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 investigated the high frequency attenuation of S-waves in Southeastern Alps and Northern External Dinarides using waveforms from 331 earthquakes (3.0〈 Mw〈 6.5). The spectral decay parameter, k, was computed using 1345 three component high quality records, collected by the Italian Strong Motion Network (RAN) and by the Short-Period Seismometric Network of North-Eastern Italy (NEI) in the period 1976-2007. Weak motion data from 11 stations of the NEI network and strong motion data collected by 5 accelerometers of the RAN were analyzed. The k parameter was estimated in the 0-250 Km distance range, in a frequency band extending from the corner frequency of the event up to 25 or 45 Hz, using the amplitude acceleration Fourier spectra of S-waves. The observed record-to-record variability of k was modeled by applying a generalized inversion procedure, using both parametric and non-parametric approaches. Our results evidence that k is independent on earthquake size, while it shows both site and distance dependence. Stations of the NEI network present the same increase of k with epicentral distance, Re, and show values of the zero-distance k parameter, k0(S), between 0.017 and 0.053 s. For the whole region, the k increase with distance can be described through a linear model with slope dk/dRe = (1.4±0.1)x10^(-4) s/Km. Assuming an average S-wave velocity, 〈Vs〉=3.34 Km/s between 5 and 15 Km depth, we estimate an average frequency independent quality factor, 〈Qi〉=2140, for the corresponding crustal layer. The non-parametric approach evidences a weak positive concavity of the curve that describes the k increase with Re at about 90 Km distance. This result can be approximated through a piecewise linear function with slopes of 1.0x10^(-4) s/Km and 1.7x10^(-4) s/Km, in accordance with a three layers model where moving from the intermediate to the bottom layer both 〈Qi〉 and 〈Vs〉 decrease. Two regional dependences were found: data from earthquakes located westward to the NEI network evidence weaker attenuation properties, probably because of S-wave reflections from different part of the Moho discontinuity under the eastern Po Plain, at about 25-30 Km depth, while earthquakes located eastward (in western Slovenia), where the Moho deepens up to 45-50 Km, evidence a higher attenuation. Moreover, the k estimates obtained with data from earthquakes located in the area of the 1998 (Mw=5.7) and 2004 (Mw=5.2) Kobarid events are 0.017 s higher than the values predicted for the whole region, probably because of the high level of fracturing that characterizes fault zones. The comparison between measured and theoretical values of k, computed at a few stations with available S-wave velocity profiles, reveals that the major contribution to the total k0(S) is due to the sedimentary column (from surface to 800 m depth). The hard rock section contribution is limited to 0.005 s, in accordance with a maximum contribution of 0.010 s predicted by the non-parametric inversion.

Description: Published

Description: 1393-1416

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: open

Description: The number of tornillo events has recently increased at the Vulcano Island, Italy. While only 15 tornillos were recorded during 2004–2006, 584 events occurred in 2007–2008. They were located just below La Fossa Crater at depths ranging between 0.1 and 1 km b.s.l. During two intervals in 2007–2008 increases in the number of tornillos took place at the same time as temperature and geochemical anomalies were observed. The spectral content of the tornillos, generally characterized by one–two dominant spectral peaks near 6 and 10 Hz, varied over time, with changes also noted in the quality factors. The simplest source mechanism proposed for tornillos is the free eigenvibration of a fluid volume within a crack or a conduit. Based on this model, we propose a causal relationship between the temperature and geochemical anomalies and the increases in numbers of tornillos. As the amount of hydrothermal fluids increases during the anomalies, the upward flux of fluids grows. The consequent changes in the pressure, temperature and dynamics of the system of cracks and conduits result in the generation of tornillos. Based on the fluid-filled crack/conduit model, the shallow depths of the sources and the values of the quality factors, the fluid within the resonant crack/conduit was inferred to be an ash–gas or water droplet–gas mixture. Moreover, the observed variations in the wavefield can be caused by small changes in the location of the source, in the source mechanism, or in the medium in between the source and the seismic station. Finally, another peculiar feature of tornillos is the amplitude modulation that can be explained as a result of a beating phenomenon.

Description: Published

Description: 377-393

Description: 3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici

Description: JCR Journal

Description: reserved

Description: The zero-lag cross-correlation technique, used for array analysis in the hypothesis of plane waves, has been modified to allow the wave front to be circular. Synthetic tests have been performed to check the capability of the method

Description: Published

Description: 159-170

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: reserved

Description: Stress can undergo rapid temporal changes in volcanic environments, and this is particularly true during eruptions. We use two independent methods, coda wave interferometry (CWI) and shear wave splitting (SWS) analysis to track stress related wave propagation effects during the waning phase of the 2002 NE fissure eruption at Mt Etna. CWI is used to estimate temporal changes in seismic wave velocity, while SWS is employed to monitor changes in elastic anisotropy. We analyse seismic doublets, detecting temporal changes both in wave velocities and anisotropy, consistent with observed eruptive activity. In particular, syn-eruptive wave propagation changes indicate a depressurization of the system, heralding the termination of the eruption, which occurs three days later.

