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.     

Structure of the vertical velocity field analyzed with a Doppler Sodar

Fifteen hours of Sodar echoes, collected during the 4th Environmental CEC Campaign at Turbigo, Italy, in September 1979, have been analyzed. We discuss the vertical profiles and the time evolution of the second and third statistical moments of the vertical component of wind velocity, measured at a rate of one height-profile every 6 s for a height range of 1000 m and a vertical resolution of 29 m. We also analyze the power spectra of the vertical velocity.     

Chemical and isotopic variations in fumarolic discharge and thermal waters at Vulcano Island (Aeolian Islands, Italy) during 1996: evidence of resumed volcanic activity

Gas samples from some fumaroles at ‘La Fossa' crater and Baia di Levante on Vulcano Island and from a diffuse soil gas emission were analysed during 1995–1996, along with water samples from thermal wells in the area of Vulcano Porto. During 1996, we observed a significant increase both in the gas/steam ratio and in the CO₂ concentration, as well as strong variations in δ¹³C_CO2, δD_H2O and δ¹⁸O_H2O of fumarolic gases. These variations are probably related to an increased inflow of deep fluids of magmatic origin. The temperatures of fumaroles did not show remarkable variations except for fumarole F11. In this case, temperature increased by about 80°C from February to August 1996. During the same period, remarkable variations in temperature, phreatic level and chemical and isotopic composition of water were also recorded in one of the geothermal wells in the Vulcano Porto area (Camping Sicilia; T60°C). The observed variations in this well are probably related to a pressure build-up, occurring at least in the surficial part of the system, because of increased gas flux and/or decreased permeability of the fumarolic degassing system. Chemical and isotopic composition of the water showed that during this evolutionary phase, the content of fumarolic condensate in this well was about 80 to 90%. Based on the observation of physical and chemical variables of the Camping Sicilia fluids, during this phase of activity, it is concluded that this area is affected by a phreatic eruption hazard if a volcanic episode with high energy discharge in a limited time span occurs. It follows that this well may be considered as a preferential point for volcanic activity monitoring, both in the case of normal routine surveillance and in the case of inaccessibility to the crater area.     

A 1D P-wave velocity model of the Gargano promontory (south-eastern Italy)

We investigate the elastic properties of the crust in the Gargano promontory, located in the northern part of the Apulia region (Southeastern Italy). Starting on April, 2013, a local-scale seismic network, composed of 12 short-period (1 Hz) seismic stations, was deployed on the Gargano promontory. Starting on October, 2013, the network was integrated with the recordings of nine seismic stations managed by the Italian Institute of Geophysics and Volcanology (INGV). The network recorded more than 1200 seismic events in about 15 months of data acquisition, with more than 700 small magnitude events localized in the Gargano promontory and surrounding areas. A Wadati-modified method allowed us to infer V_P/V_S = 1.73 for the area. A subset of about 400 events having a relatively smaller azimuthal gap (<200°) was selected to calibrate a 1D P-wave velocity model of the area, using the VELEST inversion code. The preferred model was obtained from the average of ten velocity models, each of them representing the inversion result from given initial velocity models, calibrated on previous geological and geophysical studies in the area. The results obtained under the assumption that V_P could decrease with depth are unstable, with very different depths of the top of low-velocity layers. Therefore, the velocity model was obtained from the average of the results obtained under the assumption that V_P cannot decrease with depth. A strong reduction of both RMS (about 58%) and errors on the location of the events was obtained with respect to the starting model. The final velocity model shows a strong velocity gradient in the upper 5 km of the crust and a small increase (from 6.7 to 7 km) at 30 km of depth. The epicenters of relocated events do not show clear correlations with the surface projection of known seismic faults. A cluster of the epicenters of the relocated events intersects almost perpendicularly the Candelaro fault trace at the surface.