ALBERT

Description: The persistent open-vent mild explosive activity of Stromboli volcano is episodically interrupted by more violent and dangerous explosive events (major explosions and paroxysms). According to the nature of erupted products, paroxysms can be related either to the explosion of overpressured gas pockets located in the proximity of the magma column or to the sudden uprise of hot, gas-rich magma from a deep part of the plumbing system. In both cases, these more energetic explosive events should be preceded by an escape or a preferential uprise of the highly mobile volatiles which, in turn, should produce gas leakage anomalies at the surface in sites of high vertical permeability, such as deep-reaching faults. In order to identify such gas leaking sites, a systematic CO2 soil flux survey has been carried out on the island using an accumulation chamber. Four hundred sixty-one points have been measured with a high density in the summit crater area, where high flux values have been found (10−3–10−2 cm/s). Anomalous points are concentrated along the main NE–SW axial feeding system of the volcano. CO2 soil flux decreases from the crater zone to the base of the volcanic cone, where, however, an interesting gas leakage anomaly occurs (Pizzillo mofette). In the Pizzillo area thermal water wells also occur, whose chemistry indicates an origin by sea water heated by hot gas. Soil gases have been sampled in the sites with the highest CO2 flux in the crater area and at the base of the cone. Chemical and isotopic analyses indicate the presence of a deep gas component especially in the crater zone samples (high CO2, appreciable contents of He and H2, 3He/4He values up to 3.55, δ13C of CO2=−2 ‰). The SC5 low-flux fumarole on the crater rim has shown, during six years, a remarkable persistence of temperature (93–95°C), with only minor fluctuations of chemistry. Appearance of anomalous peaks of H2 in correspondence with strombolian explosions was observed during a 2-h 30-min experiment of continuous recording of hydrogen content of the fumarole. Together with previous data on H2O, CO2 and He, this experiment confirms that strombolian blasts produce rapid fluctuations in the fumarolic gas composition. In the light of this study, SC5 fumarole and Pizzillo mofette look promising sites for the testing of a continuous geochemical monitoring system of Stromboli volcano.

Description: Gruppo Nazionale per la Vulcanologia CEC project “Pre-eruptive processes: Modelling and Parameterization”, contract no. ENV4-CT96-0259 (DG12–ESCY)

Description: Published

Description: 226-245

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: JCR Journal

Description: Sulphur behaviour and variations in redox conditions during magma differentiation and degassing in the Mt Etna (Italy) volcanic system have been explored by integrating the study of olivine-hosted melt inclusions (MIs) with an experimental survey of sulphur solubility in hydrous basaltic magmas. Sulphur solubility experiments were performed at conditions relevant to the Etnean plumbing system (1200 C, 200MPa and oxygen fugacity between NNOþ0 2 and NNOþ1 7, with NNO being the nickel–nickel oxide buffer), and their results confirm the important control of oxygen fugacity (fO2) on S abundance in mafic magmas and on S partitioning between fluid and melt phases (DSfluid/melt). The observed DSfluid/melt value increases from 5164 to 14666 when fO2 decreases from NNOþ1 760 5 to NNOþ0 3. Based on the calculated DSfluid/melt and a careful selection of previously published data, an empirical model is proposed for basaltic magmas to predict the variation of DSfluid/melt values with variations in P (25–300 MPa), T (1030–1200 C) and fO2 (between NNO– 0 8 and NNOþ2 4). Olivine-hosted melt inclusions (Fo89-91) from tephra of the prehistoric (4 ka BP) sub-plinian picritic eruption, named FS (‘Fall Stratified’), have been investigated for their major element compositions, volatile contents and iron speciation (expressed as Fe3þ/PFe ratio). These primitive MIs present S content from 235677 to 34456168 ppm, and oxygen fugacity values, estimated from Fe3þ/PFe ratios, range from NNOþ0 760 2 to NNOþ1 660 2. Iron speciation has also been investigated in more evolved and volatile-poorer Etnean MIs. The only primitive melt inclusion from the Mt Spagnolo eruption (4–15 ka BP) presents a S content of 1515649ppm and an estimated fO2 of NNOþ1 460 1. The more evolved MIs (from 2002–2003, 2006, 2008–2009 and 2013 eruptions) have S content lower than 500 ppm, and their Fe3þ/RFe ratios result in fO2 between NNO– 0 960 1 and NNOþ0 460 1. Redox conditions and S behaviour in Etnean magmas during degassing and fractional crystallization were modelled coupling MELTS code with our empirical DSfluid/melt model. Starting from an FS-type magma composition and upon decrease of T and P, fractional crystallization of olivine, clinopyroxene, spinel and plagioclase causes a significant fO2 decrease. The fO2 reduction, in turn, causes a decrease in sulphur solubility and an increase in DSfluid/melt, promoting S exsolution during magma ascent, which further enhances the reduction of fO2. For the evolved MIs of 2002–2013 eruptions, magma differentiation may therefore have played a crucial role in decreasing redox conditions and favouring efficient S degassing. Differently, during the unusual FS eruption, only limited melt evolution is observed and S exsolution seems to have been triggered by a major pressure decrease accompanied by H2O and CO2 exsolution during fast magmatic ascent.