Description: Published

Description: 1779-1788

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

Description: JCR Journal

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: The 2009 April 6, Mw= 6.3 L’Aquila earthquake occurred within a complex system of NW–SE trending normal faults in the Abruzzi Central Apennines (Italy). We analyse the coseismic deformation as measured by 〉70 global positioning system (GPS) stations, both from continuous and survey-mode networks, providing unprecedented details for a moderate normal faulting earthquake in Italy from GPS measurements. We use rectangular, uniform-slip, dislocations embedded in an elastic, homogeneous and isotropic half-space and a constrained, non-linear optimization algorithm, to solve for the best-fitting rectangular dislocation geometry and coseismic-slip distribution. We use a bootstrap approach to investigate uncertainties in the model parameters and define confidence bounds for all the inverted parameters. The rupture occurred on a N129°E striking and 50° southwestward dipping normal fault, in agreement with geological observations of surface breaks along the Paganica fault. Our distributed slip model exhibits a zone of relatively higher slip (〉60 cm) between ∼1.5 and ∼11 km depth, along a roughly downdip, NW–SE elongated patch, confined within the fault plane inverted assuming uniform-slip. The highest slip, of the order of ∼1 m, occurred on a ∼16 km2 area located at ∼5 km depth, SE of the mainshock epicentre. The analysis of model resolution suggests that slip at depth below ∼5 km can be resolved only at a spatial scale larger than 2 km, so a finer discretization of different asperities within the main patch of coseismic-slip is not allowed by GPS data. We compute the coseismic Coulomb stress changes in the crustal volume affected by the major aftershocks, and compare the results obtained from the uniform-slip and the heterogeneous-slip models. We find that most of the large aftershocks occurred in areas of Coulomb stress increase of 0.2–13 bar and that a deepening of the slip distribution down to a depth greater than 6 km in the SE part of the fault plane, in agreement with the inverted slip model, can explain the deepest, April 7, Mw 5.3 aftershock.

Description: Published

Description: 473-489

Description: 1.9. Rete GPS nazionale

Description: 3.2. Tettonica attiva

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: Published

Description: 452-462

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

Description: JCR Journal

Description: reserved

Description: The Mt. Etna eruption of July 2001 was announced by severe seismic activity and by the opening of a 7-km-long zone of densely distributed fractures. The large amount of data collected gave a unique opportunity to study the magma migration process and to infer the position and geometry of the uprising dyke. Results from multidisciplinary approaches suggest that the observed phenomenology was the result of the rapid intrusion of a vertical dyke, oriented roughly N–S and located a few km south of the summit region. To add new constraints to the dynamics of the eruption process, in this study we determine the full seismic moment tensors of 61 earthquakes, selected among those occurring between July 12 and July 18 (Md ≥ 2.2), located in a depth ranging from 1 km above sea level (a.s.l.) to 3 km below sea level (b.s.l.). At the beginning of the seismic swarm, the dominant component of the seismic source tensor is double-couple percentage (around 65 per cent on average) statistical significant at 95 per cent confidence level and in the following hours the non-double-couple components increase at the expenses of the double-couple. Such observations are related well with the system of fractures formed just before the eruption, whereas the increasing non-double-couple components can be explained as the response of the confining rocks to the rising magma and degassing processes. The type of focal mechanisms retrieved are predominantly of normal fault type (44 per cent), strike slip (30 per cent) and thrust mechanisms (9 per cent), and outline a scenario that concurs with the stress regime induced by a dyke injection. The space–time analysis of seismic source locations and source moment tensors (1) confirms the evidence of a vertical dyke emplacement that fed the 2001 lateral eruption and (2) adds new insights to support the hypothesis of the injection of a second aborted dyke, 2 km SE from the fractures zone.

