OSL chronology of Quaternary terraced deposits outcropping between Mt. Etna volcano and the Catania Plain (Sicily,southern Italy)

In this paper we tested the applicability of the Optically Stimulated Luminescence (OSL) technique through Single-Aliquot Regenerative-dose (SAR) protocol, on single grain quartz extracted from alluvial–coastal sediments. Five samples were collected from deposits belonging to a flight of seven orders of coastal–alluvial terraces outcropping in the area between Mt. Etna volcano and the Catania Plain (Sicily, southern Italy), at the front of the Sicilian fold and thrust system. After various performance tests, we obtained OSL ages ranging between 240 ± 12 and 80 ± 4 ka, consistent with the normal evolutionary model of a terraced sequence, moving from the highest to the lowest elevation. Obtained data allowed us to determine a mean uplift rate of 1.2 mm/year during the last 240 ka, mostly related to regional uplift processes coupled with sea-level changes. Moreover, terraces belonging to the two highest orders are folded, forming a large anticline. According to our results, the frontal thrust of the Sicilian chain was active between 236 and 197 ka ago, even though seismological and geodetic data suggest current activity to the back.     

Major-ion bulk deposition around an active volcano (Mt. Etna, Italy)

Bulk atmospheric deposition of major cations (Na, K, Ca, Mg) and anions (Cl, F, SO₄) were measured at 15 sites around an active volcano, Mount Etna, from 2001 to 2003. Their composition indicates several natural sources, among which deposition of plume-derived volcanogenic gas compounds is prevalent for F, Cl and S. Plume-derived acidic compounds are also responsible for the prevailing acidic composition of the samples collected on the summit of the volcano (pH in the 2.45–5.57 range). Cation species have complex origin, including deposition of plume volcanogenic ash and aerosols and soil-dust wind re-suspension of either volcanic or carbonate sedimentary rocks. Variation of the deposition rates during the March 2001–March 2003 period, coupled with previous measurements from 1997 to 2000 (Appl Geochem 16:985–1000, 2001), were compared with the variation of SO₂ flux, volcanic activity and rainfall. The deposition rate was mainly controlled by rainfall. Commonly, about 0.1–0.9% of HF, HCl and SO₂ emitted by the summit crater's plume were deposited around the volcano. We estimate that ∼2 Gg of volcanogenic sulphur were deposited over the Etnean area during the 2002–2003 flank eruption, at an average rate of ∼24 Mg day⁻¹ which is two orders of magnitude higher than that typical of quiescent degassing phases.     

Probability hazard map for future vent opening at the Campi Flegrei caldera,Italy

The Campi Flegrei caldera is a restless structure affected by general subsidence and ongoing resurgence of its central part. The persistent activity of the system and the explosive character of the volcanism lead to a very high volcanic hazard that, combined with intense urbanization, corresponds to a very high volcanic risk. One of the largest sources of uncertainty in volcanic hazard/risk assessment for Campi Flegrei is the spatial location of the future volcanic activity. This paper presents and discusses a long-term probability hazard map for vent opening in case of renewal of volcanism at the Campi Flegrei caldera, which shows the spatial conditional probability for the next vent opening, given that an eruption occurs. The map has been constructed by building a Bayesian inference scheme merging prior information and past data. The method allows both aleatory and epistemic uncertainties to be evaluated. The probability map of vent opening shows that two areas of relatively high probability are present within the active portion of the caldera, with a probability approximately doubled with respect to the rest of the caldera. The map has an immediate use in evaluating the areas of the caldera prone to the highest volcanic hazard. Furthermore, it represents an important ingredient in addressing the more general problem of quantitative volcanic hazards assessment at the Campi Flegrei caldera.     

