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.     

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.     

Seismic activity accompanying the 1974 eruption of Mt. Etna

On January 30, 1974, an explosive eruption began on the western side of Etna. The activity evolved into two eruptive periods (January 30–February 17 and March 11–29). Two spatter cones (Mount De Fiore I and Mount De Fiore II) were formed at a height of about 1650 m a.s.l. and a distance of 6 km from the summit area. The effusive activity was very irregular with viscous lava flows of modest length.A seismic network of four stations was established around the upper part of the volcano on February 3. Moreover additional mobile stations were set up at several different sites in order to obtain more detailed informations on epicenter locations and spectral content of volcanic tremor.The volcanic activity is discussed in relation to the distribution of epicenters and the time-space distribution of the spectral characteristics of volcanic earthquakes and tremor. The characteristics of the seismic activity suggest that the flank eruption of Mount Etna was probably feed by a lateral branch of the main conduit yielding the activity at the Central Crater.     

A syn-eruptive ground deformation episode measured by GPS,during the 2001 eruption on the upper southern flank of Mt Etna

Ground deformation occurring on the southern flank of Mt Etna volcano during the July–August 2001 eruption was monitored by GPS measurements along an E–W profile crossing the fissure system. This profile was measured eight times during the eruption, using the 'stop and go' semi-kinematic technique. Horizontal and vertical displacements between GPS surveys are reported for each station. The most significant event is a deformation episode occurring during the first week of the eruption, between 25–27 July. Displacements were measured on benchmarks close to the eruptive fissure and the tensile 1989 fracture. Data inversions for measured displacements were performed using the Okada model. The model shows the narrowing of the 2001 dyke accompanied by a dextral dislocation along an east-dipping fault, parallel to the 1989 fracture.Editorial responsibility: T. Druitt     

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.     

Lava-flow hazard on the SE flank of Mt. Etna (Southern Italy)

A method for mapping lava-flow hazard on the SE flank of Mt. Etna (Sicily, Southern Italy) by applying the Cellular Automata model SCIARA_-fv is described, together with employed techniques of calibration and validation through a parallel Genetic Algorithm. The study area is partly urbanised; it has repeatedly been affected by lava flows from flank eruptions in historical time, and shows evidence of a dominant SSE-trending fracture system. Moreover, a dormant deep-seated gravitational deformation, associated with a larger volcano-tectonic phenomenon, affects the whole south-eastern flank of the volcano.     

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.     

Earthen barriers to control lava flows in the 2001 eruption of Mt. Etna

Preceded by four days of intense seismicity and marked ground deformation, a new eruption of Mt. Etna started on 17 July and lasted until 9 August 2001. It produced lava emission and strombolian and phreatomagmatic activity from four different main vents located on a complex fracture system extending from the southeast summit cone for about 4.5 km southwards, from 3000 to 2100 m elevation (a.s.l.). The lava emitted from the lowest vent cut up an important road on the volcano and destroyed other rural roads and a few isolated country houses. Its front descended southwards to about 4 km distance from the villages of Nicolosi and Belpasso. A plan of intervention, including diversion and retaining barriers and possibly lava flow interruption, was prepared but not activated because the flow front stopped as a consequence of a decrease in the effusion rate. Extensive interventions were carried out in order to protect some important tourist facilities of the Sapienza and Mts. Silvestri zones (1900 m elevation) from being destroyed by the lava emitted from vents located at 2700 m and 2550 m elevation. Thirteen earthen barriers (with a maximum length of 370 m, height of 10–12 m, base width of 15 m and volume of 25 000 m³) were built to divert the lava flow away from the facilities towards a path implying considerably less damage. Most of the barriers were oriented diagonally (110–135°) to the direction of the flow. They were made of loose material excavated nearby and worked very nicely, resisting the thrust of the lava without any difficulty. After the interventions carried out on Mt. Etna in 1983 and in 1991–1992, those of 2001 confirm that earthen barriers can be very effective in controlling lava flows.     

Satellite thermal infrared observations of Mt. Etna after the 17th March 1981 eruption

The aim of this study is to estimate the feasibility of monitoring the surface temperature of an active volcano, using satellite thermal infrared data.For this, the data collected by a radiometer on board the NOAA polar-orbiting satellites over Mt. Etna were processed (geometrical and radiometric corrections) and merged with a Digital Terrain Model in order to correct for the variations of mean ground temperature and atmospheric absorption with altitude.Four pictures, or scenes, subsequent to the eruption of 17th March 1981, have been interpreted; and they display two thermal anomalies: one, near the summit zone; and the other, on the NNW slope where lava flows took place.Some elementary models of thermal evolution are proposed.     