Description: Published

Description: 951–965

Description: 3.6. Fisica del vulcanismo

Description: JCR Journal

Description: reserved

Description: In this work, we present regional maps of the inverse intrinsic quality factor (Qi −1), the inverse scattering quality factor (Qs −1) and total inverse quality factor (Qt −1) for the volcanic environment of Deception Island (Antarctica). Our attenuation study is based on diffusion approximation, which permits us to obtain the attenuation coefficients for every single couple source-receiver separately. The data set used in this research is derived from an active seismic experiment using more than 5200 offshore shots (air guns) recorded at 32 onshore seismic stations and four ocean bottom seismometers. To arrive at a regional distribution of these values, we used a new mapping technique based on a Gaussian space probability function. This approach led us to create ‘2-D probabilistic maps’ of values of intrinsic and scattering seismic attenuation. The 2-D tomographic images confirm the existence of a high attenuation body below an inner bay of Deception Island. This structure, previously observed in 2-D and 3-D velocity tomography of the region, is associated with a massive magma reservoir. Magnetotelluric studies reach a similar interpretation of this strong anomaly. Additionally, we observed areas with lower attenuation effects that bear correlation with consolidated structures described in other studies and associated with the crystalline basement of the area. Our calculations of the transport mean-free path and absorption length for intrinsic attenuation gave respective values of ≈950 m and 5 km, which are lower than the values obtained in tectonic regions or volcanic areas such as Tenerife Island. However, as observed in other volcanic regions, our results indicate that scattering effects dominate strongly over the intrinsic attenuation.

Description: This work has been partially supported by the Spanish project Ephestos, CGL2011–29499-C02–01, by the EU project EC-FP7 MEDiterranean SUpersite Volcanoes (MED-SUV), by the Basque Government researcher training program BFI09.277 and by the Regional project ‘Grupo de Investigaci´on en Geof´ısica y Sismolog´ıa de la Junta de Andaluc´ıa, RNM104.’ Edoardo del Pezzo was partly supported by DPC-INGV projects UNREST SPEED and V2 (Precursori).

Description: Published

Description: 1957-1969

Description: 3.1. Fisica dei terremoti

Description: 3.6. Fisica del vulcanismo

Description: JCR Journal

Description: restricted

Description: Post-seismic relaxation is known to occur after large or moderate earthquakes, on time scales ranging from days to years or even decades. In general, long-term deformation following seismic events has been detected by means of standard geodetic measurements, although seismic instruments are only used to estimate short timescale transient processes. Albeit inertial seismic sensors are also sensitive to rotation around their sensitive axes, the recording of very slow inclination of the ground surface at their standard output channels is practically impossible, because of their design characteristics. However, modern force-balance, broad- band seismometers provide the possibility to detect and measure slow surface inclination, through the analysis of the mass position signal. This output channel represents the integral of the broad-band velocity and is generally considered only for state-of-health diagnostics. In fact, the analysis of mass position data recorded at the time of the 2009 April 6, L’Aquila (MW = 6.3) earthquake, by a closely located STS-2 seismometer, evidenced the occurrence of a very low frequency signal, starting right at the time of the seismic event. This waveform is only visible on the horizontal components and is not related to the usual drift coupled with the temperature changes. This analysis suggests that the observed signal is to be ascribed to slowly developing ground inclination at the station site, caused by post-seismic relaxation following the main shock. The observed tilt reached 1.7 × 10−5 rad in about 2 months. This estimate is in very good agreement with the geodetic observations, giving comparable tilt magnitude and direction at the same site. This study represents the first seismic analysis ever for the mass position signal, suggesting useful applications for usually neglected data.

Description: Published

Description: 1717-1724

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: restricted

Description: Published

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

Description: 5.8. TTC - Biblioteche ed editoria

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

Description: In this paper we investigate nature and properties of narrow-band, transient seismic signals observed by a temporary array deployed in the Val Tiberina area (central Apennines, Italy). These signals are characterized by spindle-shaped, harmonic waveforms with no clear S-wave arrivals. The first portion of the seismograms exhibits a main frequency peak centred at 4.5 Hz, while the spectrum of the slowly decaying coda is peaked at about 2 Hz. Events discrimination is performed using a matched-filtering technique, resulting in a set of 2466 detections spanning the 2010 January–March time interval. From a plane-wave-fitting procedure, we estimate the kinematic properties of signals pertaining to a cluster of similar events. The repetition of measurements over a large number of precisely aligned seismograms allows for obtaining a robust statistics of horizontal slownesses and propagation azimuths associated with the early portion of the waveforms. The P-wave arrival exhibits horizontal slownesses around 0.1 s km−1, thus suggesting waves impinging at the array almost vertically. Separately, we use traveltimes measured at a sparse network to derive independent constraints on epicentral location. Ray parameters and azimuths are calibrated using slowness measurements from a local, well-located earthquake. After this correction, the joint solution from traveltime inversion and array analysis indicates a source region spanning the 1–3 km depth interval. Considerations related to the source depth and energy, and the occurrence rate which is not related to the daily and weekly working cycles, play against a surface, artificial source. Instead, the close resemblance of these signals to those commonly observed in volcanic environments suggest a source mechanism related to the resonance of a fluid–filled fracture, likely associated with instabilities in the flux of pressurized CO2.

