High precision locations of multiplets on south-eastern flank of Mt. Etna (Italy): reconstruction of fault plane geometry

On 9 January 2001, a seismic swarm, located on the south-eastern flank of Mt. Etna and with nearly identical waveforms, caused some damage to Zafferana Etnea village, 3 km from the epicentral area.An analysis of the seismicity occurring in the last 8 years in this area has revealed other earthquakes with the same characteristics; some pre-empted and followed (up to a few months) the 2001 January swarm, others were recorded more than five years beforehand.Using similarity of waveforms, these earthquakes were classified into three families.The use of a multiplet-technique has allowed to obtain the spatial distribution of the events with higher precision (mean error of 10-20 m) with respect to traditional localization techniques.Mt. Etna earthquakes relocation clearly describes the geometry of the seismogenic tectonic structure; the hypocenters lie on a NE-SW oriented plane that is coincident with one of the focal planes obtained by first-arrival polarities. This alignment is also coherent with one of the main regional tectonic trends cutting the Mt. Etna area, and can be interpreted as a right-lateral strike seismic source on the south-eastern flank of Mt. Etna, distant from eruptive centres, which repeats from time to time and is able to produce strong energy releases.     

New Insights on Mt. Etna’s Crust and Relationship with the Regional Tectonic Framework from Joint Active and Passive P-Wave Seismic Tomography

In the Central Mediterranean region, the production of chemically diverse volcanic products (e.g., those from Mt. Etna and the Aeolian Islands archipelago) testifies to the complexity of the tectonic and geodynamic setting. Despite the large number of studies that have focused on this area, the relationships among volcanism, tectonics, magma ascent, and geodynamic processes remain poorly understood. We present a tomographic inversion of P-wave velocity using active and passive sources. Seismic signals were recorded using both temporary on-land and ocean bottom seismometers and data from a permanent local seismic network consisting of 267 seismic stations. Active seismic signals were generated using air gun shots mounted on the Spanish Oceanographic Vessel ‘Sarmiento de Gamboa’. Passive seismic sources were obtained from 452 local earthquakes recorded over a 4-month period. In total, 184,797 active P-phase and 11,802 passive P-phase first arrivals were inverted to provide three different velocity models. Our results include the first crustal seismic active tomography for the northern Sicily area, including the Peloritan–southern Calabria region and both the Mt. Etna and Aeolian volcanic environments. The tomographic images provide a detailed and complete regional seismotectonic framework and highlight a spatially heterogeneous tectonic regime, which is consistent with and extends the findings of previous models. One of our most significant results was a tomographic map extending to 14 km depth showing a discontinuity striking roughly NW–SE, extending from the Gulf of Patti to the Ionian Sea, south-east of Capo Taormina, corresponding to the Aeolian–Tindari–Letojanni fault system, a regional deformation belt. Moreover, for the first time, we observed a high-velocity anomaly located in the south-eastern sector of the Mt. Etna region, offshore of the Timpe area, which is compatible with the plumbing system of an ancient shield volcano located offshore of Mt. Etna.     

Fault-controlled Soil CO2 Degassing and Shallow Magma Bodies: Summit and Lower East Rift of Kilauea Volcano (Hawaii), 1997

Soil CO₂ flux measurements were carried out along traverses across mapped faults and eruptive fissures on the summit and the lower East Rift Zone of Kilauea volcano. Anomalous levels of soil degassing were found for 44 of the tectonic structures and 47 of the eruptive fissures intercepted by the surveyed profiles. This result contrasts with what was recently observed on Mt. Etna, where most of the surveyed faults were associated with anomalous soil degassing. The difference is probably related to the differences in the state of activity at the time when soil gas measurements were made: Kilauea was erupting, whereas Mt. Etna was quiescent although in a pre-eruptive stage. Unlike Mt. Etna, flank degassing on Kilauea is restricted to the tectonic and volcanic structures directly connected to the magma reservoir feeding the ongoing East Rift eruption or in areas of the Lower East Rift where other shallow, likely independent reservoirs are postulated. Anomalous soil degassing was also found in areas without surface evidence of faults, thus suggesting the possibility of previously unknown structures. Received: November 2003, revised: January 2005, accepted: January 2005     

