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.     

The dyke swarm of Mount Calanna (Etna, Italy): an example of the uppermost portion of a volcanic plumbing system

A new multidisciplinary study, combining geology, petrography, and geochemistry, on the rocks of the isolated hill of Mount Calanna (Mount Etna, Italy) has provided evidence for the existence of a dyke swarm, formed by more than 200 dykes distributed over an area of ~0.7 km², with an intensity of intrusion up to 40%. All bodies are deeply altered, and the geological and mesostructural surveying of 132 dykes revealed that they intruded in E–W direction, with an average dip of 60°. The faults affecting the outcrop have in general an E–W strike and dip of ~55°: these have all normal motion and have been interpreted as coeval with the dykes. This interpretation contrasts with the previous hypothesis that considered Mount Calanna as a thrust resulting from compressive deformation resulting from the gravitational spreading of the volcanic edifice. Mount Calanna is here interpreted as the uppermost portion of a vertically extensive magmatic plexus that fed the eruptive activity of one (or more) eruptive center/s sited in the Valle del Bove area. Measurements of the apparent densities on 23 dykes and host rock samples give an average value of 2,420 kg/m³ for the entire complex, ~15% lower than the density expected for hawaiitic magma, placing an important constraint on the geophysical identification of similar structures. Considering that Mount Etna is not an old eroded edifice but an active and growing volcano, the exposure of this subvolcanic structure can be regarded as exceptional. Its geometry and physical characteristics can be thus regarded as an interesting example of the present-day shallow plumbing system of Mount Etna as well as of other basaltic volcanoes.     

Review of seismological studies at Mount Etna

This paper reports the present state of seismological research at Mt. Etna.A schematic classification of the earthquakes that occur on the volcano is proposed, based on both seismogram and spectrum features.We have made both focal solutions and estimates of earthquake source parameters (stress drop values between 2 and 20 bars and small source dimensions).The crust of Etna thus appears as an extremely heterogeneous medium that does not permit great stress accumulation. The coexistence of an extensional regime with an older and deeper compressive one seems confirmed at depths greater than about 7 km.Eruptive and seismic phenomena occur mainly along the principal structural trends of the volcano, but often the directions of the eruptive fractures and the earthquake concentration during the same eruption do not coincide.Tectonics seem to play an important role in controlling seismo-volcanic behaviour.     

The storage and release of magma on Mount Etna

The eruptive history of Etna during the past 450 years provides data on effusion rates, volumes of magma involved, and the nature of the eruptive conduits. These data are interpreted in terms of a two-part intravolcanic magma reservoir which feeds the flank eruptions through dike-like conduits. The structural framework of the volcano which controls the spatial distribution of eruptive sites is partly inherited from the basement and partly controlled by the central magma column and the surrounding caldera boundary faults. Hydraulic fracturing theory predicts that the central magma column will fail at depths below 1 km if the tensile strength of the conduit rocks is about 100 bars and that a peak fracturing capability will be reached between 1 and 2 km depth. This inference agrees well with the peak of flank eruptive activity at 1.4 km below the summit observed in the data on the loci of eruptions. The average flank-eruption feeding dike is defined and shown to be capable of the observed maximum effusion rates (20–100 m³ s⁻¹) from magmatic pressure differences of 30–150 bars     

Estimations of mercury fluxes emitted by Mount Etna Volcano

A sampling and measuring device which enables the assessment of atmospheric particulate and gaseous mercury concentrations has been tested on Mount Etna Volcano. Particulate matter is collected on a Whatman GF/C of 1.0 µm pore-size, gaseous mercury species on a Au-column. The analysis is carried out in two steps: (1) the mercury species collected on the filter or the Au-column are transferred to a fixed analytical Au-column; (2) mercury liberated from this column during the second step is detected with a Mercury Vapour Monitor. Average concentrations of gaseous and particulate mercury in ambient sampling sites on Mount Etna are 3.8 ng m^?3 and 0.49 ng m^?3 respectively. Average concentrations of gaseous and particulate mercury in the plume of Bocca Nuova on Mount Etna are 15 ng m^?3 and 24 ng m^?3 respectively. An estimation of the total mercury discharge from Mount Etna amounts to 2.5 10^?2 tons day^?1.     