Three-dimensional model of the feeder conduit of the 1983 eruption of Mt. Etna volcano,from ground deformation measurements

A levelling network and a horizontal distance measuring network down Mt. Etna's south flank were measured before, during and after the 1983 eruption. We analyse the large movements recorded, using theoretical models of surface displacement, to determine the shape and position of the feeder conduit that supplied magma to the eruption. The results indicate that a dyke, the top surface extending nearly horizontally, connects the eruption site to a point near the Southeast Crater. The top of the dyke lies at an altitude of 2400 m to 2500 m, and dips between 75° and 90° to the west.More tentative evidence indicates that the top of the active dyke dropped by about 150 m between the 25th and the 95th day of the eruption, and that a small branching dyke 1 km long was emplaced to the east of the main dyke, but did not attain the surface. The implications of these observation are discussed.     

Shifting styles of basaltic explosive activity during the 2002–03 eruption of Mt. Etna, Italy

The 2002–03 flank eruption of Etna was characterized by two months of explosive activity that produced copious ash fallout, constituting a major source of hazard and damage over all eastern Sicily. Most of the tephra were erupted from vents at 2750 and 2800 m elevation on the S flank of the volcano, where different eruptive styles alternated. The dominant style of explosive activity consisted of discrete to pulsing magma jets mounted by wide ash plumes, which we refer to as ash-rich jets and plumes. Similarly, ash-rich explosive activity was also briefly observed during the 2001 flank eruption of Etna, but is otherwise fairly uncommon in the recent history of Etna. Here, we describe the features of the 2002–03 explosive activity and compare it with the 2001 eruption in order to characterize ash-rich jets and plumes and their transition with other eruptive styles, including Strombolian and ash explosions, mainly through chemical, componentry and morphology investigations of erupted ash. Past models explain the transition between different styles of basaltic explosive activity only in terms of flow conditions of gas and liquid. Our findings suggest that the abundant presence of a solid phase (microlites) may also control vent degassing and consequent magma fragmentation and eruptive style. In fact, in contrast with the Strombolian or Hawaiian microlite-poor, fluidal, sideromelane clasts, ash-rich jets and plumes produce crystal-rich tachylite clasts with evidence of brittle fragmentation, suggesting that high groundmass crystallinity of the very top part of the magma column may reduce bubble movement while increasing fragmentation efficiency.     

Relationship between the flank sliding of the South East Crater (Mt. Etna, Italy) and the paroxysmal event of November 16, 2006

Over the last 2 years, the south east crater (SEC) at Mt. Etna (Italy) has been characterised by the intermittent emission of lava flows, often accompanied by paroxysmal episodes with violent strombolian activity and/or lava fountains. One of the most intense and intriguing episodes occurred on November 16, 2006 during strombolian and effusive activity from the very top of the SEC. At the eastern base of the crater, a violent and short-lasting outburst generated a 300-m-high eruptive curtain of finely fragmented magma and steam, which collapsed in a few seconds giving rise to a small but significant pyroclastic flow. The paroxysm was preceded by progressive sliding of the eastern flank of the SEC edifice, which led to the formation of a large niche. Previously published models explain the outburst as due to rapid vaporisation of water contained in sediments on contact with hot lava flowing along the flank of the SEC or, alternatively, to gravitational collapse of a destabilised portion of the eastern flank of the crater. However, several lines of evidence suggest that these models do not adequately explain the paroxysm, which we propose was associated with the explosive emission of magma during the rapid opening of an ESE-WNW-oriented fracture at the eastern base of the SEC. Moreover, geochemical data of the products associated with this episode show that they are the most primitive and gas-rich among those erupted during the entire 2006 period. We suggest that the paroxysm of November 16, 2006, which occurred at base of the SEC, was a magma outburst due to rapid ground fracturing related to a drop in confining pressure upon the underlying dyke consequent to the sliding of the eastern flank of the SEC cone. Specifically, the unloaded shallow portion of the feeding dyke did not restrain the arrival of a new batch of gas-rich magma, which worked its way out by fracturing the base of the niche at its weakest point. This paroxysm appears significant because it reveals the potential development of syn-eruptive dynamics acting directly on the uppermost portion of the plumbing system which is able to generate pyroclastic flows even on basaltic volcanoes such as Mount Etna.     

Features of volcanic tremors on Mt. Etna (Sicily) during the March–August 1983 eruption

The seismic analysis of the volcanic tremors preceding and accompanying the Etnean eruption of March–August, 1983 has shown a significant variation in the spectral content before the beginning of the eruption, the tremor peaks at 1.4 and 1.6 Hz — which might be associated with the feeding pipes of the NE crater (Schick et al., 1982a) — being the dominant feature of the spectra.A model of eruption mechanism is proposed where a feeder dyke would connect the NE crater with the effusive fracture.     

Methodology for the construction of earth barriers to divert lava flows: the Mt. Etna 1983 eruption

This paper gives a critical analysis of the technical solutions adopted for erecting earthworks to divert lava flows on the basis of the experience obtained recently during the 1983 eruption of Mount Etna. The purpose of the paper is to explain questions of general nature and throw light on problems that must still be solved, which render it difficult to extrapolate the adopted solutions to other volcanic eruptions having radically different characteristics. An account of the earthworks made at the various sites to the east and the west of the lava flow is given, and an analysis is made of the planning choices, the decisions taken during the work, the daily output and thence the relative costs. The works carried out are summarized in Table 1 giving characteristics and costs of the works and times and sizes of the equipments used. The following works were carried out:

4 barriers for a total length of ca. 1700 m with a height variable from 8 to 20 m.