Ground deformation modeling of flank dynamics prior to the 2002 eruption of Mt. Etna

On 22 September 2002, 1 month before the beginning of the flank eruption on the NE Rift, an M-3.7 earthquake struck the northeastern part of Mt. Etna, on the westernmost part of the Pernicana fault. In order to investigate the ground deformation pattern associated with this event, a multi-disciplinary approach is presented here. Just after the earthquake, specific GPS surveys were carried out on two small sub-networks, aimed at monitoring the eastern part of the Pernicana fault, and some baselines belonging to the northeastern EDM monitoring network of Mt. Etna were measured. The leveling route on the northeastern flank of the volcano was also surveyed. Furthermore, an investigation using SAR interferometry was performed and also the continuous tilt data recorded at a high precision sensor close to the epicenter were analyzed to constrain the coseismic deformation. The results of the geodetic surveys show a ground deformation pattern that affects the entire northeastern flank of the volcano, clearly shaped by the Pernicana fault, but too strong and wide to be related only to an M-3.7 earthquake. Leveling and DInSAR data highlight a local strong subsidence, up to 7 cm, close to the Pernicana fault. Significant displacements, up to 2 cm, were also detected on the upper part of the NE Rift and in the summit craters area, while the displacements decrease at lower altitude, suggesting that the dislocation did not continue further eastward. Three-dimensional GPS data inversions have been attempted in order to model the ground deformation source and its relationship with the volcano plumbing system. The model has also been constrained by vertical displacements measured by the leveling survey and by the deformation map obtained by SAR interferometry.     

Volcanic tremor at Mt. Etna: Inferences on magma dynamics during effusive and explosive activity

During 1999, the volcanic activity at Mt. Etna was both explosive and effusive at the summit craters: Strombolian activity, lava fountains and lava flows affected different areas of the volcano, involving three of the four summit craters. Results from analysis of the 1999 volcanic tremor features are shown at two different time scales. First, the long-term time variation of the features of the volcanic tremor (including spectral and polarization parameters), during the entire year, was compared with the evolution of the eruptive activity. This approach demonstrated the good agreement between tremor data and observed eruptive activity; the activation of different tremor sources was suggested. Then, a more refined analysis of the volcanic tremor, recorded during 14 lava fountain eruptions, was performed. In particular, a shift of the dominant frequencies towards lower values was noted which corresponds with increasing explosive activity. Similar behaviour in the frequency content has already been observed in other explosive eruptions at Mt. Etna as well as on other volcanoes. This behaviour has been explained in terms of either an increase in the tremor source dimension or a decrease in the sound speed in the magma within the conduit. These results confirm that the volcanic tremor is a powerful tool for better understanding the physical processes controlling explosive eruptions at Mt. Etna volcano.     

Lava tubes,terraces and megatumuli on the 1614–24 pahoehoe lava flow field,Mount Etna,sicily

The 1614–1624 lava flow of Mt. Etna was formed during a long-duration flank eruption involving predominantly pahoehoe flows which produced unusual surface features including mega-tumuli (here defined) and terraces. Detailed mapping of the flow units, surface features, and associated tubes reveals a complex sequence of emplacement for the field. The stair-stepped terraces appear to have been formed as a consequence of self-damming of tube-fed flows which developed «perched» ponds of lava. Surges of lava through tubes elevated sections of crusted lava at the distal ends of the flow to generate tumuli, some as high as 130 m, as a consequence of pressure via «hydrostatic head» conditions within the tube. Although pahoehoe lavas and the related features described here are atypical of Mt. Etna, they may reflect styles of eruption and lava emplacement found on volcanoes elsewhere.     

