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  • 1
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    Acoustic Emission spectra classification from rock samples of Etna basalt in deformation-decompression laboratory experiments (2011)
    Springer
    Details
    Publication Date: 2020-02-24
    Description: Recent laboratory experiments on Etna basalt have permitted the generation of an extensive catalogue of acoustic emissions (AE) during two key experimental phases. Firstly, AE have been generated during triaxial compressional tests and formation of a complex fracture/damage zone. Secondly, rapid fluid decompression through the damage/shear zone after failure. We report new results from an advanced analysis method using AE spectrograms, allowing us to qualitatively identify high and low frequency events; essentially comparable to seismicity in volcanic areas. Our analysis, for the first time, quantitatively classifies ‘families’ of AE events belonging to the same experimental stage without prior knowledge. We then test the method using the AE catalogue for verification, which is not possible with field data. FFT spectra, obtained from AE, are subdivided into equal log intervals for which a local slope is calculated. Factor analysis has been then applied, in which we use a data matrix of columns representing the variables considered (frequency data averaged in bins) vs. rows indicating each AE data set. Factor analysis shows that the method is very effective and suitable for reducing data complexity, allowing distinct factors to be obtained. We conclude that most of the data variance (information content) can be well represented by three factors only, each one representing a well defined frequency range. Through the factor scores it is possible to represent data in a lower dimension factor space. Classification is then possible by identifying clusters of AE belonging to the same experimental stage. This allows us to propose a deformation/decompression interpretation based solely on the AE frequency analysis and to identify a third type of AE related to fluid movements in the deformation stage.
    Description: Published
    Description: 201-211
    Description: 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive
    Description: open
    Keywords: acoustic emissions ; 04. Solid Earth::04.06. Seismology::04.06.08. Volcano seismology ; 04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks ; 05. General::05.01. Computational geophysics::05.01.04. Statistical analysis
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: book chapter
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  • 2
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    Effect of phase transitions on seismic properties of metapelites: A new high-temperature laboratory calibration (2013)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2013-03-22
    Description: We report new measurements of the seismic properties of an upper amphibolite facies metapelite from the Serie dei Laghi basement (southern Alps, northern Italy), presenting evidence for the α-β transition using elastic wave velocity methods and under in-situ conditions. Measurements were made using an internally heated gas apparatus under hydrostatic conditions to 500 MPa and temperatures to 1075 K. P-wave and S-wave velocities were measured parallel and normal to both lineation and foliation at room temperature. An average V p (at 400 MPa) of 6.4 km/s and V p anisotropy of 15% were found, and an average V s1 of 3.6 km/s (8.2% anisotropy) and V s2 of 3.5 km/s (4.1% anisotropy) were also measured. To observe the effects of the α-β quartz transition under hydrostatic conditions, elastic wave velocity was monitored continuously as the sample was heated to 1075 K. At 400 MPa, V p was seen to decrease monotonically with increasing temperature to 950 K; as temperature continued to increase, V p rapidly increased, until 1075 K. The effect was found to be reversible and is interpreted in terms of the α-β transition in quartz, in broad agreement with previous studies. A simple interpretation of the data, taking into account a field case from southern Tuscany as well as natural and elevated geothermal gradient, suggests that thermal and hydraulically driven fracture is a likely cause of "bright spots" seen in these areas. With knowledge of the depth of such features, the transition additionally allows the temperature at depth to be estimated.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 3
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    Reconstructing magma failure and the degassing network of dome-building eruptions (2013)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2013-03-22
    Description: Volcanic eruptions are regulated by the rheology of magmas and their ability to degas. Both detail the evolution of stresses within ascending subvolcanic magma. But as magma is forced through the ductile-brittle transition, new pathways emerge as cracks nucleate, propagate, and coalesce, constructing a permeable network. Current analyses of magma dynamics center on models of the glass transition, neglecting important aspects such as incremental strain accommodation and (the key monitoring tool of) seismicity. Here, in a combined-methods study, we report the first high-resolution (20 μm) neutron-computed tomography and microseismic monitoring of magma failure under controlled experimental conditions. The data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain-to-failure and importantly, the geometry and efficiency of the permeable fracture network that regulates degassing events. Extrapolation of our findings yields magma ascent via strain localization along conduit margins, thereby providing an explanation for gas-and-ash explosions along arcuate fractures at active lava domes. We conclude that a coupled deformation-seismicity analysis holds a derivation of fracture mechanisms and network, and thus holds potential application in forecasting technologies.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 4
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    Decoupling of paramagnetic and ferrimagnetic AMS development during the experimental chemical compaction of illite shale powder (2013)
    Oxford University Press
    Details
    Publication Date: 2013-02-15
    Description: Inclination shallowing of detrital remanent magnetization in sedimentary strata has solely been constrained for the mechanical processes associated with mud deposition and shallow compaction of clay-rich sediment, even though a significant part of mud diagenesis involves chemical compaction. Here we report, for the first time, on the laboratory simulation of magnetic assemblage development in a chemically compacting illite shale powder of natural origin. The experimental procedure comprised three compaction stages that, when combined, simulate the diagenesis and low-grade metamorphism of illite mud. First, the full extent of load-sensitive mechanical compaction is simulated by room temperature dry axial compression. Subsequently, temperature controlled chemical compaction is initiated by exposing the sample in two stages to amphibolite or granulite facies conditions (temperature is 490 to 750°C and confining pressure is 170 or 300 MPa) both in the absence (confining pressure only) and presence of a deformation stress field (axial compression or confined torsion). Thermodynamic equilibrium in the last two compaction stages was not reached, but illite and mica dehydroxylation initiated, thus providing a wet environment. Magnetic properties were characterized by magnetic susceptibility and its anisotropy (AMS) in both high- and low-applied field. Acquisition of isothermal remanent magnetization (IRM), stepwise three-component thermal de-magnetization of IRM and first-order reversal curves were used to characterize the remanence-bearing minerals. During the chemical compaction experiments ferrimagnetic iron-sulphides formed after reduction of magnetite and detrital pyrite in a low sulphur fugacity environment. The degree of low-field AMS is unaffected by porosity reduction from 15 to ~1 per cent, regardless of operating conditions and compaction history. High-field paramagnetic AMS increases with compaction for all employed stress regimes and conditions, and is attributed to illite transformation to iron-bearing mica. AMS of authigenic iron-sulphide minerals remained constant during compaction indicating an independence of ferrimagnetic fabric development to chemical compaction in illite shale powder. The decoupling of paramagnetic and ferrimagnetic AMS development during chemical compaction of pelite contrasts with findings from mechanical compaction studies.
    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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  • 5
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    Volcanic conduit failure as a trigger to magma fragmentation (2011)
    Springer
    Details
    Publication Date: 2011-09-27
    Print ISSN: 0258-8900
    Electronic ISSN: 1432-0819
    Topics: Geosciences
    Published by Springer
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