo et al. 2004). Castro et al. (1993) first obtained a frequency-dependent quality factor of coda waves (QC) and calculated the average attenuation law for the area. Giampiccolo et al. (2002, 2004) investigated in more detail both the frequency and lapse time dependence of QC by using different data sets. Their results show a good agreement with those obtained by Castro et al. (1993). Moreover, a clear increase of QC with lapse time and, therefore, with depth was observed. Giampiccolo et al. (2003) and de Lorenzo et al. (2004) estimated the quality factor of P-waves (QP) by applying the pulse-broadening method (Wu & Lees 1996) whereas Giampiccolo et al. (2003) estimated the attenuation of S waves (Q−1 S ) by applying the frequency decay method (see Bianco et al. 1999) in the low- (below the corner frequency) and high- (above the corner frequency and below the cut-off filter) frequency ranges. The obtained results suggested that attenuation at higher frequencies is less pronounced than at lower ones. However, the detailed frequency-dependent nature of QP and QS was not resolved yet by the above quoted studies. It isworth stressing that attenuation estimated from direct S waves contains the combined effects of scattering and intrinsic loss. Scattering attenuation is described by the quality factor Q−1 s and is due to the presence of inhomogeneities. Therefore, it depends on the spatial structure of the heterogeneities in the medium and on the size of the velocity and density fluctuations. Intrinsic absorption Q−1 i is caused by the anelasticity of the medium and depends on viscous processes between the rock matrix and liquid inclusions, such as pore fluids, and on movements of dislocations through the mineral grains (Goric & Muller 1987). Quantifying the relative contribution of scattering and intrinsic attenuation has been a subject of considerable interest among seismologists and different methods have been developed (e.g. Wu 1985; Hoshiba et al. 1991; Wennerberg 1993). In the present paper we will estimate the quality factor of S waves (QS) in the lithosphere beneath southeastern Sicily and clarify its frequency dependence by means of the coda-normalization method (Aki 1980), applied in the frequency range 1.5–15 Hz.We will also obtain a separate estimate of intrinsic and scattering attenuation by applying the multiple lapse time window analysis (MLTWA) technique by Hoshiba et al. (1991). This method gives information about the temporal change of seismic energy during a wave’s propagation by considering the energy in multiple consecutive time windows as a function of the hypocentral distance. Under the assumptions of multiple and isotropic scattering and uniformdistribution of scatterers, two attenuation parameters will be calculated: the seismic albedo B0, defined as the dimensionless ratio of the scattering loss to total attenuation (B0 = Q−1 s /Q−1 T ) and the inverse of the extinction length L−1 e that is the inverse of the distance (in kilometres) over which the primary S-wave energy is decreased by e−1. B0 ranges between 0 and 1 and was proposed byWu (1985) to describe the proportions of energy loss dominated by intrinsic attenuation (B0 〈 0.5) or scattering attenuation (B0 〉 0.5). The estimated scattering (Q−1 s ) and intrinsic (Q−1 i ) attenuation mechanisms in the frequency range 1.5–15 Hz will be discussed and compared with previous results obtained by Giampiccolo et al. (2004). Estimates of total attenuation Q−1 T will be compared with the coda-Q values (QCobs) obtained by Giampiccolo et al. (2004) with the expected coda-Q (QCexp) calculated in this study by using Hoshiba (1991) relationship. Finally, since the MLTWA technique has been widely applied to several areas in the world (e.g. Mayeda et al. 1992; Hoshiba 1993; Akinci et al. 1995; Pujades et al. 1997; Akinci & Eydo˘gan 2000; Bianco et al. 2002; Ugalde et al. 2002; Bianco et al. 2005), differences and analogies observed among southeastern Sicily and other tectonic settings will be discussed.

Description: Published

Description: 211-222

Description: reserved

Description: The time delays and polarizations of shear wave splitting above small earthquakes show variations before the 2001 July 17–August 9 2001 flank eruption on Mt Etna, Sicily. Normalized time delays, measured by singular value decomposition, show a systematic increase starting several days before the onset of the eruption. On several occasions before the eruption, the polarization directions of the shear waves at Station MNT, closest to the eruption, show 90◦- flips where the faster and slower split shear waves exchange polarizations. The last 90◦-flip being 5 days before the onset of the eruption. The time delays also exhibit a sudden decrease shortly before the start of the eruption suggesting the possible occurrence of a ‘relaxation’ phenomena, due to crack coalescence. This behaviour has many similarities to that observed before a number of earthquakes elsewhere.