Compositionally zoned crystals and real-time degassing data reveal changes in magma transfer dynamics during the 2006 summit eruptive episodes of Mt. Etna

One of the major objectives of volcanology remains relating variations in surface monitoring signals to the magmatic processes at depth that cause these variations. We present a method that enables compositional and temporal information stored in zoning of minerals (olivine in this case) to be linked to observations of real-time degassing data. The integrated record may reveal details of the dynamics of gradual evolution of a plumbing system during eruption. We illustrate our approach using the 2006 summit eruptive episodes of Mt. Etna. We find that the history tracked by olivine crystals, and hence, most likely the magma pathways within the shallow plumbing system of Mt. Etna, differed considerably between the July and October eruptions. The compositional and temporal record preserved in the olivine zoning patterns reveal two mafic recharge events within months of each other (June and September 2006), and each of these magma supplies may have triggered the initiation of different eruptive cycles (July 14–24 and August 31–December 14). Correlation of these observations with gas monitoring data shows that the systematic rise of the CO₂/SO₂ gas values is associated with the gradual (pre- and syn-eruptive) supply of batches of gas-rich mafic magma into segments of Etna’s shallow plumbing system, where mixing with pre-existing and more evolved magma occurred.     

First historical evidence of a significant Mt. Etna eruption in 1224

The 1224 Mt. Etna eruption is a significant event both in terms of the mass of erupted materials and because it involved the lower eastern slope of the volcano, reaching down to the sea. Nevertheless, it is unknown to current historical catalogues. According to the historical sources, only two other lava flows actually reached as far as the sea: in 396 BC, just north of the present-day inhabited area of Acireale, according to the geological data alone, and in 1669, when the lava covered the south-eastern flank of Mt. Etna and damaged Catania. We present and discuss the two medieval sources that attest to the eruption of 1224 and make available the original texts. Furthermore, through the close analysis of the historical and topographic context of the Etna area, taking account of the roads and ports in the early 13th century, we have tried to single out the possible area of the lava's outlet into the sea in 1224 on historical grounds. A repeat of an eruption similar to that of 1224 would have a serious impact today as the coast is densely populated.     

Contribution des Sondes Aérologiques Motorisées à l’Etude de la Physico-Chimie des Panaches Volcaniques

Our knowledge of the physico-chemical properties of volcanic plumes remains at an embryonic state, mainly because of the prohibitive cost of measurements made from aircraft, which alone can provide samples representative of the total aerosol and gaseous emission from a volcanic source. The authors show that a small Remotely Piloted Vehicle (R.P.V.), similar in design to the one they used for some probative flights over Mt. Etna, may be of great use in this field. They can, for instance, insure frequent measurement of the SO₂/HCl ratio in volcanic emanations close to active craters at times when the eruptive vents themselves cannot be sampled directly. In June 1978, we were thus able to measure a mean ratio of SO₂/HCl (7.6) in the Mt. Etna volcanic plume. In addition, a correlation spectrometer was in operation during the airborne sampling and allowed us to obtain order of magnitude values for the Mt. Etna discharge in water vapour (170000 tons/day), sulphur dioxide (1700 t/d), hydrochloric acid (340 t/d) and hydrofluoric acid (40 t/d). Widespread use of such R.P.V’s would insure the collection of a great deal of physicochemical data from volcanic plumes, data which are presently lacking and which could extensively enhance the efficiency of chemical methods of volcano surveillance.     

Seismic activity preceding the 1983 eruption of Mt. Etna

The March–August, 1983 eruption of Mt. Etna can be considered as one of the most important in the last years.The analysis of seismic activity during the three months immediately before the eruption showed interesting variations of theb coefficient, in the frequency-magnitude relationship, that have been linked to possible changes of the stress field in the Etnean region.The eruption start was also preceded by a strong seismic crisis with epicenters mostly on the southern, eastern and southwestern flanks of the volcano, and characterized by the shallowness of most of the events (h3 km).The data analysis has led to a hypothesis on the eruption occurrence based on a model of dynamic evolution of the stress field acting on Mt. Etna.     

