INGV Osservatorio Vesuviano

Seismic data recorded in the 2–30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source‐centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and ∼160 m northwest of the active vents. The source mechanisms include both moment‐tensor and single‐force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio ν = 1/3, a value appropriate for hot rock. Both imaged cracks dip ∼60° to the northwest and strike northeast–southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are ∼200 m 3 for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10 8 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston‐like rise of a slug of gas in the conduit.

Recent examinations of the chemical fluxes through convergent plate margins suggest the existence of significant mass imbalances for many key species: only 20–30% of the to‐the‐trench inventory of large‐ion lithophile elements (LILE) can be accounted for by the magmatic outputs of volcanic arcs. Active serpentinite mud volcanism in the shallow forearc region of the Mariana convergent margin presents a unique opportunity to study a new outflux: the products of shallow‐level exchanges between the upper mantle and slab‐derived fluids. ODP Leg 125 recovered serpentinized harzburgites and dunites from three sites on the crests and flanks of the active Conical Seamount. These serpentinites have U‐shaped rare earth element (REE) patterns, resembling those of boninites. U, Th, and the high field strength elements (HFSE) are highly depleted and vary in concentration by up to 2 orders of magnitude. The low U contents and positive Eu anomalies indicate that fluids from the subducting Pacific slab were probably reducing in nature. On the basis of substantial enrichments of fluid‐mobile elements in serpentinized peridotites, we calculated very large slab inventory depletions of B (79%), Cs (32%), Li (18%), As (17%), and Sb (12%). Such highly enriched serpentinized peridotites dragged down to depths of arc magma generation may represent an unexplored reservoir that could help balance the input‐output deficit of these elements as observed by Plank and Langmuir (1993, 1998) and others. Surprisingly, many species thought to be mobile in fluids, such as U, Ba, Rb, and to a lesser extent Sr and Pb, are not enriched in the rocks relative to the depleted mantle peridotites, and we estimate that only 1–2% of these elements leave the subducting slabs at depths of 10 to 40 km. Enrichments of these elements in volcanic front and behind‐the‐front arc lavas point to changes in slab fluid composition at greater depths.

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Major and trace element, Sr–Nd–Pb isotope and mineral chemical at least two distinct geochemical components: a mid-ocean ridge basalt (MORB) source, relatively depleted component, and adata are presented for mafic and felsic volcanic rocks from the island of Pantelleria. The mafic rocks, mostly basalts, range from hy- HIMU-like enriched component. A further enriched component, normative transitional basalts, through alkali basalts, to basanites. similar to the Enriched Mantle 1 (EM 1) component, could also Clinopyroxene in the mafic rocks varies in composition from Al, Ti- have been involved. According to geophysical data, the lithosphere poor diopside to Al, Ti-rich augite. These two populations can be is thinned beneath the island, and the asthenospheric mantle rises present simultaneously in the same sample and even in the same to a depth of 60 km. Rare earth element data require residual garnet crystal, suggesting polybaric fractionation in the pressure range 0–4 in the source and constrain the melting process to a depth of kbar, or mixing between basaltic magmas with different degrees of 70–80 km. The petrological and geochemical data suggest that the alkalinity. On the basis of their major and trace element and mafic magmas are generated within the asthenospheric mantle, from Sr–Nd–Pb isotope composition and age of eruption, two groups of a deep plume bringing the HIMU–EM 1 isotopic and trace element basalts are distinguished: a high TiO2–P2O5 group, erupted before signatures. Interaction of these OIB-like magmas with the shallower 50 ka BP, and a low TiO2–P2O5 group, erupted after 50 ka BP, asthenospheric mantle, providing a depleted MORB signature, separated by a caldera collapse. The felsic volcanic rocks have gives rise to magmas with the observed isotopic and geochemical