The Biancavilla-Montalto ingnimbrite (Etna,Sicily)

Stratigraphic, granulometric, and SEM grain texture studies have been carried out on pyroclastic deposits in the Biancavilla and Montalto areas on the lower SW flank of Etna. These studies support the interpretation that the deposits were emplaced as pyroclastic flows during the final stage of the Ellittico eruptive activity (14.18±26 ka). Based on the high percentage of juvenile components and granulometric characteristics, the deposits are classified as normal ignimbrite. Four separate flow units have been distinguished based on the presence of fine-grained basal layers. A multivariate G-mode statistical analysis has been used to discriminate particle morphology populations between flow units and infer temporal changes in the nature of the eruptive processes. The initial flow (I) was emplaced at a high temperature and involved a juvenile gas phase exolved from the magma. Subsequent flows (II, III, IV) exhibit evidence for the interaction of external water.     

Debris-avalanche deposits of the Milo Lahar sequence and the opening of the Valle del Bove on Etna volcano (Italy)

Previously undescribed debris-avalanche deposits occur in two locations downslope from the open end of the Valle del Bove. These outcrops comprise unstratified, ungraded deposits of metre-scale lava blocks in a matrix of weathered and fractured lava clasts. The avalanche deposits are unconformably overlain by matrix- to clast-supported conglomerates, representing debris-flow and interbedded fluvial deposits, that constitute most of the Milo Lahar sequence. We present evidence that the Milo Lahar sequence, which crops out just at the exit of the Valle del Bove, formed during the opening and enlargement of this depression. The presence of the avalanche deposits at the base of the Milo Lahar sequence indicates that catastrophic landslides were involved in the formation of the Valle del Bove. The composition of lavas in the debris avalanche deposits is similar to that of most of the Ellittico volcanic sequence exposed along the northern wall of the Valle del Bove. Radiocarbon dates of 8400 and 5300 years BP from the base and top, respectively, of the debris-flow sequence indicate that the Milo Lahars are correlative with the exposed part of the Chiancone deposit. The basal lahars of the Chiancone, which contain lava blocks whose compositions partially overlap that of blocks in the avalanche deposits, may have formed by water concentration in the distal end of the avalanche causing transformation to debris, or alternatively by reworking of the avalanche deposit.     

Ground and marine magnetic surveys of the lower eastern flank of Etna volcano (Italy)

In 1996 and 1997, two high-resolution magnetic surveys, one on land and the other at sea, were carried out on the lower eastern flank of Mount Etna. The magnetic surveys, covering an area of about 400 km², aimed to elucidate the relationships between the main tectonic and morphologic features of this flank of Mount Etna. Major features include widespread NNW- and NNE-trending active faults and the Valle del Bove, a depression considered to be the source area of the Chiancone deposit, the largest Etnean volcaniclastic sequence. Magnetic surveys show anomalies that roughly follow the trend of active main structures. Although few magnetization measurements are available for the most representative outcrops of the lower eastern side of Mount Etna, interpretation of the anomalies defines the underground geometry of the Chiancone deposit and its relationship with volcano stratigraphic units and the underlying sedimentary rocks. In particular, a volume of about 14 km³ was ascribed to the Chiancone deposit. Such a large amount of material was likely produced by a catastrophic event, and deposited at different periods at the exit of the Valle del Bove in an area produced by the interaction, on a regional scale, of the main tectonic structures affecting this flank of the volcano.     

Cross-correlation analysis of seismic and volcanic data at Mt Etna volcano,Italy

Seismic data from the MVT-SLN sesmic station located 7 km from the summit area of Mt Etna volcano, which has been operating steadily for the last two decades, have been analysed together with the volcanic activity during the same period. Cross-correlation techniques are used to investigate possible relationships between seismic and volcanic data and to evaluate the statistical significance of the results. A number of significant correlations have been identified, showing that there is an evident relation between seismic events and flank eruptions, and a less clear relation with summit activity, which appears more linked to tremor rather than to the low-frequency events. Particularly interesting are the low-frequency events whose rate of occurrence increases, starting from 17 to 108 days, prior to the onset of the flank activity and are candidates for a useful precursor. On the other hand, a tendency towards the increase in both the duration and the occurrence rate of transients in the volcanic tremor was observed before the onset of summit eruptions. As a result of this study different stages in the volcanic activity of Mt Etna, represented by changes in the characteristics of the recorded seismic phenomena, are identified.     