Volcanic Tremor at Mt. Etna,Italy, Preceding and Accompanying the Eruption of July – August, 2001

The July 17 – August 9, 2001 flank eruption of Mt. Etna was preceded and accompanied by remarkable changes in volcanic tremor. Based on the records of stations belonging to the permanent seismic network deployed on the volcano, we analyze amplitude and frequency content of the seismic signal. We find considerable changes in the volcanic tremor which mark the transition to different styles of eruptive activity, e.g., lava fountains, phreatomagmatic activity, Strombolian explosions. In particular, the frequency content of the signal decreases from 5 Hz to 3 Hz at our reference station ETF during episodes of lava fountains, and further decreases at about 2 Hz throughout phases of intense lava emission. The frequency content and the ratios of the signal amplitude allow us to distinguish three seismic sources, i.e., the peripheral dike which fed the eruption, the reservoir which fed the lava fountains, and the central conduit. Based on the analysis of the amplitude decay of the signal, we highlight the migration of the dike from a depth of ca. 5 km to about 1 km between July 10 and 12. After the onset of the effusive phase, the distribution of the amplitude decay at our stations can be interpreted as the overall result of sources located within the first half kilometer from the surface. Although on a qualitative basis, our findings shed some light on the complex feeding system of Mt. Etna, and integrate other volcanological and geophysical studies which tackle the problem of magma replenishment for the July–August, 2001 flank eruption. We conclude that volcanic tremor is fundamental in monitoring Mt. Etna, not only as a marker of the different sources which act within the volcano edifice, but also of the diverse styles of eruptive activity. An erratum to this article is available at .     

Towards an Automatic Monitoring System of Infrasonic Events at Mt. Etna: Strategies for Source Location and Modeling

Active volcanoes characterized by open conduit conditions generate sonic and infrasonic signals, whose investigation provides useful information for both monitoring purposes and studying the dynamics of explosive processes. In this work, we discuss the automatic procedures implemented for a real-time application to the data acquired by a permanent network of five infrasound stations running at Mt. Etna volcano. The infrasound signals at Mt. Etna consist in amplitude transients, called infrasound events. The adopted procedure uses a multi-algorithm approach for event detection, counting, characterization and location. It is designed for an efficient and accurate processing of infrasound records provided by single-site and array stations. Moreover, the source mechanism of these events can be investigated off-line or in near real-time by using three different models: (1) Strombolian bubble; (2) resonating conduit and (3) Helmholtz resonator. The infrasound waveforms allow us to choose the most suitable model, to get quantitative information about the source and to follow the time evolution of the source parameters.     

Coseismic and Aseismic Tilt Variations on Mount Etna

?—?In the last ten years (1990–1999), 21 discrete variations of continuous tilt signal have been recorded on Mount Etna, among which one episode was caused by the opening of the eruptive fracture. The remaining 20 anomalies can be classified into two categories: the first comprises 5 “instantaneous” tilt variations recorded in correspondence to the most energetic seismic events (M _L ?≥?3.3) localized on the high western part of the volcano; the second consists of 15 transient anomalies ranging from some hours to 1–2 days, observed at different times at the various tilt stations, with no correlation to seismic events or other evident volcanic episodes. The aseismic variations propagate through the volcanic edifice with a velocity between 4.5–6.0?km/day. Modeling studies suggest that the deformation is generated by a tensile source located 3–6?km SW from Etna volcano summit and 5–10?km depth.     

Review of Microgravity Observations at Mt. Etna: A Powerful Tool to Monitor and Study Active Volcanoes

Microgravity observations at Mt. Etna have been routinely performed as both discrete (since 1986) and continuous (since 1998) measurements. In addition to describing the methodology for acquiring and reducing gravity data from Mt. Etna, this paper provides a collection of case studies aimed at demonstrating the potential of microgravity to investigate the plumbing system of an active volcano and detect forerunners to paroxysmal volcanic events. For discrete gravity measurements, results from 1994–1996 and 2001 are reported. During the first period, the observed gravity changes are interpreted within the framework of the Strombolian activity which occurred from the summit craters. Gravity changes observed during the first nine months of 2001 are directly related to subsurface mass redistributions which preceded, accompanied and followed the July-August 2001 flank eruption of Mt. Etna. Two continuous gravity records are discussed: a 16-month (October 1998 to February 2000) sequence and a 48-hour (26–28 October, 2002) sequence, both from a station within a few kilometers of the volcano's summit. The 16-month record may be the longest continuous gravity sequence ever acquired at a station very close to the summit zone of an active volcano. By cross analyzing it with contemporaneous discrete observations along a summit profile of stations, both the geometry of a buried source and its time evolution can be investigated. The shorter continuous sequence encompasses the onset of an eruption from a location only 1.5 km from the gravity station. This gravity record is useful for establishing constraints on the characteristics of the intrusive mechanism leading to the eruption. In particular, the observed gravity anomaly indicates that the magma intrusion occurred “passively” within a fracture system opened by external forces.     

