Sebastiano Imposa

Mt. Etna: a model for the 1989 eruption

Abstract During the 1989 eruption of Mt. Etna, one of the most important in the last 20 years in terms of effusion rate, a pair of fractures formed on the slopes of the Southeast Crater cone and propagated in a few days trending ca. northeast and southeast, with large quantities of magma being drained from the northern fracture. In contrast the southern fracture, after a short initial eruptive phase, grew by 5 km in five days without eruptive or seismic activity. When the fracture system crossed the southern scarp of the Valle del Bove wall, some 6 km SE of the active crater, an intense seismic swarm started in the same area. This lasted about four days and consisted of several hundred events clustering in a small focal volume. Although such a sequence of failure events contains elements of compatibility with a shallow dyke intrusion, it did not lead to the expected flank eruption which would have threatened settlements just a few kilometres from the distal end of the fracture. The question arises, therefore, whether or not the fracture system was dyke-related, and if so to what extent. We attempt here to answer such a question by integration and interpretation of the available field observation and geophysical data. Indeed, field observation constraints on the southern fracture system and the typical depth (> 2 km) of the swarm do not contain evidence for shallow dyke emplacement beneath the lowermost part of the SSE fracture. Conversely, the inferred direction and shape of the tremor source, the location and extent of a gravity anomaly observed a few months before the eruption, the near-horizontal migration of the seismic focii during the swarm, and the change with time of related focal mechanisms, lead us to hypothesize that the extreme development of the fracture system at its south was due to a deeper intrusion beneath the southern slope of the volcano.

The role of regional tectonics, magma pressure and gravitational spreading in earthquakes of the eastern sector of Mt. Etna volcano (Italy)

Data concerning M ⩾ 3.3 earthquakes that occurred in the eastern sector of Mt. Etna volcano (Sicily, Italy) in the period 1984–1989 are here presented and discussed. Only events with reliable focal mechanisms and detailed macroseismic investigations have been considered. Instrumental information come from local seismic networks, both permanent and temporary, run by the University of Catania (UCT), the Seismological Observatory of Acireale (SOACR), the CNRS (Grenoble, France) and the Institut de Physique du Globe de Paris (IPG, France). Observations regarding the macroseismic effects have been collected by means of field recognitions mainly carried out by the authors. All available data are analyzed in light of the more recent interpretations on the kinematic behaviour of the shallowest structures of Etna, with particular reference to its eastern flank which, according to Borgia et al. (1992), is likely affected by a seaward gravitational migration, driven by the sliding of this sector of the volcanic apparatus over its clay-rich substratum. The results obtained support the hypothesis that the shallow seismicity of the eastern flank of Mt. Etna is related to a complex stress field due to the combined effects of: (a) the tectonics associated with the interaction between the African and Eurasian plates; (b) the rise of magma into the crust; and (c) the gravitative seaward sliding of the eastern sector of the volcano. In particular, we hypothesize that: (1) tectonic forces caused the end of the 1984 eruption, by means of a “locking mechanism”; (2) the increment of magma pressure at depth is periodically responsible for local re-orientation of the stress field acting on the volcano, which also produces earthquakes of high intensity; (3) shallow earthquakes (z < = 5 km), showing eastward compression mechanism, could be related to the gravitational sliding of unbuttressed sectors of the volcanic body accompanying the emplacement of intrusive dykes.

Feeding Mechanism of Eruptive Activity at Mt. Etna Based on Seismological and Petrological Data

Seismic and petrological data are analyzed with reference to the eruptive phenomena at Mt. Etna from 1974–1984. Seismological data show that most earthquakes occur within the upper 16 km of the crust and very few below 30 km. Focal solutions for events at depth between 7 and 16 km show evidence of both normal and thrust faults, due to an unstable stress field, whereas at shallower levels only normal mechanisms are observed. In most cases, enhanced seismic activity preceded the beginning of adventive eruptions, but neither upward migration nor clear concentration of epicenters near the eruptive area (except for the shallowest events; h < 1 km) was observed.

Upwards migration of seismic focii: a forerunner of the 1989 eruption of Mt Etna (Italy)

Data from a portable array of three-component digital stations, run at Mt Etna from 1988 to early 1990, highlight the seismic behaviour of the volcano before the 1989 eruption, one of the most significant in terms of energy of the last two decades. After a two-year period of weak and discotinuous seismicity, the depth of the seismically active volumes was observed to become shallower a few months before the volcanic event. The overall migration of the events, inferred by hypocentral locations and decreases of S-P time differences at two stations, agrees with other geophysical forerunners and allows further insights into the changes in the stress field leading to the eruption.

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 (h≤3 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.

An approach to problems on energy sources at Mount Etna based on seismological and volcanological data

Based on seismograms from the most continuously operating station of the seismic network at Mt. Etna (Monte Vetore, 1665 m a.s.l.), the energy release patterns through time have been obtained for volcanic tremors and earthquakes which occurred between 1978–1982. Both energies range between 1011∓1013 J/a, but their release patterns are not strictly correlated to each other. Considering only the eruptions which occurred during the same time span, the amount of thermal energy released and the associated potential energy needed to raise the magma to the surface from a 2-km-deep hydrostatic equilibrium level were estimated to be about 1017 and 1015 J/a, respectively. The computed energies for earthquakes and tremors are at least one order of magnitude less than what is needed to match a model of magma transport based solely on the jerky propagation of melt-filled cracks. The energy needed to raise the magma to the surface could be supplied by expanding gases in the upper levels of the magma column and/or by tectonic stresses acting on shallow batches of magma.