Mapping and surveillance of active fissure zones on a volcano by the self-potential method, Etna, Sicily

The self-potential (S.P.) method, often employed in base-metal exploration, has successfully been applied to volcanological problems.Test surveys on the site of Mount Etna (Sicily) have revealed some positive anomalies as high as a few hundred millivolts over recent and old eruptive fissures. The S.P. technique allows one to determine the precise location of the fissures.The interpretation of the geophysical results (including S.P.), taking into account the structure of Mount Etna, has shown two important zones upon which several S.P. investigations have been carried out.Studies of the causes of these S.P. anomalies have shown the importance of the streaming potential as the principal source mechanism.     

Anomalous soil CO2 degassing in relation to faults and eruptive fissures on Mount Etna (Sicily, Italy)

 The relationships between soil gas emissions and both tectonic and volcano-tectonic structures on Mt. Etna have been studied. The investigation consisted of soil CO₂ flux measurements along traverses orthogonal to the main faults and eruptive fissures of the volcano. Anomalous levels of soil degassing were found mainly in coincidence with faults, whereas only 49% of the eruptive fissures were found to produce elevated CO₂ soil fluxes. This result suggests that only zones of strain are able to channel deep gases to the surface. According to this hypothesis, several previously unknown structures are suggested. Based on our geochemical data, new structural maps of different areas of Etna are proposed. The soil CO₂ fluxes observed in this study are higher than those measured in a 1987 study, and they are consistent with the higher level of volcanic unrest during the current study. Received: 20 March 1998 / Accepted: 17 June 1998     

Building vulnerability and seismic risk analysis in the urban area of Mt. Etna volcano (Italy)

The tectonic system of the eastern flank of Mt. Etna volcano (Sicily, Italy) is the source of most of the strongest earthquakes occurring in the area over the last 205 years. A total of 12 events with epicentre intensities ≥VIII EMS have occurred at Mt. Etna, 10 of which were located on the eastern flank. This indicates a mean recurrence time of about 20 years. This area is highly urbanised, with many villages around the volcano at altitudes up to 700 m a.s.l. The southern and eastern flanks are particularly highly populated areas, with numerous villages very close to each other. The probabilistic seismic hazard due to local faults for Mt. Etna was calculated by adopting a site approach to seismic hazard assessment. Only the site histories of local volcano-tectonic earthquakes were considered, leaving out the effects due to strong regional earthquakes that occurred in north-eastern and south-eastern Sicily. The inventory used in this application refers to residential buildings. These data were extracted from the 1991 census of the Italian National Institute of Statistics, and are grouped according to the census sections. The seismic vulnerability of the elements at risk belonging to a given building typology is described by a vulnerability index, in accordance with a damage model based on macroseismic intensities. For the estimation of economic losses due to physical damage to buildings, an integrated impact indicator was used, which is equivalent to the lost building volume. The expected annualised economic earthquake losses were evaluated both in absolute and in relative terms, and were compared with the geographical distribution of seismic hazard and with similar evaluations of losses for other regions.     

