Increasing rates of volcano-tectonic (VT) seismicity are one of the most common precursors before volcanic eruptions. Previous quantitative studies of VT precursors have focussed on volcanoes in subduction zones, with compressional background stress fields. To compare these studies with similar precursors observed before eruptions in extensional stress fields, VT seismicity was examined before ten flank eruptions at Mt Etna, in Sicily, between 1977 and 2008. Thirty percent of the eruptions were preceded by exponential increases in VT seismicity, with durations of months to years. Two exponential accelerations were also identified that did not result in an eruption. The locations of earthquakes suggest that pre-eruptive and non-eruptive VT sequences were both the result of the pressurisation of a magma chamber, causing stress changes in the surrounding crust and volcanic edifice. The behaviour of precursory sequences was further investigated through a series of rock-physics experiments in the laboratory. Samples of Etna basalt were deformed at a constant strain rate. Fracturing was recorded as acoustic emissions, which are analogues of VT events. Using a novel experimental design, experiments were performed in extensional, as well as in compressional, stress fields. In all cases, sample failure was preceded by exponential increases in the total number of acoustic emissions with time. The results are well described by a theoretical model for which the exponential trends reflect the probability of cracking under a constant strain rate and are characterised by the ratio of applied differential stress to a characteristic thermodynamic stress, calculated independently from the rock’s composition and the pressure and temperature of the experiment. The experimental results support the interpretation that exponential VT trends in the field can be used to follow the pressurisation of a magma body and, hence, they can potentially be used to improve forecasts of eruptions at volcanoes, regardless of the prevailing stress field.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:626566 |
| Date | January 2014 |
| Creators | Wall, R. J. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1426966/ |
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