Source parameters of seismic events at Mount Etna volcano, Italy, during the outburst of the 1991–1993 eruption

Abstract A volcanic eruption lasting 472 days occurred at Etna, Italy, between December 1991 and March 1993 on the upper eastern flank of the volcano. This activity produced a lava flow of more than 7 km in length which stopped at an elevation of about 750 m above sea-level, causing some damage at the periphery of the village of Zafferana. We have estimated the source parameters of seismic events which preceded and accompanied the outburst of the eruptive activity. Source parameters have been determined from the SH-displacement spectra using the source model of Brune. We have also calculated the relations between the equivalent Wood-Anderson magnitude ( M WAeq ) of shocks, the local coda-duration magnitude ( M D ), the seismic moment ( M o ), the stress drop (Δσ), and the radius ( r ) of the seismic sources. Finally, we compared our results with those obtained in a reference extensional region (Swabian Jura, Southwest Germany) to speculate on the rheological properties of the Etnean crust with respect to that of a well-studied non-volcanic area with similar structural and tectonic features. The comparison suggests that at Etna regional stress affects a highly fractured medium, where the reactivation, the coalescence and the propagation of pre-existing fractures and faults are the dominant processes in the formation of new dislocation structures. This hypothesis is strengthened by the repetitive occurrence of volcanic eruptions along the same structural trends, as is deduced from the very recent eruptive history of Etna. Frequent temporal changes in the orientation of the main stress axes occur, as is suggested by the azimuthal dispersion of the main structural lines and the superimposition with time of extensional and shear mechanisms along the same tectonic alignment.

Volcanic tremor at Mt. Etna: state-of-the-art and perspectives

Volcanic tremor on Etna seems to have its origin within the main magma feeding system. On the basis of both spectral analyses at two permanent seismic stations and periodical measurements along the slopes of the volcano, two distinct sources are proposed. The former, characterized by low frequency contents (f<1.5 Hz), is located in a 2 km deep flat magma chamber, whereas the latter source seems to be linked to the upper part of the active vents.Turbulent motions in the magma-gas mixture, induced by escaping gases within the conduits, is one proposed cause of volcanic tremor on Etna (Seidlet al., 1981).From spectral analyses we propose approximate models of the feeding system of the main summit craters.Time variations of tremor energy were also investigated, and no regular patterns have been observed for the studied eruptions.More systematic information seems to be needed for a better knowledge of both the source model and location, and correlation between tremor features and volcanic activity.

Evaluation of the stability of a rock cliff by means of geophysical and geomechanical surveys in a cultural heritage site (south-eastern Sicily)

An integrated geophysical and geomechanical study is proposed for the evaluation of the fracturing condition and the stability of a rock cliff in the cultural heritage site of Ispica (southeastern Sicily).The site comprises several worship constructions such as the Friars Minor Monastery, the oldest construction of the village and a Church. The buildings stand on the edge of a 35 m high calcareous cliff, showing instability conditions related to the presence of deep fracturing and karst features. Due to possible future rockfalls the slope is expected to be subjected to retreat phenomena, which will threat the structures on top.The study of the seismic wave speed and of the environmental noise (microtremors), through active and passive seismic survey, proved useful in highlighting the spatial variation of the degree of fracturing in the rock mass. Moreover, the resonance frequency of the site allowed estimating the frequencies at which the ground motion is amplified by stratigraphic features.The stability analysis pointed out the critical condition of the slope, affected by different potential failure mechanisms with very low safety factors and a high probability of failure.All the considerations are contextualized in the wide Hyblean panorama, where important tectonic structures are responsible of the actual geological setting and of the seismicity of the area, as well as of the geomechanical condition of the slopes.

Earth tides and Etnean volcanic eruptions: an attempt at correlation of the two phenomena during the 1983, 1985 and 1986 eruptions

Abstract An attempt has been made to correlate volcanic events on Mt. Etna and Moon-Sun attraction during the eruptions of 28 March–6 August 1983, 12 March–13 July 1985 and 30 October 1986. The possibility of a ‘trigger’ effect has been supposed for eruptive phenomena that originate by small and continuous external forces under particular stress conditions on the volcanic structure. The parameters compared, for the two phenomena, are the hourly mean tremor amplitude recorded at Serra La Nave station and the hourly value of the vertical and of the two partial horizontal north-south and east-west components of the tidal acceleration induced by the Moon-Sun system. The relationship between two series of values (the former for the tremor amplitude and the latter for one of the tidal acceleration components) were then tested by statistical methods based on the information theoretic measure of interdependence between any pair of signal sequences. The results show an effect of the horizontal tidal components on well-known tectonic structures, which may be widened or closed; in contrast, there is little evidence that the uprise of magma masses might depend on the vertical tidal attraction.