Description: Published

Description: egaa095

Description: 4V. Processi pre-eruttivi

Description: JCR Journal

Description: The scaling of earthquake parameters with seismic moment and its interpretation in terms of self- similarity is still debated in the literature. We address this question by examining a worldwide compilation of corner frequency-based and elastic rebound theory (ERT)-based fault slip, area and stress drop values for earthquakes ranging in magnitude from -0.7 to 7.8. We find that corner frequency estimates of slip (and stress drop) scale differently than those inferred from the ERT approach, where the latter deviates from the generally accepted constant stress drop behavior of so- called self-similar scaling models. We also find that average slips from finite-source models are consistent with corner frequency scaling, whereas peak slip values are more consistent with the ERT scaling. The different scaling of corner frequency- and ERT-based estimates of slip and stress drop with earthquake size is interpreted in terms of heterogeneity of the rupture process. ERT-based estimates of stress drop decrease with seismic moment suggesting a self-affine behavior. Despite the inferred heterogeneity at all scales, we do not observe a clear effect on the Brune stress drop scaling with earthquake size.

Description: Published

Description: 1771–1781

Description: 2T. Deformazione crostale attiva

Description: JCR Journal

Description: The volcano Chichontepeque (San Vicente) is one of the nine recent volcanoes making up the El Salvador sector of the WNW-ESE-trending active Central American volcanic belt. Thermal activity is at present reduced to a few thermal springs and fumaroles. The most important manifestations (Agua Agria and Los Infernillos Ciegos) are boiling springs and fumaroles located on the northern slope of the volcano (850 m a.s.l.) along two radial faults. The chloride acid waters of the Los Infernillos area are partly fed by a deep hydrothermal aquifer (crossed at 1100–1300 m by a geothermal exploration well), which finds a preferential path to the surface through the radial fault system. C02 is the most important gas (〉90%) of the Los Infernillos Ciegos and Agua Agria fumaroles. Part of the Los Infernillos gases may also come from a deeper, hotter source, given their high HCl/Stot. ratio and their more reducing conditions. The application of geothermometric and geobarometric methods to the gases and thermal waters suggests that both thermal areas are linked to the identified 1100–1300 m reservoir, whose temperature (250°C), lateral extension and chemical composition, as resulting from this study, are of interest for industrial development.

Description: Salvadorian State Agency for Electric Power (CEL)

Description: Published

Description: 83-97

Description: 1TR. Georisorse

Description: JCR Journal

Description: All the actions carried out in 1992 to protect the village of Zafferana Etnea from being invaded by lava are described. An earthen barrier 234 m long and 21 m high was firstly built in January 1992 by accumulating with mechanical escavators 370,000 m3 of earth, scoriae and stones. This embankment contained the lava for about one month and was overflowed by April 9, 1992. Three additional smaller earthen barriers (lenght: 90–160 m; height: 6–12 m) were built in April to gain time while the lava front was descending towards Zafferana from the overflowed first embankment. The major effort of the 1992 operation consisted of several attempts at stopping the lava front advance by diverting the flow out from the natural and extensively tunnelled channel through a skylight near the vent. The main intervention point was located in Valle del Bove at an elevation of 2000 m, at 8 km from Zafferana, in a zone almost unaccessible from land: helicopters were hence extensively used during the whole operation. Initial interventions called for attempts at plugging a tunnel by dumping into it linked concrete blocks, hedgehogs and blasted portions of the solid levee. Each intervention caused the partial obstruction of the tunnelled channel, which determined major increases of lava overflow in Valle del Bove and the consequent halt of the most advanced fronts. However, benefits were of brief duration, at the most two weeks of respite, before new lava fronts approached again and again the outskirts of Zafferana. The final successful intervention was carried out on May 27–29. An artificial channel was dug departing from the natural one. The solid separation levee was thinned to 3 m and blasted by 7000 kg of explosives. After the explosion, of the lava flowed spontaneously in the artificial channel and then the total diversion was obtained, the tunnel being plugged by dumping into the natural flow 230 m3 of lava boulders. As a consequence of the intervention the active natural lava front, that on May 27 was only 850 m from Zafferana, came to an halt, as did the entire flow downhill from the diversion point, bringing back the situation as it was five months earlier, a few days after the beginning of the eruption, with the new front of the diverted flow at 6–7 km from Zafferana. In June 1992, the effusion rate halved from 30 to 15 m3/s and with this reduced thrust the lava was no longer capable of covering long distances. Five months after the conclusive intervention, the diverted lava continues to flow over its initial natural field but remaining confined in the upper Valle del Bove, without any new threat to Zafferana.

Description: Published

Description: 1-34

Description: 5V. Processi eruttivi e post-eruttivi

Description: JCR Journal