Description: Published

Description: 918-928

Description: 1.1. TTC - Monitoraggio sismico del territorio nazionale

Description: JCR Journal

Description: reserved

Description: The peculiar source characteristics of long-period seismic events (time persistency of the source, low-frequency peaks in the source spectrum, absence of high-frequency radiation) prevent the formation of a definite high-frequency coda in the seismograms. In contrast, this is well formed in volcano–tectonic quakes. For this reason, the widely used duration magnitude scale that is based on the proportionality between the energy and the coda duration cannot be used for long-period estimation. In observatory practice, the long-period magnitude is sometimes estimated using the same duration magnitude scale, leading to confusing results. In this report, we show a new method to estimate the magnitude of long-period events that generally occur for volcanoes, with some application examples from data for Mt Etna (Italy), Colima Volcano (Mexico) and Campi Flegrei (Italy).

Description: Published

Description: 911-919

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: restricted

Description: The complex volcanic system of Tenerife Island is known to have a highly heterogeneous character, as recently confirmed by velocity tomography.We present new information derived from intrinsic quality factor inverse maps (Qi −1), scattering quality factor inverse maps (Qs −1) and total quality factor inverse maps (Qt −1) obtained for the same region. The data set used in this work is the result of the analysis of an active seismic experiment carried out, using offshore shots (air guns) recorded at over 85 onshore seismic stations. The estimates of the attenuation parameters are based on the assumption that the seismogram energy envelopes are determined by seismic energy diffusion processes occurring inside the island. Diffusion model parameters, proportional to Qi −1 and to Qs −1, are estimated from the inversion of the energy envelopes for any source–receiver couple. They are then weighted with a new graphical approach based on a Gaussian space probability function, which allowed us to create ‘2-D probabilistic maps’ representing the space distribution of the attenuation parameters. The 2-D images obtained reveal the existence of a zone in the centre of the island characterized by the lowest attenuation effects. This effect is interpreted as highly rigid and cooled rocks. This low-attenuation region is bordered by zones of high attenuation, associated with the recent historical volcanic activity. We calculate the transport mean free path obtaining a value of around 4 km for the frequency range 6–12 Hz. This result is two orders of magnitude smaller than values calculated for the crust of the Earth. An absorption length between 10 and 14 km is associated with the average intrinsic attenuation parameter. These values, while small in the context of tectonic regions, are greater than those obtained in volcanic regions such as Vesuvius or Merapi. Such differences may be explained by the magnitude of the region of study, over three times larger than the aforementioned study areas. This also implies deeper sampling of the crust, which is evidenced by a change in the values of seismic attenuation. One important observation is that scattering attenuation dominates over the intrinsic effects, Qi being at least twice the value of Qs.

Description: This work has been partially supported by the Spanish project Ephestos, CGL2011-29499-C02-01, by the EU project EC-FP7 MEDiterranean SUpersite Volcanoes (MED-SUV), by the Basque Government researcher training program BFI09.277 and by the Regional project ‘Grupo de Investigaci´on en Geof´ısica y Sismolog´ıa de la Junta de Andaluc´ıa, RNM104’. EdP has been partly supported by DPC-INGV projects UNREST SPEED and V2 (Precursori).

Description: Published

Description: 1942-1956

Description: 3.1. Fisica dei terremoti

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: restricted

Description: The identification of a source model for the catastrophic 1908 December 28 Messina earth- quake (Mw = 7.2) has been the subject of many papers in the last decades. Several authors proposed different models on the basis of seismological, macroseismic and geodetic data sets; among these models, remarkable differences exist with regard to almost all parameters. We selected a subset of six models among those most cited in literature and used them to model the post-seismic sea level variation recorded at the tide gauge station of Messina (until 1923), to attempt an independent discrimination among them. For each model, we assumed a simple rheological structure and carried out a direct-search inversion of upper crust thickness and lower crust viscosity to fit the post-seismic sea level signal. This approach enabled us to iden- tify a class of fault geometries which is consistent with the post-seismic signal at the Messina tide gauge and with the known structural and rheological features of the Messina strait

Description: Published

Description: 611-622

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: open

Description: We investigated the relationship between volcano-seismic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2006, and the dynamics of the hydrothermal system. During the period of study, three episodes of increasing numbers of volcano-seismic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Three classes of seismic events (long period, high frequency and monochromatic events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5–1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system.