Spatial variations of seismogenic stress orientations in Sicily,south Italy

Seismogenic stress orientations are estimated in the lithosphere of Sicily by inversion of 131 local earthquake focal mechanisms (FMs) selected from the literature. An average misfit F = 14.5° between stress tensor and FMs indicates that the entire set of earthquakes is generated by a highly heterogeneous stress field. Detailed analysis of stress tensors and related earthquake misfits obtained for tens of subsets based on spatial separation of data allowed us to identify two main stress domains in the study region: (i) a compressional domain, including Etna, western Sicily and the southern Tyrrhenian sea offshore Sicily, where the σ₁ orientation roughly changes from NW–SE in the Etna area and western Sicily to NNE–SSW at the northeastern edge of the domain (Eolian Islands) and (ii) an extensional domain in northeastern Sicily between the Eolian Islands and Etna, where σ₃ is oriented analogously to previously estimated in the easterly confining extensional area of the Messina Straits. General agreement is found between this stress pattern and the GPS crustal displacement vectors reported in the most recent literature for the study region. Moreover, stress inversion enables us to locate the extensional domain more accurately than the presently available GPS data. Finally, the stress orientations estimated in the southern and western sectors of the study area (Etna and western Sicily on and offshore) match well with the displacement fields predicted for Sicily by large-scale models of plate motion, conversely a mismatch is observed in the northeastern sector (Eolian Islands and northeastern Sicily). We suggest that the joint action of Africa–Eurasia convergence and Ionian subduction trench retreat (rollback of the subducting slab) may explain the stress pattern detected in Sicily in the present study. Work for computation of a finite-element regional geodynamic model based on geophysical and geological data collected over the last few years has recently started with the purpose of quantitatively checking this hypothesis.     

The occurrence of Mt Barca flank eruption in the evolution of the NW periphery of Etna volcano (Italy)

Geological surveys, tephrostratigraphic study, and ⁴⁰Ar/³⁹Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.     

Chlorine and fluorine in tholeiitic and alkaline lavas of Etna (Sicily)

Chlorine and fluorine were measured from whole-rock samples from tholeiites (150,000 years old) to present-day hawaiites. The overall range of F content is from 240 to 985 ppm, with a slight decrease of F/Th and F/Cl ratios, from tholeiites to hawaiites. Chlorine is positively correlated with Th (Cl/Th=100) as well as K₂O and P₂O₅, as the differentiation progresses, and, increases from 220 ppm in tholeiites to 2410 ppm in mugearites. Data obtained from present-day hawaiites indicate that Cl lost by degassing during eruption is limited. However, Cl contents of some whole-rock samples are inconsistent with previous results published for melt inclusions of phenocrysts from the same historic hawaiite samples and suggesting outgassing of Cl prior to the eruption. This implies that apparent correlation between Cl and Th can be considered as the result of superimposition of several petrogenetic processes. Results point out the richness of Etnean tholeiites in chlorine compared to M.A.R. basalts. Such a character could have been inherited from the mantle source or during the mantle source melting.     

Statistical analysis of earthquake activity at Etna volcano (March 1981 eruption)

Seismic activity that preceded, accompanied, and followed the 17–23 March 1981 Etnean eruption has been statistically analyzed.On the grounds of both time evolution of seismicity and catalogue completeness, three time intervals have been defined (12 February–2 March, 12–17 March, 19–31 March) and for each of these periods both the b coefficient of theGutenberg-Richter's (1956) relationship and the E parameter of the cluster size (Shlien andToksoz, 1970) have been calculated.No significant variations were observed between the first and third periods, while lower values of bothb andE coefficients were found in the second one. These findings might indicate that changes in the seismicity features occur just before the eruption start.Small but fast variations in the stress field acting on the volcano might originate this type of seismic activity, while the importance of the tectonic control on volcanic phenomena seems to be confirmed.     

Decadal evolution of a degassing magma reservoir unravelled from fire fountains produced at Etna volcano (Italy) between 1989 and 2001