2 channels for a total length of 1000 m.

many emergency measures during night and day-time also on fresh lava.

During 50 days of 13 working hours per day a total volume of ca. 750,000 m³ of material was moved and ca. 10 km of service roads were built. The total cost of the above mentioned works was 3,678 millions of Italian liras. The final result fulfilled the planning forecasts: all the earthworks erected carried out their task of guiding and diverting the lava flows. Amongst other things, the study of the evolution of eruption has shown that preventive and emergency works can successfully be programmed. Finally, the technological aspects of the works are discussed with particular reference to their planning, site organization, determination of type and number of vehicles to be employed, output to expect and thus, for a correcta priori estimate of the advantage-cost ratio, a forecast of the expenses involved. The paper aims at giving information that may be of use during emergency situations and at stimulating the study of an operations program for mitigating the effects of lava flows in areas of volcanic risk.     

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.     

The contemporaneous emission of low-K and high-K trachybasalts and the role of the NE Rift during the 2002 eruptive event,Mt. Etna,Italy

Mount Etna volcano erupted almost simultaneously on its northeastern and southern flanks between October 27 and November 3, 2002. The eruption on the northeastern flank lasted for 8 days, while on the southern flank it continued for 3 months. The northeastern flank eruption was characterized by the opening of a long eruptive fracture system between 2,900 and 1,900 m.a.s.l. A detailed survey indicates that the fractures’ direction shifted during the opening from N10W (at the NE Crater, 2,900 m) to N45E (at its lowest portion, 1,900 m) and that distinct magma groups were erupted at distinct fracture segments. Based on their petrological features, three distinct groups of rocks have been identified. The first group, high-potassium porphyritic (HKP), is made up of porphyritic lavas with a Porphyritic Index (P.I.) of 20–32 and K₂O content higher than 2 wt%. The second group is represented by lavas and tephra with low modal phenocryst abundance (P.I. < 20) named here oligo-phyric (low-phyric), and K₂O content higher than 2 wt% (HKO, high-potassium oligophyric). The third group, low-potassium oligophyric (LKO), consists of tephra with oligophyric texture (P.I. < 20) but K₂O content < 2 wt%. K-rich magmas (HKP and HKO) are similar to the magma erupted on the southern flank, and geochemical variations within these groups can be accounted for by a variable degree of fractionation from a single parent magma. The K-poor magma (LKO), erupted only in the upper segment of the fracture, cannot be placed on the same liquid line of descent of the HK groups, and it is similar to the magmas that fed the activity of Etna volcano prior to the eruption of 1971. This is the first time since then that a magma of this composition has been documented at Mt. Etna, thus providing a strong indication for the existence of distinct batches of magma whose rise and differentiation are independent from the main conduit system. The evolution of this eruption provides evidence that the NE Rift plays a very active role in the activity of Mt. Etna volcano, and that its extensional tectonics allows the intrusion and residence of magma bodies at various depths, which can therefore differentiate independently from the main open conduit system.     

Seismic activity accompanying the outbreak of the 1991–1993 eruption of Mt. Etna (Italy)

The character and location of seismic activity accompanying the onset of the 1991–1993 eruption at Mt. Etna are compatible with the surface evidence of the volcanic pile rupture. Both the epicentral distribution and the focal mechanisms of a swarm that occurred on December 14, 1991, agree with magma ascent occurring along the main NNW-SSE-trending structure of the volcano and the consequent opening of a system of effusive fissures with the same trend. A typical mainshock-aftershock sequence, recorded the day after and indicating transcurrent displacement occurring along the second-principal structure of Etna (NE-SW), depicts the tectonic response of the volcanic pile and the underlying basement to major stresses applied by the magma push.     

Three-dimensional velocity structure and seismicity of Mt. Etna Volcano, Italy

A new set of three-dimensional velocity models beneath Mt. Etna volcano is derived in the present work. We have used P- and S-wave arrivals from local earthquakes recorded at permanent and temporary seismic networks installed since 1980. A set of 1249 earthquakes recorded at more than four seismic stations was selected for traveltime inversion. The velocity models obtained by using different data selection criteria and parametrization display similar basic features, showing a high P-wave velocity at shallow depth in the SE quadrant, in close connection with a high gravimetric Bouguer anomaly. This area shares a low V_p/V_s ratio. High P-wave velocities and high V_p/V_s ratios are obtained along the central conduits, suggesting the presence of dense, intrusive magmatic bodies extending to a depth of about 20 km. The central intrusive core is surrounded by lower P-wave velocities. The relocated earthquake hypocenters also display the presence of an outward dipping brittle region, away from the central conduits, surrounding a ductile zone spatially related to the high P-wave velocity anomalies located in proximity to the central craters.     

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.