Geophysical evidence of the preparedness for a volcanic eruption: El Hierro 2011-2012
[eng] The observational data recorded by the monitoring networks during El Hierro 2011-2012 volcanic unrest and eruption, allowed us to conduct a study of the volcanic eruptive phenomena with that wide view. The lack of previous instrumental data describing such a process in the Canaries gave us the...
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| Formato: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2017 |
| País: | España |
| Recursos: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/119772 |
| Acesso em linha: | https://hdl.handle.net/2445/119772 http://hdl.handle.net/10803/461580 |
| Access Level: | acceso abierto |
| Palavra-chave: | Vulcanologia Sismologia Geofísica Hierro (Canàries) Volcanology Seismology Geophysics Hierro (Canary Islands) |
| Resumo: | [eng] The observational data recorded by the monitoring networks during El Hierro 2011-2012 volcanic unrest and eruption, allowed us to conduct a study of the volcanic eruptive phenomena with that wide view. The lack of previous instrumental data describing such a process in the Canaries gave us the invaluable chance of analyzing for the first time, novel geophysical signals registered before, during and after an eruptive event, allowing the study of evidence of the energy involved in the emplacement and migration of magma through the lithosphere. The aim of this PhD Thesis is to make a comprehensive revision of the physics of this eruptive process and its causes, through the analysis and processing of geophysical data which has been recorded since the time of the first instrumental record to exist in the area, to the present. Three main aspects are considered, focusing on the eruptive phenomenon of El Hierro at different temporal and spatial scales, going from the general to the specific. The first aspect is the study of the potential relation between the regional geodynamics and the eruptive event of El Hierro 2011–2012. This potential relation, through the analysis of long-time series of geophysical observational data, has not been studied before. Highly reliable seismic and geodetic data acquired from 1996 to 2014 is analyzed, covering: from the North Atlantic Ridge to the West, to the Azores-Gibraltar boundary to the North (including the NW African margin) and the Canary Islands. A joint regional- and local scale analysis based on this data enabled the identification of early signs of anomalous tectonic activity from 2003 onwards, the intensity of which increased in2007,accelerating three months before the onset of the volcanic eruption on El Hierro in October 2011. The second aspect is the study of the precursory signals recorded during the El Hierro volcanic episode (19 July to 10 October, 2011), and their correct interpretation when only limited data is available. The seismic and deformation data corresponding to the pre-eruptive unrest is reanalyzed using novel methods, taking into account new information about the internal structure of the island. Results indicate that important changes in the medium properties and in the magmatic mechanism occurred throughout the process, identifying different phases with distinct types of fracturing. A triggered shear seismicity is observed on pre-existent faults caused by the magma pressure on the structure underneath the island, and the crossing of the Moho discontinuity. The third aspect is the detailed study of the type and geometry of the irreversible mechanism, acting during the final phase of the unrest episode (6-10 October, 2011) and during the first days of tremor (10-15 October, 2011) recording on the seismic monitoring stations. Application of time varying fractal analysis to the seismic data and the characterization of the seismicity pattern and the strain and the stress rates, allows the identification of different stages in the source mechanism, and to infer the geometry of the path used by the magma and associated fluids to reach the Earth’s surface. |
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