Combined discrete and continuous gravity observations at Mount Etna

Systematic investigation of discrete gravity measurements has continued at Mount Etna since 1986. The network now covers an area of 400 km² with about 70 stations 0.5–3 km apart. Mass redistributions occurring at depths ranging between about 8 km below sea level and a few hundred metres below the surface (magma level changes within the shallower parts of the feeding conduits) have been identified from these data. Conventional (discrete) microgravity monitoring on a network of stations furnishes only instantaneous states of the mass distribution at continuously active systems. In order to obtain information on the rate at which the volcanic processes (and thus mass transfers) occur, three stations for continuously recording gravity where installed on Mount Etna in 1998. A 16-month long sequence from one of the continuously running stations (PDN, located 2 km from the active northeast crater at the summit of Etna volcano) is presented. After removing the effects of Earth Tide and tilt, the correlation of the residual gravity sequence with simultaneous recordings of meteorological parameters acquired at the same station was analysed. Once the meteorological effects have also been removed, continuous gravity changes are within 10 μGal of gravity changes measured using conventional microgravity observations at sites very close to the continuous station. This example shows how discrete and continuous gravity observations can be used together at active volcanoes to get a fuller and more accurate picture of the spatial and temporal characteristics of volcanic processes.     

A multi-disciplinary study of the 2002–03 Etna eruption: insights into a complex plumbing system

The 2002–03 Mt Etna flank eruption began on 26 October 2002 and finished on 28 January 2003, after three months of continuous explosive activity and discontinuous lava flow output. The eruption involved the opening of eruptive fissures on the NE and S flanks of the volcano, with lava flow output and fire fountaining until 5 November. After this date, the eruption continued exclusively on the S flank, with continuous explosive activity and lava flows active between 13 November and 28 January 2003. Multi-disciplinary data collected during the eruption (petrology, analyses of ash components, gas geochemistry, field surveys, thermal mapping and structural surveys) allowed us to analyse the dynamics of the eruption. The eruption was triggered either by (i) accumulation and eventual ascent of magma from depth or (ii) depressurisation of the edifice due to spreading of the eastern flank of the volcano. The extraordinary explosivity makes the 2002–03 eruption a unique event in the last 300 years, comparable only with La Montagnola 1763 and the 2001 Lower Vents eruptions. A notable feature of the eruption was also the simultaneous effusion of lavas with different composition and emplacement features. Magma erupted from the NE fissure represented the partially degassed magma fraction normally residing within the central conduits and the shallow plumbing system. The magma that erupted from the S fissure was the relatively undegassed, volatile-rich, buoyant fraction which drained the deep feeding system, bypassing the central conduits. This is typical of most Etnean eccentric eruptions. We believe that there is a high probability that Mount Etna has entered a new eruptive phase, with magma being supplied to a deep reservoir independent from the central conduit, that could periodically produce sufficient overpressure to propagate a dyke to the surface and generate further flank eruptions.Editorial responsibility: J. Donnelly-Nolan     

Doppler radar sounding of volcanic eruption dynamics at Mount Etna

Besides their common use in atmospheric studies, Doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field. We present the observations made with a mid-power UHF Doppler radar (Voldorad) during a 7-h Strombolian eruption at the SE crater of Mount Etna on 11–12 October 1998. Main characteristics of radar echoes are retrieved from analysis of Doppler spectra recorded in the two range gates on either side of the jet axis. From the geometry of the sounding, the contribution of uprising and falling ejecta to each Doppler spectrum can be discriminated. The temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before, during and after its paroxysm. Moreover, during the sharp decrease of eruptive activity following the paroxysm, detailed analysis of video (from camera recording), radar and seismic measurements reveals that radar and video signals start to decrease simultaneously, approximately 2.5 min after the tremor decline. This delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the SE crater. Detailed analysis of eruptive processes has been also made with Voldorad operating in a high sampling rate mode. Signature of individual outburst is clearly identified on the half part of Doppler spectra corresponding to rising ejecta: temporal variations of the backscattered power exhibit quasi periodic undulations, whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time. Periodicity of power variations (between 3.8 and 5.5 s) is in agreement with the occurrence of explosions visually observed at the SE vent. Maximum vertical velocities of over 160 m s^–1 were measured during the paraoxysmal stage and the renewed activity. Finally, by using a simplified model simulating the radar echoes characteristics, we show that when Voldorad is operating in high sampling rate mode, the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam.Editorial responsibility: A. Woods     

Deformation of Mount Etna, 1971–1974

Electro-optical distance measurements made on the summit of Mt. Etna from 1971 to 1974 show evidence of large surface deformation of the volcano. This deformation cannot be satisfactorily analysed in terms of the models of subsurface magma reservoirs of various geometries that have been previously used, as they have, for instance, on Kilauea in Hawaii. A model that gives a better fit between the observed and computed data involves horizontal, radial strain about an open, cylindrical magma column. In this model, strain is inversely proportional to the square of the distance from the centre of the deformation. This strain pattern is probably confined to the immediate vicinity of the summit vents and is of a different nature lower down the volcano. Tiltmeter, precise levelling and distance measurement data collected over the period of a small flank eruption in January–March 1974 indicate that the eruption was fed by magma through a conduit from the summit reservoir system of the Chasm and Bocca Nuova. Inflation of the summit around the Northeast Crater, which had been measured since 1971, continued despite the flank eruption, and eruptive activity was resumed at the Northeast Crater in September 1974.     