Description: Published

Description: 803-816

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

Description: After an earthquake, rapid, real-time assessment of hazards such as ground shaking and tsunami potential is important for early warning and emergency response. Tsunami potential depends on sea floor displacement, which is related to the length, L, width, W, mean slip, D, and depth, z, of earthquake rupture. Currently, the primary discriminant for tsunami potential is the centroid-moment tensor magnitude, MwCMT, representing the seismic potency LWD, and estimated through an indirect, inversion procedure. The obtained MwCMT and the implied LWD value vary with the depth of faulting, assumed earth model and other factors, and is only available 30 min or more after an earthquake. The use of more direct procedures for hazard assessment, when available, could avoid these problems and aid in effective early warning. Here we present a direct procedure for rapid assessment of earthquake tsunami potential using two, simple measures on P-wave seismograms – the dominant period on the velocity records, Td, and the likelihood that the high-frequency, apparent rupture-duration, T0, exceeds 50-55 sec. T0 can be related to the critical parameters L and z, while Td may be related to W, D or z. For a set of recent, large earthquakes, we show that the period-duration product TdT0 gives more information on tsunami impact and size than MwCMT and other currently used discriminants. All discriminants have difficulty in assessing the tsunami potential for oceanic strike-slip and back-arc or upper-plate, intraplate earthquake types. Our analysis and results suggest that tsunami potential is not directly related to the potency LWD from the “seismic” faulting model, as is assumed with the use of the MwCMT discriminant. Instead, knowledge of rupture length, L, and depth, z, alone can constrain well the tsunami potential of an earthquake, with explicit determination of fault width, W, and slip, D, being of secondary importance. With available real-time seismogram data, rapid calculation of the direct, period- duration discriminant can be completed within 6-10 min after an earthquake occurs and thus can aid in effective and reliable tsunami early warning.

Description: In press

Description: 1.1. TTC - Monitoraggio sismico del territorio nazionale

Description: JCR Journal

Description: open

Description: Many observations made on volcanic areas showed that a non-uniform distribution of coda wave energy is present between regions with differing geological structures. This observation was carried out in La Reunion and interpreted by Aki and Ferrazzini as due to a trapping process of the scattered waves in the volcanic cone, named by these authors as ‘coda localization’. The same properties were found in Mt Vesuvius examining codas from shots fired for active tomography. In this paper, we experimentally check whether the scattered wave field produced by local VT earthquakes is inhomogeneous as found for codas from shots in the same volcano. We examine the Log plot of S-wave direct amplitude normalized for the coda amplitude at a given lapse time (coda normalization) for all the recording stations and the site transfer functions calculated for the same stations using both the direct S-wave spectra and the coda wave spectra. Results show that the Log normalized amplitude increases with distance, the opposite of what commonly observed in non-volcanic zones. This effect may be caused by an effective inhomogeneity of the scattering wave field or, in different words, by a total-Q increasing with depth.

Description: Published

Description: 926–934

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: reserved

Description: The availability of continuous GPS measurements during or soon after significant (Mw 〉 6) seismic events is important to record the coseismic displacements, the initial postseismic evolution and to evaluate their relative contribution to the overall crustal deformation and total moment release (both seismic and aseismic). Here we present the result of the analysis of continuously operating GPS permanent stations already active or rapidly deployed after the Mw 6.3 2009 April 6th L'Aquila earthquake. In contrast to the observations made for previous earthquakes in Italy, our observations capture the 2009 mainshock allowing an improved temporal resolution on the early postseismic deformation. In order to better define initial postseismic displacements and investigate sub-daily station motions we calculate epoch-by-epoch (0.1 - 30 sec) position time series with the new "carrier range" data type based on the JPL GIPSY-OASIS package. This new approach is based on the calibration of carrier phase data of each station using estimates of one-way carrier phase biases from an ambiguity-fixed network of ~3,500 stations worldwide [see Blewitt, Bertiger and Weiss, 2009 Fall AGU Meeting]. Carrier range data (a precise pseudorange data type) were constructed for GPS stations in the epicentral area, and were processed without carrier phase bias estimation. Time-dependent postseismic displacements were then modeled with a logarithmic time-dependent function. Since postseismic deformation begins immediately after the mainshock and is large within the first day following the mainshock, the actual estimate of amount of coseismic deformation depends upon the temporal character of the deformation and the availability of high-rate GPS time series immediately after the mainshock. The results of our analysis are then used to characterize the characteristics of the initial postseismic evolution after the 2009 mainshock and to investigate the time-dependent distribution of afterslip on the fault.

Description: Unpublished

Description: San Francisco, USA

Description: 1.9. Rete GPS nazionale

Description: open