CO2 output and δ13C(CO2) from Mount Etna as indicators of degassing of shallow asthenosphere

 An estimated average CO₂ output from Etna's summit craters in the range of 13±3 Mt/a has recently been determined from the measured SO₂ output and measured CO₂/SO₂ molar ratios. To this amount the CO₂ output emitted diffusely from the soil (≈ 1 Mt/a) and the amount of CO₂ dissolved in Etna's aquifers (≈ 0.25 Mt/a) must be added. Data on the solubility of CO₂ in Etnean magmas at high temperature and pressure allow the volume of magma involved in the release of such an amount of this gas to be estimated. This volume of magma (≈ 0.7 km³/a) is approximately 20 times greater than the volume of magma erupted annually during the period 1971–1995. On the basis of C-isotopic data of CO₂ collected in the Etna area and of new hypotheses on the source of Mediterranean magmas, significant contributions of CO₂ from non-magmatic sources to the total output from Etna are unlikely. Such large outputs of CO₂ and also of SO₂ from Etna could be due to an anomalously shallow asthenosphere beneath the volcano that allows a continuous escape of gases toward the surface, even without migration of magma. Received: 7 August 1996 / Accepted: 9 November 1996     

Geochemistry of some volcanic rocks from south-eastern sicily: Rare earth and other trace element distribution

Some trace element data for volcanic rocks found at different levels, from Tertiary to Holocene, in south-eastern Sicily (Iblean Plateau and Mt. Etna) are presented and discussed in the present paper in order to better the information about the origin and relationships of the various rock types. Four groups of volcanic rocks have been recognized on the basis of their major element chemistry: 1) low-K tholeiites, 2) associated alkali basalts to nephelinites of the Iblean Plateau (Upper Pliocene to Lower Pleistocene), 3) the basal subalkaline lavas of Mt. Etna, and 4) the alkalic suite rocks that make up the bulk of the volcano. The distribution of Rb, Sr, Ni, Cr, Co, Cu, REE, Th and Sc suggests:

1. an origin of the Iblean magmas by a different degree of partial melting of a Rb-poor and possibly slightly hetereogeneous mantle;
2. quite distinct source compositions for the Etnean magmas, relative to those of the Iblean area, on the basis of their Rb and Sr contents;
3. an origin of the alkalic rocks of Mt. Etna from independently generated magma(s) rather than by crystal fractionation of the Etnean subalkaline magmas or of a magma having the geochemical features of the Iblean alkali basalts; evidence for this is given by the distribution features of the incompatible elements showing an origin for these rocks from compositionally different parent magmas and/or an evolution under widely variable environmental conditions;
4. the primary character for the chemical differences observed in some of the Etnean subalkaline rocks that can be accounted for by different physico-chemical conditions at their source rather than by crystal fractionation processes.

     

A model for eruptive mechanisms of Mt. Etna from the study of seismicity from 1978 to 1983

The papers deals with the seismic activity occurred on Mt. Etna from 1978 to 1983, and special emphasis is given to the seismicity linked to eruptive phenomena that took place during that period.Location of epicentres and hypocentres of all earthquakes occurred during the considered years and in association with each eruption has shown to be a useful tool to investigate relationships between seismicity and characteristics of various eruptions.A preliminary model is proposed to explain seismo-eruptive mechanisms controlling the uprise of magma and subsequent eruptions of Mt. Etna. The complexity of phenomena observed in the Etnean area could be interpreted as the result of the combined effect of regional stress field and local changes of it due to the volcano structural inhomogeneities. Thus, the earthquakes occurring in the studied area may cause either partial intrusion of magma at various depth, or final opening of surface fractures and subsequent output of lava.     

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.     

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.