Documenting surface magmatic activity at Mount Etna using ATSR remote sensing

Analysis of thermally generated night-time volcanic radiances recorded with a 1-km pixel size at 1.6 and 11 µm during 1991-1993 and 1996-1999 for Mount Etna shows that lava flows extending beyond the summit craters can be distinguished from vent activity. The two phenomena plot in different regions of feature space when the mean volcanic radiance (per anomalous pixel) at 11 µm is plotted against the mean volcanic radiance at 1.6 µm. The distinct feature space characteristics of lava flow fields are apparent within 1-2 days of the onset of each effusive event. Such a plot also enables lava flow fields being fed by open channels to be distinguished from tube-fed flow fields. Rank order analysis of the total 1.6-µm volcanic radiance series shows that vent activity and lava flows belong to different populations, and offers further scope for remotely identifying changes in eruptive state.     

Chaos and stochasticity in volcanic eruptions the case of Mount Etna and Vesuvius

The series of historical eruptions of Mount Etna and Vesuvius volcanoes are analyzed to verify the presence of low-dimensional chaos in the mechanism driving eruptive activity. A recently developed optimal methodology which is efficient on relatively small sets of data is used. The results indicate that there is no evidence of low-dimensional chaos in the sequences considered, and the mechanism appears better described by a classical stationary stochastic process.     

The seismic detection of a magma chamber beneath Mount Etna,sicily

Macroseismic studies, linear refraction profiles, and a two-dimensional seismic array study on Mount Etna, have all detected anomalous low velocity zones beneath the volcano. Seismic travel time delays together with high frequency attenuation observations confirm the presence of a large volume of partial melt beneath the volcano. A simplified three-dimensional model of the main storage system of Mount Etna is presented.Paper presented at the Symposium Volcanoes of the Earth and Planets, held at the University of Lancaster, March 17, 1981.     

Distortion of the geomagnetic field in volcanic terrains: an experimental study of the Mount Etna stratovolcano

The inclination (I), declination (D) and total intensity (F) of the geomagnetic field were measured on Mount Etna in 1989-1991 at a dozen sites previously sampled for archeomagnetic studies. The purpose of the work was to determine the variations of these parameters at 30 cm above ground level, and how the distortion from the main field can affect the archeomagnetic record of volcanic rocks. Ten measurements were usually performed at each site with a three-component flux-gate magnetometer, whose estimated precision is ±0.2° on direction and ±50 nT on intensity. This was considered sufficient on volcanic areas with highly magnetized rocks and where the geomagnetic gradient may be in excess of 1000 nT/m. Results averaged for each site generally show small variations in intensity (±3% of the total field) and direction (±1.5°). The averaged values of the 12 sites (I=52.6°, D=0.3°, F=44010 nT) are very close to those measured in sedimentary terrain away from the volcano (I=52.9°, D=0.35°, F=44110 nT), themselves consistent with the interpolated IGRF in eastern Sicily. The largest deviations of the geomagnetic direction have been observed on four sites, three of them located on the South flank between 1900 and 700 m elevation. It is suggested that these anomalies are mainly related to dyke swarms which are common within the South Rift Zone of Mount Etna. Our findings show that reliable archeomagnetic results can be obtained from volcanic rocks, provided that lavas of the same eruption are sampled on several sites distributed over the largest possible area.     

Prehistoric dyke trends on Mount Etna; implications for magma transport and storage

The distributions and alignments of over 200 prehistoric dykes exposed in the walls of the Valle del Bove caldera on Mount Etna have been plotted, and samples collected from some 10% of those occurring in the southern wall. Important tectonic trends are reflected by the dykes, along which magma movement was facilitated prior to the formation of the caldera. Close directional relations between the dyke trends and the orientations of historic fissures on the volcano, point to the existence of a plexus of interconnecting subsurface fissures immediately to the south-east of the summit. A model is envisaged within which magma enters this «clearing house» from depth, and is distributed via fissures to other parts of the volcano including the summit region. Here, the interaction of fissures with the conduits of the summit craters is put forward as a mechanism to explain the behaviour of recent activity.     

