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: We studied the ongoing tectonics of the region extending between the Aeolian Arc and the Ionian Sea offshore, including the southern Calabria and the north-eastern Sicily, through an in-depth analysis of seismological data. For this area, recent studies have shown a very complex tectonic framework, fragmented into crustal blocks separated by seismically active belts and characterized by neighbouring collisional, subduction and active volcanic (Etna and Aeolian Arc) domains. Contraction affects mainly the western sector with an E-W oriented compressive belt extending from the Aeolian archipelago to the Ustica Island. Conversely, the eastern sector (i.e. NE Sicily and western Calabria) is dominated by Late Quaternary extensional deformation. The definition of the seismogenic sources in this area is a difficult task and a matter of intense debate, mainly because morphological expressions of the faults are not evident, since the terrains traverse make it more difficult to keep track of faulting activity, and because there are difficulties in recognizing geological formations and tectonic structures at the great depth reaching the Tyrrhenian and Ionian Seas. In our study, a selected dataset consisting of more than 4000 small-to moderate-magnitude earthquakes (1.0≤ML≤4.8), collected in two decades by a local seismic network, were used to perform a simultaneous inversion of both 3D velocity structure and earthquake locations, in order to trace the characteristics of the faulting systems. The obtained velocity images and the foci distribution depict relevant structural features at depth. In particular, velocity anomalies and hypocentres highlight some WNW-ESE to NW-SE lineaments between the Aeolian Islands and the Ionian Sea. In addition, the fault plane solutions for the best recorded earthquakes were determined and used to resolve the current local stress fields and to characterize the faulting regime of the main seismogenic sources. The results were combined to achieve a coherent geodynamic scheme and to better characterize the active tectonics of the region.

Description: Published

Description: http://www.geoscienze2014.it/

Description: 1T. Geodinamica e interno della Terra

Description: open

Description: The adaptive mesh double-difference tomography algorithm (tomoADD) was applied to absolute and differential P, S and S–P data to determine three dimensional VP, VS and VP/VS variations and event locations in southeastern Sicily (Italy). The obtained velocity images highlight vertical and lateral heterogeneities that can be associated with different geological units and main tectonic features. In particular, the sharp velocity contrasts are consistent with previously recognized active faults, allowing us to better determine their shapes and geometries at depth. Moreover, a striking correspondence between areas ruptured by earthquakes and velocity anomalies is observed. In fact, seismicity is mostly confined in the high velocity volumes and/or along the high–low velocity boundaries at mid-crustal depths, whereas it tends to avoid regions with lower than average VP and VS values and higher VP/VS ratios.

Description: Published

Description: 74-85

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: reserved

Description: We analysed the seismic activity preceding and accompanying the onset of the 2008 Mt. Etna eruption. Since January 2008, a clear seismic evidence of a magmatic unrest of the volcano was observed. Seismicity was firstly located in the southwestern sector of the volcano, at depth ranging between 10 and 20 km, along two tectonic structures (NE-SW and NNW-SSE) usually associated with deeper magmatic recharge mechanisms (Figs. 1, 2). Afterwards, the seismicity was located along the shallower portions of the main structures of the northeastern and southern flanks of the volcano (Figs. 1, 2). On May 13, 2008 an intense seismic swarm (about 230 events in 7 hours) announced the beginning of the eruption (Fig. 1, white circles). In order to provide seismological constraints to the magmatic unrest of the volcano, 336 earthquakes recorded from January 2007 to May 2008 (magnitude greater than 1.0) were selected for stress and strain tensors computation and 3D velocity and attenuation structure determination. This in order to individuate possible stress variations caused by the activation of magmatic sources which can be well evidenced by 3D tomographic images.