A multiparticle thermofluid dynamic model was developed to assess the effect of a range of particle size on the transient two‐dimensional behavior of collapsing columns and associated pyroclastic flows. The model accounts for full mechanical and thermal nonequilibrium conditions between a continuous gas phase and N solid particulate phases, each characterized by specific physical parameters and properties. The dynamics of the process were simulated by adopting a large eddy simulation approach able to resolve the large‐scale features of the flow and by parametrizing the subgrid gas turbulence. Viscous and interphase effects were expressed in terms of Newtonian stress tensors and gas‐particle and particle‐particle coefficients, respectively. Numerical simulations were carried out by using different grain‐size distributions of the mixture at the vent, constitutive equations, and numerical resolutions. Dispersal dynamics describe the formation of the vertical jet, the column collapse and the building of the pyroclastic fountain, the generation of radially spreading pyroclastic flows, and the development of thermal convective instabilities from the fountain and the flow. The results highlight the importance of the multiparticle formulation of the model and describe several mechanical and thermal nonequilibrium effects. Finer particles tend to follow the hot ascending gas, mainly in the phoenix column and, secondarily, in the convective plume above the fountain. Coarser particles tend to segregate mainly along the ground both in the proximal area close to the crater rim because of the recycling of material from the fountain and in the distal area, because of the loss of radial momentum. As a result, pyroclastic flows were described as formed by a dilute fine‐rich suspension current overlying a dense underflow rich in coarse particles from the proximal region of the flow. Nonequilibrium effects between particles of different sizes appear to be controlled by particle‐particle collisions in the basal layer of the flow, whereas particle dispersal in the suspension current and ascending plumes is determined by the gas‐particle drag. Simulations performed with a different grain‐size distribution at the vent indicate that a fine‐grained mixture produces a thicker and more mobile current, a larger runout distance, and a greater elutriated mass than the coarse‐grained mixture.

Phlegrean Fields is an active and densely populated caldera near Naples (Italy). Two major unrest episodes characterized its recent history, each leading to remarkable ground uplift and followed by slow subsidence. Fumaroles near the caldera centre underwent important chemical changes during these volcanic crises. Based on published data we show that a correlation exits between ground displacement and gas composition. Numerical modelling of hydrothermal circulation shows that periods of enhanced fluid injection at the base of the hydrothermal system, are consistent with the observed chemical variations. The model predicts an average increase in pore pressure and temperature within the system, suggesting potential effects on ground deformation. Literature data and simulation results show that periods of intense magmatic degassing could explain most of the features characterizing recent bradyseismic crises and should be considered a potential trigger for the unrest at Phlegrean Fields, as well as at other calderas in the world.

Abstract We present a new strategy for reliable automatic classification of local seismic signals and volcano-tectonic earthquakes (VT). The method is based on a supervised neural network in which a new approach for feature extraction from short period seismic signals is applied. To reduce the number of records required for the analysis we set up a specialized neural classifier, able to distinguish two classes of signals, for each of the selected stations. The neural network architecture is a multilayer perceptron (MLP) with a single hidden layer. Spectral features of the signals and the parameterized attributes of their waveform have been used as input for this network. Feature extraction is done by using both the linear predictor coding technique for computing the spectrograms, and a function of the amplitude for characterizing waveforms. Compared to strategies that use only spectral signatures, the inclusion of properly normalized amplitude features improves the performance of the classifiers, and allows the network to better generalize. To train the MLP network we compared the performance of the quasi-Newton algorithm with the scaled conjugate gradient method. We found that the scaled conjugate gradient approach is the faster of the two, with quite equally good performance. Our method was tested on a dataset recorded by four selected stations of the Mt. Vesuvius monitoring network, for the discrimination of low magnitude VT events and transient signals caused by either artificial (quarry blasts, underwater explosions) and natural (thunder) sources. In this test application we obtained 100 % correct classification for one of the possible pairs of signal types (VT versus quarry blasts). Because this method was developed independently of this particular discrimination task, it can be applied to a broad range of other applications.