Description: Published

Description: 959-967

Description: open

Description: Seismic coda of regional and local earthquakes recorded at a couple of broad band seismometers located at the bottom of a 125 m deep borehole and up-hole at surface show interesting spectral features. We observe strong similarity between the waveforms recorded from up- and down-hole sensors at low frequency (0.1 -3 Hz) and measurable differences in the higher frequency limit. We interpret this observation assuming that at high frequency the up-hole coda is produced by body-tosurface wave scattering in the near surface. We compare the experimental results with numerical simulations done using the Monte Carlo scheme of Yoshimoto et al. (2000) carried out in the assumption of velocity and scattering coefficient which smoothly vary with depth, with the addition of a body-to-surface wave conversion for the energy particles which reach the surface. The comparison of the experimental coda envelopes with those obtained through numerical simulation allow for a quantification of the turbidity parameter at surface.

Description: Published

Description: Perugia,, Italy

Description: 3.1. Fisica dei terremoti

Description: open

Description: The attenuation of seismic waves is one of the basic physical parameters used in seismological studies and earthquake engineering, which is closely related to the seismicity and regional tectonic activity of a particular area. In this work, the seismic attenuation in the Straits of Messina, affected by several and strong historical earthquakes, was studied using waveforms recorded by a local seismic network composed of seven stations. We measured: the coda quality factor (Qc) in the Single Scattering model hypothesis; the direct quality factor (Qd) applying the Coda Normalization method for S-waves; and the intrinsic and scattering quality factor (Qi and Qs) by the Multiple Lapse Time Window Analysis (MLTWA) method. Coda Q values were obtained using different lapse times (40, 60 and 80 s) for the frequency bands centred at 1.5, 3, 6 and 12 Hz. Our findings indicate that Qc increases with increasing lapse time and that Qc is frequency dependent. This behaviour is usually correlated to the degree of tectonic complexity and to the presence of heterogeneities at several scales. In fact, by using the Coda Normalization method we obtained low Qd values, as expected for a heterogeneous and active zone. Finally, by the MLTWA method we observe that the contribution of the scattering attenuation (Qs −1) prevails on the intrinsic absorption (Qi −1) until 3 Hz. Conversely, Qi −1 and Qs −1 seem to be of the same order in the higher frequency bands.

Description: Published

Description: 173-185

Description: reserved

Description: Mt. Etna lies in front of the southeast-verging Apennine-Maghrebian fold-and-thrust belt, where the NNW-trending Malta Escarpment separates the Sicilian continental crust from the Ionian Mesozoic oceanic basin, presently subducting beneath the Calabrian arc (Selvaggi and Chiarabba, 1995). Seismic tomographic studies indicate the presence of a mantle plume beneath the volcano with a Moho transition at depth less than 20 km (Nicolich et al.,2000; Barberi et al., 2006). Geophysical and geological evidences suggest that the Mt. Etna magma ascent mechanism is related to the major NNW-trending lithospheric fault (Doglioni et al., 2001). However, the reason for the Mt. Etna mantle plume draining and channeling the magma from the upper mantle source to the surface is not yet clear. All models proposed in literature (Rittmann, 1973; Tanguy et al., 1997; Monaco et al.; 1997; Gvirtzman and Nur, 1999; Doglioni et al., 2001) do not explain why such a mantle plume has originated in this anomalous external position with respect to the arc magmatism and back-arc spreading zones associated with the Apennines subduction. Some ideas on the subduction rollback must be better developed through the comparison with new regional tomographic studies that are being released. Moreover, tomographic studies reveal a complex and large plumbing system below the volcano from -2 to -7 km a.s.l., wide up to 60 km2 that reduces itself in size down to -18 km of depth close to the apex of the mantle plume. Chiocci et al. (2011) found a large bulge on the underwater continental margin facing Mt. Etna, and suggested that the huge crystallized magma body intruded in the middle and upper continental crust was able to trigger an instability process involving the Sicilian continental margin during the last 0.1 Ma. This phenomenon induces the sliding of the volcano eastern flank observed since the 90s (Borgia et al, 1992; Lo Giudice and Rasà, 1992) because the effects of the bulge collapse are propagating upslope, and the continuous decompression at the volcano summit favors the ascent of basic magma without lengthy storage in the upper crust, as one might expect in a compressive tectonic regime. Taken together, these new evidences (tomographic, tectonic, volcanic) are concerned with the exceptional nature of Mt. Etna and raise the need to explain the origin of the mantle plume that supplies its volcanism. The lower crust and the uppermost mantle need to be better resolved in future experiments and studies. The use of regional and teleseismic events for tomography and receiver function analyses is required to explore a volume that has only marginally been investigated to date. The relation between the magma source in the mantle and the upper parts of the system, as well as the hypothesis above reported on the relation between tectonics and volcanism and the role of lithospheric faults, could be resolved only by applying seismological techniques able to better constrain broader and deeper models. Finally, although the recent tomographic inversions have progressively improved our knowledge of Etna’s shallow structure, highlighting a complex pattern of magma chambers and conduits with variable dimensions, the geometry of the conduits and the dimensions and shapes of small magmatic bodies still require greater investigation. Their precise definition is crucial to delineate a working model of this volcano in order to understand its behaviour and evolution. For this purpose, at least within the volcanic edifice, the precise locations of the seismo-volcanic signals can be considered a useful tool to constrain both the area and the depth range of magma degassing and the geometry of the shallow conduits. In this work, we furnish evidences that the tremor and LP locations allowed to track magma migration during the initial phase of the 2008-2009 eruption and in particular the initial northward dike intrusion, also confirmed by other geophysical, structural and volcanological observations (Aloisi et al., 2009; Bonaccorso et al., 2011), and the following fissure opening east of the summit area at the base of SEC. All these evidences, obtained by the marked improvement in the monitoring system together with the development of new processing techniques, allowed us to constrain both the area and the depth range of magma degassing, highlighting the geometry of the magmatic system feeding the 2008-2009 eruption.