Between 1989 and 2001, five eruptions at Etna displayed a regular alternation between repose periods and episodes rich in gas, termed quasi-fire fountains and consisting of a series of Strombolian explosions sometimes leading to a fire fountain. This behaviour results from the coalescence of a foam layer trapped at the top of the reservoir which was periodically rebuilt prior to each episode (Vergniolle and Jaupart, J Geophys Res 95:2793–2809, 1990). Visual observations of fire fountains are combined with the foam dynamics to estimate the five degassing parameters characteristic of the degassing reservoir, i.e. the number of bubbles, gas volume fraction, bubble diameter, reservoir thickness and reservoir volume. The study of decadal cycles of eruptive patterns (Allard et al., Earth Sci Rev 78:85–114, 2006) suggests that the first eruption with fire fountains occurred in 1995 while the last one happened in 2001. The number of bubbles and the gas volume fraction increase smoothly from the beginning of the cycle (1995) to its end (2001). The increasing number of bubbles per cubic metre, from 0.61–20×10⁵ to 0.1–3.4×10⁹, results from cooling of the magma within the reservoir. The simultaneously decreasing bubble diameter, from 0.67–0.43 to 0.30–0.19 mm, is related to the decreasing amount of dissolved volatiles. Meanwhile, the thickness and the volume of the degassing reservoir diminish, from values typical of the magma reservoir to values characteristic of a very thin bubbly layer, marking the quasi-exhaustion of volatiles. The magma reservoir has a slender vertical shape, with a maximum thickness of 3,300–8,200 m and a radius of 240 m (Vergniolle 2008), making its detection from seismic studies difficult. Its volume, at most 0.58–1.4 km³, is in agreement with geochemical studies (0.5 km³) (Le Cloarec and Pennisi, J Volcanol Geotherm Res 108:141–155, 2001). The time evolution of both the total gas volume expelled per eruption, and the inter-eruptive gas flux results from the competition between the increasing number of bubbles and the decreasing bubble diameter. The smooth temporal evolution of the five degassing parameters also points towards bubbles being produced by a self-induced mechanism within the magma reservoir rather than by a magmatic reinjection prior to each eruption. The decadal cycles are therefore initiated by a magmatic reinjection, in agreement with a typical return time of 14–80 years (Albarède 1993). Hence, the 1995 eruption results from a fresh magma being newly emplaced while the magma from the following eruptions is progressively depleted in volatiles species until reaching a state of quasi-exhaustion in 2001. A magmatic reinjection of 0.13–0.6 km³ every few decades is sufficient to explain the expelled gas volume, including SO₂. A scenario is also proposed for the alternation between gas-rich summit eruptions and gas-poor flank eruptions which are observed during decadal cycles. The scenario proposed for Etna could also be at work at Piton de la Fournaise and Erta ’Ale volcanoes.     

Attenuation of body waves in Southeastern Sicily (Italy)

The attenuation of P- and S-waves in Southeastern Sicily was estimated by applying two different methods in time and frequency domains. We analyzed waveforms from about 290 local events (0.6≤M_L≤4.6) recorded at a three-component digital network.By applying the pulse broadening method to the first P-wave pulse, we found an average Q_p value of ca. 140. The application of the frequency decay method provided a Q_p value of ca. 120, in the low-frequency band (3-9 Hz). Conversely, in the high frequency range (16-27 Hz) the average Q_p is significantly larger (ca. 640). The frequency decay method was also applied to S-waves spectra. In the low frequency range (2-5 Hz) the estimated average Q_s is ca. 190. As for Q_p, also Q_s, in the high frequency range (16-27 Hz), is larger (ca. 700). These results evidenced a frequency dependence of both the quality factors Q_p and Q_s, as commonly observed in tectonically active zones characterized by high degree of heterogeneity.     

Characterization of shape and terminal velocity of tephra particles erupted during the 2002 eruption of Etna volcano,Italy

In this paper, we present a complete morphological characterization of the ash particles erupted on 18 December 2002 from Etna volcano, Italy. The work is based on the acquisition and processing of bidimensional digital images carried out by scanning electron microscopy (SEM) to obtain shape parameters by image analysis. We measure aspect ratio (AR), form factor (FF), compactness (CC), and rectangularity (RT) of 2065 ash particles with size between 0.026 and 1.122 mm. We evaluate the variation of these parameters as a function of the grain-size. Ash particles with a diameter of <0.125 mm vary from mostly equant to very equant, ash particles between 0.125 and 0.250 mm have an intermediate shape, and particles with diameters >0.250 mm are subelongate. We find that, on average, particles with a diameter of <0.250 mm are subrounded, particles between 0.250 and 0.50 mm are subangular, and particles >0.50 mm are angular. Using this morphological analysis and an empirical relation between the drag coefficient (C _D) and Reynolds number (R _e) of Wilson and Huang (Earth Planet Sci Lett 44:311–324, 1979), we calculate the terminal settling velocities (V _WH). The comparisons between these velocities and those calculated with the formula of Kunii and Levenspiel (Fluidization engineering. Wiley, New York, pp 97, 1969) (V _KL), which considers ash particles as spheres, show that V _KL are in average 1.28 greater than V _WH. Hence, we quantify the systematic error on the spatial distribution of the mass computed around the volcano carried out by tephra dispersal models when the particles are assumed to be spherical.     