Field measurements conducted on several active volcanoes in Italy, the Lesser Antilles, and Indonesia demonstrate the common Occurrence of diffuse soil gas emanations from the volcanic piles, at distances from active craters or fumarolic zones. These emanations consist essentially of carbon dioxide and rare gases and their genetic link with crater fumaroles and/or magma degassing at depth can be verified both chemically and isotopically. We emphasize here the potential use of these fluids for continuous volcano monitoring and eruption forecasting.

Abstract Campi Flegrei is an active volcanic area situated in the Campanian Plain (Italy) and dominated by a resurgent caldera. The great majority of past eruptions have been explosive, variable in magnitude, intensity, and in their vent locations. In this hazard assessment study we present a probabilistic analysis using a variety of volcanological data sets to map the background spatial probability of vent opening conditional on the occurrence of an event in the foreseeable future. The analysis focuses on the reconstruction of the location of past eruptive vents in the last 15 ka, including the distribution of faults and surface fractures as being representative of areas of crustal weakness. One of our key objectives was to incorporate some of the main sources of epistemic uncertainty about the volcanic system through a structured expert elicitation, thereby quantifying uncertainties for certain important model parameters and allowing outcomes from different expert weighting models to be evaluated. Results indicate that past vent locations are the most informative factors governing the probabilities of vent opening, followed by the locations of faults and then fractures. Our vent opening probability maps highlight the presence of a sizeable region in the central eastern part of the caldera where the likelihood of new vent opening per kilometer squared is about 6 times higher than the baseline value for the whole caldera. While these probability values have substantial uncertainties associated with them, our findings provide a rational basis for hazard mapping of the next eruption at Campi Flegrei caldera.

We study the statistical properties of time distribution of seismicity in California by means of a new method of analysis, the diffusion entropy. We find that the distribution of time intervals between a large earthquake (the main shock of a given seismic sequence) and the next one does not obey Poisson statistics, as assumed by the current models. We prove that this distribution is an inverse power law with an exponent mu=2.06+/-0.01. We propose the long-range model, reproducing the main properties of the diffusion entropy and describing the seismic triggering mechanisms induced by large earthquakes.

The Somma–Vesuvius volcanic complex and surroundings are characterized by topographic relief of over 1000 m and strong 3-D structural variations. This complexity has to be taken into account when monitoring the background volcano seismicity in order to obtain reliable estimates of the absolute epicentres, depths and focal mechanisms for events beneath the volcano. We have developed a 3-D P-wave velocity model for Vesuvius by interpolation of 2-D velocity sections obtained from non-linear tomographic inversion of the Tomoves 1994 and 1996 active seismic experiment data. The comparison of predicted and observed 3-D traveltime data from active and passive seismic data validate the 3-D interpolated model. We have relocated about 400 natural seismic events from 1989 to 1998 under Vesuvius using the new interpolated 3-D model with two different VP/VS ratios and a global search, 3-D location method. The solution quality, station residuals and hypocentre distribution for these 3-D locations have been compared with those for a representative layered model. A relatively high VP/VS ratio of 1.90 has been obtained. The highest-quality set of locations using the new 3-D model falls in a depth range of about 1–3.5 km below sea level, significantly shallower than the 2–6 km event depths determined in previous studies. The events are concentrated in the upper 2 km of the Mesozoic carbonate basement underlying the Somma–Vesuvius complex. The first-motion mechanisms for a subset of these events, although highly variable, give a weak indication of predominantly N–S to near-vertical directions for the tension axes, and ESE–WNW near-vertical directions for the compression axes.

Campi Flegrei (Phlegrean Fields) is a Holocene caldera located west of the city of Naples in an area of regional extension [ Finetti and Morelli, 1974; Scandone, 1979]. The erupted products range in composition from K basalts to alkali trachyte and phonolite. The complex has been active since at least 47,000 yr B.P. [ Capaldi et al., 1985], and it is surrounded by three other quaternary volcanic centers.