Description: Published

Description: 73-104

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: open

Description: We present the first application of a time reverse location method in a volcanic setting, for a family of long-period (LP) events recorded on Mt Etna. Results are compared with locations determined using a full moment tensor grid search inversion and cross-correlation method. From 2008 June 18 to July 3, 50 broad-band seismic stations were deployed on Mt Etna, Italy, in close proximity to the summit. Two families of LP events were detected with dominant spectral peaks around 0.9 Hz. The large number of stations close to the summit allowed us to locate all events in both families using a time reversal location method. The method involves taking the seismic signal, reversing it in time, and using it as a seismic source in a numerical seismic wave simulator where the reversed signals propagate through the numerical model, interfere constructively and destructively, and focus on the original source location. The source location is the computational cell with the largest displacement magnitude at the time of maximum energy current density inside the grid. Before we located the two LP families we first applied the method to two synthetic data sets and found a good fit between the time reverse location and true synthetic location for a known velocity model. The time reverse location results of the two families show a shallow seismic region close to the summit in agreement with the locations using a moment tensor full waveform inversion method and a cross-correlation location method.

Description: Published

Description: 452-462

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: Active volcanoes generate sonic and infrasonic signals, whose investigation provides useful information for both monitoring purposes and the study of the dynamics of explosive phenomena. At Mt. Etna volcano (Italy), a pattern recognition system based on infrasonic waveform features has been developed. First, by a parametric power spectrum method, the features describing and characterizing the infrasound events were extracted: peak frequency and quality factor. Then, together with the peak-to-peak amplitude, these features constituted a 3-D ‘feature space’; by Density-Based Spatial Clustering of Applications with Noise algorithm (DBSCAN) three clusters were recognized inside it. After the clustering process, by using a common location method (semblance method) and additional volcanological information concerning the intensity of the explosive activity, we were able to associate each cluster to a particular source vent and/or a kind of volcanic activity. Finally, for automatic event location, clusters were used to train a model based on Support Vector Machine, calculating optimal hyperplanes able to maximize the margins of separation among the clusters. After the training phase this system automatically allows recognizing the active vent with no location algorithm and by using only a single station.

Description: Published

Description: 253-264

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: We present the first application of a time reverse location method in a volcanic setting, for a family of long-period (LP) events recorded on Mt Etna. Results are compared with locations determined using a full moment tensor grid search inversion and cross-correlation method. From 2008 June 18 to July 3, 50 broad-band seismic stations were deployed on Mt Etna, Italy, in close proximity to the summit. Two families of LP events were detected with dominant spectral peaks around 0.9 Hz. The large number of stations close to the summit allowed us to locate all events in both families using a time reversal location method. The method involves taking the seismic signal, reversing it in time, and using it as a seismic source in a numerical seismic wave simulator where the reversed signals propagate through the numerical model, interfere constructively and destructively, and focus on the original source location. The source location is the computational cell with the largest displacement magnitude at the time of maximum energy current density inside the grid. Before we located the two LP families we first applied the method to two synthetic data sets and found a good fit between the time reverse location and true synthetic location for a known velocity model. The time reverse location results of the two families show a shallow seismic region close to the summit in agreement with the locations using a moment tensor full waveform inversion method and a cross-correlation location method.

Description: In press

Description: (11)

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved