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  • 1
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    Hydrogeology of Stromboli volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms, self-potential, soil temperature and soil CO2 concentration measurements (2011)
    Wiley Blackwell
    Details
    Publication Date: 2022-06-13
    Description: To gain a better insight of the hydrogeology and the location of the main tectonic faults of Stromboli volcano in Italy, we collected electrical resistivity measurements, soil CO2 concentrations, temperature and self-potential measurements along two profiles. These two profiles started at the village of Ginostra in the southwest part of the island. The first profile (4.8 km in length) ended up at the village of Scari in the north east part of the volcano and the second one (3.5 km in length) at Forgia Vecchia beach, in the eastern part of the island. These data were used to provide insights regarding the position of shallow aquifers and the extension of the hydrothermal system. This large-scale study is complemented by two high-resolution studies, one at the Pizzo area (near the active vents) and one at Rina Grande where flank collapse areas can be observed. The Pizzo corresponds to one of the main degassing structure of the hydrothermal system. The main degassing area is localized along a higher permeability area corresponding to the head of the gliding plane of the Rina Grande sector collapse. We found that the self-potential data reveal the position of an aquifer above the villages of Scari and San Vincenzo. We provide an estimate of the depth of this aquifer from these data. The lateral extension of the hydrothermal system (resistivity ∼15–60 ohm m) is broader than anticipated extending in the direction of the villages of Scari and San Vincenzo (in agreement with temperature data recorded in shallow wells). The lateral extension of the hydrothermal system reaches the lower third of the Rina Grande sector collapse area in the eastern part of the island. The hydrothermal body in this area is blocked by an old collapse boundary. This position of the hydrothermal body is consistent with low values of the magnetization (〈2.5 A m−1) from previously published work. The presence of the hydrothermal body below Rina Grande raises questions about the mechanical stability of this flank of the edifice.
    Description: INSU-CNRS, Laboratoire GeoSciences Reunion-IPGP, INGV, DOE (Energy Efficiency and Renewable Energy, Geothermal Technologies Program, awardDE-FG36–08GO018195)
    Description: Published
    Description: 1078-1094
    Description: 1.8. Osservazioni di geofisica ambientale
    Description: JCR Journal
    Description: restricted
    Keywords: Tomography. ; Electrical properties ; Hydrogeophysics ; Volcano monitoring ; 04. Solid Earth::04.08. Volcanology::04.08.99. General or miscellaneous
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 2
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    Spatial and temporal dynamics of gas-related processes in the Sea of Marmara monitored with ocean bottom seismometers (2018)
    Oxford University Press
    Details
    Publication Date: 2018
    Description: 〈span〉〈div〉Abstract〈/div〉In the Sea of Marmara, areas of gas seepage or cold seeps are tightly related to the faults system and understanding the spatial and temporal dynamics in gas-related processes is crucial for geohazard mitigation. Although acoustic surveys proved to be efficient in detecting and locating cold seeps, temporal variability or trends in the gas-related processes are still poorly understood. Two arrays of 10 ocean bottom seismometers were deployed in the western part of the Sea of Marmara in 2011 and 2014, respectively. In addition to the local seismic events, the instruments recorded a large number of short duration events and long-lasting tremors. Short duration events are impulsive signals with duration 〈 1 s, amplitude well above the noise level and a frequency spectrum with one or two narrow peaks. They are not correlated from one site to another, suggesting a very local source. Tremors consist of sequences of clustered impulsive signals lasting for minutes to more than an hour with a multi-peak frequency spectrum. Based on evidence of known seepage and by analogy with volcanic and hydrothermal models, we suggest that short duration events and tremors are associated with gas migration and seepage. There is a relationship between tremors associated with gas emission and the local seismicity, although not systematic. Rather than triggering gas migration out of the seabed, locally strong earthquakes act as catalysts when gas is already present or gas emission is already initiated.〈/span〉
    Print ISSN: 2051-1965
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
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    Seismicity and active accretion processes at the ultraslow-spreading Southwest and intermediate-spreading Southeast Indian ridges from hydroacoustic data (2016)
    Oxford University Press
    Details
    Publication Date: 2016-07-03
    Description: Volcanic and tectonic events are the main processes involved in the generation of the oceanic crust and responsible for the seismicity associated with seafloor spreading. To monitor this activity, usually not or poorly detected by land-based seismological stations, we deployed from February 2012 to February 2013 a network of autonomous hydrophones to compare the behaviour of the ultraslow-spreading Southwest Indian ridge (SWIR) with that of the intermediate-spreading Southeast Indian ridge (SEIR). The rate of seismicity is similar for both ridges, suggesting that there is no systematic relationship between seismicity and spreading rates. The along-axis distribution of the seismic events, however, does differ, reflecting the rate dependence of accretion modes. Earthquakes are sparse and regularly spaced and scattered along the SWIR, reflecting prevailing tectonic processes. By contrast, along the SEIR, events are irregularly distributed and focus at ridge-segment ends and transforms faults, reflecting the ridge segmentation; only two swarms occurred at a segment centre and are probably caused by a magmatic event. This seismicity distribution thus looks controlled by segment-scale crustal heterogeneities along the SEIR and by regional-scale contrasting accretion processes along the SWIR, probably driven by different lithospheric and asthenospheric dynamics on either side of the Melville fracture zone. The comparison of hydroacoustic and teleseismic catalogues shows that, along these spreading ridges, the background seismicity observed in 1 yr by a hydroacoustic network is representative of the seismicity observed over two decades by land-based networks.
    Keywords: Marine Geosciences and Applied Geophysics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 4
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    Spatial and temporal dynamics of gas-related processes in the Sea of Marmara monitored with ocean bottom seismometers (2018)
    Oxford University Press
    Details
    Publication Date: 2018
    Description: 〈span〉〈div〉SUMMARY〈/div〉In the Sea of Marmara, areas of gas seepage or cold seeps are tightly related to the faults system and understanding the spatial and temporal dynamics in gas-related processes is crucial for geohazard mitigation. Although acoustic surveys proved to be efficient in detecting and locating cold seeps, temporal variability or trends in the gas-related processes are still poorly understood. Two arrays of 10 ocean bottom seismometers were deployed in the western part of the Sea of Marmara in 2011 and 2014, respectively. In addition to the local seismic events, the instruments recorded a large number of short duration events and long-lasting tremors. Short duration events are impulsive signals with duration 〈1 s, amplitude well above the noise level and a frequency spectrum with one or two narrow peaks. They are not correlated from one site to another, suggesting a very local source. Tremors consist of sequences of clustered impulsive signals lasting for minutes to more than an hour with a multipeak frequency spectrum. Based on evidence of known seepage and by analogy with volcanic and hydrothermal models, we suggest that short duration events and tremors are associated with gas migration and seepage. There is a relationship between tremors associated with gas emission and the local seismicity, although not systematic. Rather than triggering gas migration out of the seabed, locally strong earthquakes act as catalysts when gas is already present or gas emission is already initiated.〈/span〉
    Print ISSN: 2051-1965
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    Syn-eruptive, soft-sediment deformation of deposits from dilute pyroclastic density current: triggers from granular shear, dynamic pore pressure, ballistic impacts and shock waves (2015)
    Copernicus
    Details
    Publication Date: 2015-05-21
    Description: Soft-sediment deformation structures can provide valuable information about the conditions of parent flows, the sediment state and the surrounding environment. Here, examples of soft-sediment deformation in deposits of dilute pyroclastic density currents are documented and possible syn-eruptive triggers suggested. Outcrops from six different volcanoes have been compiled in order to provide a broad perspective on the variety of structures: Soufrière Hills (Montserrat), Tungurahua (Ecuador), Ubehebe craters (USA), Laacher See (Germany), and Tower Hill and Purrumbete lakes (both Australia). The variety of features can be classified in four groups: (1) tubular features such as pipes; (2) isolated, laterally oriented deformation such as overturned or oversteepened laminations and vortex-shaped laminae; (3) folds-and-faults structures involving thick (〉30 cm) units; (4) dominantly vertical inter-penetration of two layers such as potatoids, dishes, or diapiric flame-like structures. The occurrence of degassing pipes together with basal intrusions suggest fluidization during flow stages, and can facilitate the development of other soft-sediment deformation structures. Variations from injection dikes to suction-driven, local uplifts at the base of outcrops indicate the role of dynamic pore pressure. Isolated, centimeter-scale, overturned beds with vortex forms have been interpreted to be the signature of shear instabilities occurring at the boundary of two granular media. They may represent the frozen record of granular, pseudo Kelvin–Helmholtz instabilities. Their recognition can be a diagnostic for flows with a granular basal boundary layer. Vertical inter-penetration and those folds-and-faults features related to slumps are driven by their excess weight and occur after deposition but penecontemporaneous to the eruption. The passage of shock waves emanating from the vent may also produce trains of isolated, fine-grained overturned beds that disturb the surface bedding without occurrence of a sedimentation phase in the vicinity of explosion centers. Finally, ballistic impacts can trigger unconventional sags producing local displacement or liquefaction. Based on the deformation depth, these can yield precise insights into depositional unit boundaries. Such impact structures may also be at the origin of some of the steep truncation planes visible at the base of the so-called "chute and pool" structures. Dilute pyroclastic density currents occur contemporaneously with seismogenic volcanic explosions. They can experience extremely high sedimentation rates and may flow at the border between traction, granular and fluid-escape boundary zones. They are often deposited on steep slopes and can incorporate large amounts of water and gas in the sediment. These are just some of the many possible triggers acting in a single environment, and they reveal the potential for insights into the eruptive and flow mechanisms of dilute pyroclastic density currents.