Abstract The properties of the tremor wave field at Stromboli are analyzed using data from small-aperture arrays of short-period seismometers deployed on the north flank of the volcano. The seismometers are configured in two semi-circular arrays with radii of 60 and 150 m and a linear array with length of 600 m. The data are analyzed using a spatiotemporal correlation technique specifically designed for the study of the stationary stochastic wave field of Rayleigh and Love waves generated by volcanic activity and by scattering sources distributed within the island. The correlation coefficients derived as a function of frequency for the three components of motion clearly define the dispersion characteristics for both Rayleigh and Love waves. Love and Rayleigh waves contribute 70% and 30%, respectively, of the surface-wave power. The phase velocities of Rayleigh waves range from 1000 m/sec at 2 Hz to 350 m/sec at 9 Hz, and those for Love waves range from 800 to 400 m/sec over the same frequency band. These velocities are similar to those measured near Puu Oo on the east rift of Kilauea Volcano, Hawaii, although the dispersion characteristics of Rayleigh waves at Stromboli show a stronger dependence on frequency. Such low velocities are consistent with values expected for densely cracked solidified basalt. The dispersion curves are inverted for a velocity model beneath the arrays, assuming those dispersions represent the fundamental modes of Rayleigh and Love waves.

Combining tectonics, with seismological and geochemical data, we reconstruct the deformation history of the presently narrow Calabrian slab and the path of mantle circulation during the last 10 Ma. We show that during the slab deformation the mantle laterally flowed inside the back arc region permitting its retrograde motion and giving a seismological and volcanological record after 1–2 myr.

The peculiarity of the quiescent La Fossa volcano is the occurrence of “crises” characterized by strong increases of fumarole T and output and by chemical changes indicative of an increasing input of magmatic fluids. Several surveys carried out during a new “crisis” began in November 2004 indicate that the total diffuse CO 2 emission for the crater area increases by one order of magnitude during crises (up to 1600 ton·d −1 in December 2005). Concern exists on the possibility that these crises be related to an unrest process leading to eruption. The repetition along decades of the same gas compositional variations during crises, their temporal coincidence with increases of the local shallow seismicity, and the lack of any significant ground motion, rather suggest that they correspond to moments of increasing volatile release from a stationary magma system.

Carbon dioxide (CO 2 ) diffuse degassing structures (DDS) at Furnas volcano (São Miguel Island, Azores) are mostly associated with the main fumarolic fields, evidence that CO 2 soil degassing is the surface expression of rising steam from the hydrothermal system. Locations with anomalous CO 2 flux are mainly controlled by tectonic structures oriented WNW–ESE and NW–SE and by the geomorphology of the volcano, as evidenced by several DDS located in depressed areas associated with crater margins. Hydrothermal soil CO 2 emissions in Furnas volcano are estimated to be ∼968 t d −1 . Discrimination between biogenic and hydrothermal CO 2 was determined using a statistical approach and the carbon isotope composition of the CO 2 efflux. Different sampling densities were used to evaluate uncertainty in the estimation of the total CO 2 flux and showed that a low density of points may not be adequate to quantify soil emanations from a relatively small DDS. Thermal energy release associated with diffuse degassing at Furnas caldera is about 118 MW (from an area of ∼4.8 km 2 ) based on the H 2 O/CO 2 ratio in fumarolic gas. The DDS also affect Furnas and Ribeira Quente villages, which are located inside the caldera and in the south flank of the volcano, respectively. At these sites, 58% and 98% of the houses are built over hydrothermal CO 2 emanations, and the populations are at risk due to potential high concentrations of CO 2 accumulating inside the dwellings.