Dike emplacement and flank instability at Mount Etna: Constraints from a poro-elastic-model of flank collapse

Many volcanic edifices are subject to flank failure, usually produced by a combination of events, rather than any single process. From a dynamic point of view, the cause of collapse can be divided into factors that contribute to an increase in shear stress, and factors that contribute to the reduction in the friction coefficient μ of a potential basal failure plane. We study the potential for flank failure at Mount Etna considering a schematic section of the eastern flank, approximated by a wedge-like block. For such geometry, we perform a (steady state) limit equilibrium analysis: the resolution of the forces parallel to the possible basal failure plane allows us to determine the total force acting on the potentially unstable wedge. An estimate of the relative strength of these forces suggests that, in first approximation, the stability is controlled primarily by the balance between block weight, lithostatic load and magmatic forces. Any other force (sea load, hydrostatic uplift, and the uplift due to mechanical and thermal pore-fluid pressure) may be considered of second order. To study the model sensitivity, we let the inferred slope α of the basal surface failure vary between ?10° and 10°, and consider three possible scenarios: no magma loading, magmastatic load, and magmastatic load with magma overpressure. We use error propagation to include in our analysis the uncertainties in the estimates of the mechanics and geometrical parameters controlling the block equilibrium. When there is no magma loading, the ratio between destabilizing and stabilizing forces is usually smaller than the coefficient of friction of the basal failure plane. In the absence of an initiating mechanism, and with the nominal values of the coefficient of friction μ = 0.7 ± 0.1 proposed, the representative wedge will remain stable or continue to move at constant speed. In presence of magmastatic forces, the influence of the lateral restraint decreases. If we consider the magmastatic load only, the block will remain stable (or continue to move at constant speed), unless the transient mechanical and thermal pressurization significantly decrease the friction coefficient, increasing the instability of the flank wedge for α > 5° (seaward dipping decollement). When the magma overpressure contribution is included in the equilibrium analysis, the ratio between destabilizing and stabilizing forces is of the same order or larger than the coefficient of friction of the basal failure plane, and the block will become unstable (or accelerate), especially in the case of the reduction in friction coefficient. Finally, our work suggests that the major challenge in studying flank instability at Mount Etna is not the lack of an appropriate physical model, but the limited knowledge of the mechanical and geometrical parameters describing the block equilibrium.     

Ground deformation and gravity changes accompanying the March 1981 eruption of Mount Etna

We present reults from simultaneous precise levelling and gravity surveys on Mount Etna covering the period August 1980–August 1981. The flank eruption of March 1981 erupted 18–35 × 10⁵m³ of lava. Following it, upward movements of more than 17 cm were observed close to the new fissure and a broad, apparently independent, uplift of 5 cm was observed 4 km to the west. A zone of about 2 cm depression to the east of the fissure is insufficient to account for the volume of magma erupted. Gravity results show positive changes of up to 63 microgal, and display good positive correlation with elevation changes. Both sets of measurements appear to be due to new intrusion of magma rather than subsurface magma drainage. Ground deformation close to the new fissure is well modelled by intrusion of a dyke in the zone 100–500 m below the surface, striking along the fissure and of dip between 75–90°. The gravity changes are modelled as due to a deeper intrusion of magma, along the same line but some 1500 m below the surface. The changes were not present immediately after the eruption but occurred during the ensuing 5 months. It is proposed that this introduction of matter occurred by crack propagation along the fissure in the aftermath of the eruption. Towards the west of the fissure, and some 4 km west of the summit, ground deformation is modelled by intrusion of a dyke in the zone 300–1500 m below the surface and dipping at 80–85°. Again, gravity changes appear to be due to magma intrusion at greater depth, close to sea level. In this case gravity changes are interpreted as due to magma density changes, as a result of pressure increase in a larger scale fissure zone. This same pressure increase may be forcing the new intrusion close to the surface, and makes this part of the volcano a region of especially high risk.     

Evaluation and restoration of the 1970 volcanic gas analyses from Mount Etna, Sicily

The 1970 Mount Etna volcanic gas analyses (Huntingdon, 1973) are among the most reduced volcanic gas samples ever reported. They contain 20–40% H₂, 2–3.5% CO, and 2–5% H₂S. Calculated oxygen fugacities for most of the analyses are well below quartz-fayalite-magnetite, several are more reduced than magnetite-wustite and all are many orders of magnitude less than those measured by Sato and Moore (1973) in the gas-streams of the collection sites at the time the samples were taken. The analyses show no similarity to calculated equilibrium compositions at any temperature. Deviations between analytical and equilibrium compositions indicate the gases have undergone extensive reduction involving mainly loss of oxygen. There also is limited evidence of sulfur loss. The reduced analyses are not the products of unusually reduced lavas, but originated from reactions of the erupted gases with the metal sampling device used in the collection procedure. The oxygen deficiencies of the analyses have been restored using the atomic hydrogen, carbon and sulfur data of Huntingdon and the oxygen fugacity data of Sato and Moore. The restored analyses are much more representative of the erupted gases which were remarkably rich in CO₂ (15–35%) and SO₂ (15–35%), and they show relatively steady compositions at each collection site over periods of observation ranging from hours to days.