    Print ISSN: 1869-9510
    Electronic ISSN: 1869-9529
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 6
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    Syn-eruptive, soft-sediment deformation of dilute pyroclastic density current deposits: triggers from granular shear, dynamic pore pressure, ballistic impacts and shock waves (2014)
    Copernicus
    Details
    Publication Date: 2014-12-16
    Description: Soft-sediment deformation produces intriguing sedimentary structures and can occur in diverse environments and from a variety of triggers. From the observation of such structures and their interpretation in terms of trigger mechanisms, valuable information can be extracted about former conditions. Here we document examples of syn-eruptive deformation in dilute pyroclastic density current deposits. Outcrops from 6 different volcanoes have been compiled in order to provide a broad perspective on the variety of structures: Ubehebe craters (USA), Tungurahua (Ecuador), Soufrière Hills (Montserrat), Laacher See (Germany), Tower Hill and Purrumbete lake (both Australia). Isolated slumps as well as sinking pseudonodules are driven by their excess weight and occur after deposition but penecontemporaneous to the eruption. Isolated, cm-scale, overturned beds with vortex forms have been interpreted to be the signature of shear instabilities occurring at the boundary of two granular media. They may represent the frozen record of granular, pseudo Kelvin–Helmholtz instabilities. Their recognition can be a diagnostic for flows with a granular basal boundary layer. The occurrence of degassing pipes together with basal intrusive dikes suggest fluidization during flow stages, and can facilitate the development of Kelvin–Helmholtz structures. The occurrence at the base of flow units of injection dikes in some outcrops compared with suction-driven local uplifts in others indicates the role of dynamic pore pressure. Variations of the latter are possibly related to local changes between depletive and accumulative dynamics of flows. Ballistic impacts can trigger unconventional sags producing local displacement or liquefaction. Based on the deformation depth, these can yield precise insights into depositional unit boundaries. Such impact structures may also be at the origin of some of the steep truncation planes visible at the base of the so-called "chute and pool" structures. Finally, the passage of shock waves emanating from the vent may be preserved in the form of trains of isolated, fine-grained overturned beds which may disturb the surface bedding without occurrence of a sedimentation phase in the vicinity of a vent. Dilute pyroclastic density currents occur contemporaneously with seismogenic volcanic explosions. They are often deposited on steep slopes and can incorporate large amounts of water and gas in the sediment. They can experience extremely high sedimentation rates and may flow at the border between traction, granular and fluid-escape boundary zones. These are just some of the many possible triggers acting in a single environment, and reveal the potential for insights into the eruptive mechanisms of dilute pyroclastic density currents.
    Electronic ISSN: 1869-9537
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 7
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    Hydrogeology of Stromboli volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms, self-potential, soil temperature and soil CO2 concentration measurements (2011)
    Oxford University Press
    Details
    Publication Date: 2011-09-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 8
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    Crustal seismic structure and anisotropy of Madagascar and south-eastern Africa using receiver function harmonics: interplay of inherited local heterogeneities and current regional stress (2021)
    Oxford University Press
    Details
    Publication Date: 2021-03-26
    Description: Summary This study investigates the seismic structure and anisotropy in the crust beneath Madagascar and south-eastern Africa, using receiver functions. The understanding of seismic anisotropy is essential for imaging past and present deformation in the lithosphere-asthenosphere system. In the upper mantle, seismic anisotropy mainly results from the orientation of olivine, which deforms under tectonic (fossil anisotropy) or flow processes (in the asthenosphere). In the crust, the crystallographic alignment of amphiboles, feldspars(plagioclase) or micas or the alignment of heterogeneities such as fractures, add to a complex geometry, which results in challenges to understanding the Earth's shallow structure. The decomposition of receiver functions into back-azimuth harmonics allows to characterize orientations of lithospheric structure responsible for azimuthally-varying seismic signals, such as a dipping isotropic velocity contrasts or layers of azimuthal seismic anisotropy. By analysing receiver function harmonics from records of 48 permanent or temporary stations this study reveals significant azimuthally-varying signals within the upper crust of Madagascar and south-eastern Africa. At 30 stations crustal anisotropy dominates the harmonics while the signature of a dipping isotropic contrast is dominant at the remaining 18 stations. However, all stations’ back-azimuth harmonics show complex signals involving both dipping isotropic and shallow anisotropic contrasts or more than one source of anisotropy at shallow depth. Our calculated orientations for the crust are therefore interpreted as reflecting either the average or the interplay of several sources of azimuthally-varying signals depending of their strength. However, comparing information between stations allows us to draw the same conclusions regionally: in both southern Africa and Madagascar our measurements reflect the interplay between local, inherited structural heterogeneities and crustal seismic anisotropy generated by the current extensional stress field imposed by the southward propagation of the East-African Rift System. A final comparison of our crustal orientations with SKS orientations attributed to mantle deformation further probes the interplay of crustal and mantle anisotropy on SKS measurements.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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