During September–October 1997, 21 three‐component broadband seismometers were deployed on Stromboli Volcano at radial distances of 0.3–2.2 km from the active crater to investigate the source mechanisms of Strombolian explosions. In the 2–50 s band, the very‐long period (VLP) signals associated with explosions are consistent with two stationary sources repeatedly activated in time. VLP particle motions are essentially linear and analyses of semblance and particle motions are consistent with a source centroid offset 300 m beneath and 300 m northwest of the active vents. Similar VLP waveforms are observed at all 21 stations, indicating that the seismograms are source‐dominated. The VLP ground displacement response to each explosion may be qualitatively interpreted as: (1) pressurization of the conduit associated with the ascent of a slug of gas; (2) depressurization of the conduit in response to mass withdrawal during the eruption; and (3) repressurization of the conduit associated with the replenishment of the source with fluid.

Super-eruptions, orders of magnitude larger than eruptions experienced in historic times, have devastated wide areas by pyroclastic flows, covered continent-size areas by ash fallout, and injected large quantities of aerosols into the stratosphere affecting global climate. The Youngest Toba Tuff (YTT) is the largest known super-eruption in the Quaternary. Here we reconstructed the ultra-distal volcanic ash dispersal during this super-eruption using a computational ash dispersal model, which provides insights into the eruption dynamics and the impact of the event. The method uses a 3D time-dependent tephra dispersion model, a set of wind fields, and several tens of thickness measurements of the YTT tephra deposit. Results reveal that the YTT eruption dispersed ~8600 km3 (~3800 km3 dense rock equivalent, DRE) of ash, covering ~40 million km2 with more than 5 mm of ash. These new fallout volume estimations indicate that the total volume of the material erupted (including the massive pyroclastic density current, 1500 km3 DRE, deposits on Sumatra) was ~5300 km3 DRE. Simulation results indicate that the eruption had a very large mass flow rate and that the umbrella cloud, associated with the eruption plume, spread as an enormous gravity current around the neutral buoyancy level. The YTT tephra forms a key chronostratigraphic marker in the sedimentary sequences, and is particularly useful for constraining the age of the palaeoenvironmental and archaeological records, and synchronizing these archives to investigate temporal relationships. These new constraints on the extent of the YTT deposit are therefore particularly useful for cryptotephra studies that aim to find non-visible tephra layers for these chronological purposes. This method used to constrain volcanological parameters of eruptions in the past provides insights into the dispersal processes, and allows the amount of volatiles released to be estimated which is crucial to assessing the impact of such events.

Scattered waves observed at the seismographs of the National Italy's seismic network have been used to investigate the intrinsic dissipation and scattering properties of the lithosphere under the Southern Apennines, Italy. First, we investigate the coda-Q properties, then we apply the MLTW analysis in the hypothesis of velocity and scattering coefficient constant with depth, and finally we interpret these results with the aid of numerical simulations in a medium with depth dependent velocity and scattering coefficient.

Abstract Hazardous effects of tephra fallout on Montserrat include roof collapse, aviation threats, health hazards from respirable crystalline silica, crop pollution, road safety and lahar generation. An advection-diffusion model was developed to investigate tephra dispersal from dome collapses and Vulcanian explosions, which generated most of the fallout tephra during the 1995-1999 eruptive period of Soufrière Hills Volcano. Wind field, atmospheric diffusion, gravity settling, aggregation and elutriation processes are considered. Computed isomass maps compare well with field observations and require aggregation of fine ash for good agreement. Probability maps were also compiled. Individual probability maps (for individual dome collapses and Vulcanian explosions) are based on the statistics of wind profiles and show that fallout tephra generated by individual eruptive events on a Montserrat scale do not cause serious damage in any area on Montserrat. Cumulative probability maps (for a given scenario of activity) are generated by sampling statistical distributions of wind profiles and eruptive events over an extended period of time. They show that persistent tephra fallout can accumulate enough material to cause roof collapses and serious damage to vegetation in the SW part of the island, and minor damage to vegetation in the north, as also confirmed by field data.