Petrologic evidence of a complex plumbing system feeding the July–August 2001 eruption of Mt. Etna,Sicily, Italy

After the major 1991–1993 eruption, Mt. Etna resumed flank activity in July 2001 through a complex system of eruptive fissures cutting the NE and the S flanks of the volcano and feeding effusive activity, fire fountains, Strombolian and minor phreatomagmatic explosions. Throughout the eruption, magmas with different petrography and composition were erupted. The vents higher than 2,600 m a.s.l. (hereafter Upper vents, UV) erupted porphyritic, plagioclase-rich trachybasalt, typical of present-day summit and flank activity. Differently, the vents located at 2,550 and 2,100 m a.s.l. (hereafter Lower vents, LV) produced slightly more primitive trachybasalt dominated by large clinopyroxene, olivine and uncommon minerals for Etna such as amphibole, apatite and orthopyroxene and containing siliceous and cognate xenoliths. Petrologic investigations carried out on samples collected throughout the eruption provided insights into one of the most intriguing aspects of the 2001 activity, namely the coeval occurrence of distinct magmas. We interpret this evidence as the result of a complex plumbing system. It consists in two separate magma storage systems: a shallow one feeding the activity of the UV and a deeper and more complex storage related to the activity of LV. In this deep storage zone, which is thermally and compositionally zoned, the favourable conditions allow the crystallization of amphibole and the occurrence of cognate xenoliths representing wall cumulates. Throughout 2001 eruption, UV and LV magmas remain clearly distinct and ascended following different paths, ruling out the occurrence of mixing processes between them. Furthermore, integrating the 2001 eruption in the framework of summit activity occurring since 1995, we propose that the 2001 magma feeding the vents lower than 2,600 m a.s.l. is a precursor of a refilling event, which reached its peak during the 2002–2003 Etna flank eruption.     

Hazards from pyroclastic density currents at Mt. Etna (Italy)

Despite the recent recognition of Mount Etna as a periodically violently explosive volcano, the hazards from various types of pyroclastic density currents (PDCs) have until now received virtually no attention at this volcano. Large-scale pyroclastic flows last occurred during the caldera-forming Ellittico eruptions, 15–16 ka ago, and the risk of them occurring in the near future is negligible. However, minor PDCs can affect much of the summit area and portions of the upper flanks of the volcano. During the past ~ 20 years, small pyroclastic flows or base-surge-like vapor and ash clouds have occurred in at least 8 cases during summit eruptions of Etna. Four different mechanisms of PDC generation have been identified during these events: (1) collapse of pyroclastic fountains (as in 2000 and possibly in 1986); (2) phreatomagmatic explosions resulting from mixing of lava with wet rock (2006); (3) phreatomagmatic explosions resulting from mixing of lava with thick snow (2007); (4) disintegration of the unstable flanks of a lava dome-like structure growing over the rim of one of the summit craters (1999). All of these recent PDCs were of a rather minor extent (maximum runout lengths were about 1.5 km in November 2006 and March 2007) and thus they represented no threat for populated areas and human property around the volcano. Yet, events of this type pose a significant threat to the lives of people visiting the summit area of Etna, and areas in a radius of 2 km from the summit craters should be off-limits anytime an event capable of producing similar PDCs occurs. The most likely source of further PDCs in the near future is the Southeast Crater, the youngest, most active and most unstable of the four summit craters of Etna, where 6 of the 8 documented recent PDCs originated. It is likely that similar hazards exist in a number of volcanic settings elsewhere, especially at snow- or glacier-covered volcanoes and on volcano slopes strongly affected by hydrothermal alteration.