Description: Published

Description: Rome

Description: 3.1. Fisica dei terremoti

Description: open

Description: The present study is mainly focused to determine the hypocentral locations and the velocity structure between the northeastern Sicily and southern Calabria, including the Aeolian Archipelago with particular regard to Stromboli volcano. The main goals are: i) to explore the differences between relative and absolute earthquake locations, ii) to identify spatial clusters in the seismicity, in order to investigate on the most active seismogenic zones and structures and iii) to recognize the presence of low velocity regions beneath the Aeolian Archipelago related to the presence of partial melting. We applied the TomoDD algorithm which use both absolute and differential arrival times to simultaneously determine event locations and Vp and Vp/Vs velocity models. 1,304 well located events have been considered during the 1994-2006 period. This code was applied to the catalog phase data, combining 11,261 P-wave and 5,751 S-wave arrival time differences. On the basis of the distribution of the events, a denser grid with respect to previous studies was used. This allowed a higher resolution of the velocity model and to find a better correspondence with the most active seismogenic zones and structures. Moreover, being the velocity structure of Stromboli poorly defined, a particular attention was devoted to this area. Tomographic images beneath this volcano revealed the presence of a low Vp and Vs (average values around 5.4 and 3.1 Km/s, respectively) elongated volume in the crust, suggesting the existence of partial melting. This volume extending until 17 km of depth, overlies a high Vp and Vs (values greater than 7.5 and 4.5 km /s, respectively) region that can be related to the Moho.

Description: Published

Description: Reykjavik

Description: open

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: The continuous volcanic and seismic activity at Mount Etna makes this volcano an important laboratory for seismological and geophysical studies. We used repeated three-dimensional tomography (4D tomography) to detect variations in elastic parameters during different volcanic cycles in the period November 2000–May 2008, that includes several flank eruptions. The use of a large number of permanent seismic stations and the abundance of local earthquakes, occurring both before and during the eruptions, guarantee consistent and high-resolution velocity models. First, we performed a tomographic inversion of the whole data set to define the 3D P-wave velocity (VP) and the structure of the P- to S-wave velocity ratio (VP/VS). A total of ca. 3,000 well constrained earthquakes (root mean square time residuals ≤ 0.4 s; horizontal and vertical hypocentral location errors ≤ 1.5 km; azimuthal gap of the stations ≤ 180°), ca. 40,000 P-wave arrivals, and ca. 9,000 S-wave arrivals were inverted to model a grid, 2 km by 2 km by 1 km spaced, with the use of SIMULPS-14 software. Then, on the basis of geophysical and geochemical observations indicating some cyclic recharging and discharging (eruptions) phases, we inverted different sub-periods to investigate time variations in the elastic parameters. The observed time changes of velocity-oriented anomalies suggest that four-dimensional tomography could provide a basis for more efficient volcano monitoring and short- and midterm eruption forecasting.

Description: Published

Description: San Diego (California)

Description: restricted

Description: We performed an in-depth analysis of the ongoing tectonics of a large sector of southern Sicily, including the Hyblean Foreland and the front of the Maghrebian Chain, as well as the Ionian Sea offshore, through the integration of seismic and GPS observations collected in the nearly two decades. In particular, a dataset consisting of more than 1100 small-to moderate-magnitude earthquakes (1.0 ≤ ML ≤ 4.6) has been used for local earthquake tomography in order to trace the characteristics of the faulting systems, and for focal mechanisms computation to resolve the current local stress field and to characterise the faulting regime of the investigated area. In addition, GPS measurements, carried out on both episodic and continuous stations, allowed us to infer the main features of the current crustal deformation pattern. Main results evidence that the Hyblean Plateau is subject to a general strike–slip faulting regime, with a maximum horizontal stress axis NW–SE to NNW–SSE oriented, in agreement with the Eurasia–Nubia direction of convergence. The Plateau is separated into two different tectonic crustal blocks by the left-lateral strike–slip Scicli–Ragusa Fault System. The western block moves in agreement with central Sicily while the eastern one accommodates part of the contraction arising from the main Eurasia–Nubia convergence. Furthermore, we provided evidences leading to consider the Hyblean–Maltese Escarpment Fault System as an active boundary characterised by a left-lateral strike–slip motion, separating the eastern block of the Plateau from the Ionian basin. All these evidences lend credit to a crustal segmentation of the southeastern Sicily.

Description: Published

Description: 137-149

Description: 1T. Geodinamica e interno della Terra

Description: JCR Journal

Description: open

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