Fracture characterization at Nabro Volcano after June 2011 eruption

Thomas Loriaux*1, Berhe Goitom1, Michael Kendall1, James Hammond2, Clive Oppenheimer3, Ghebrebrhan Ogubazghi4, James Verdon1, Alan Baird1, James Wookey1

 1School of Earth Sciences, University of Bristol, UK (, 2Department of Earth and Planetary Sciences, Birkbeck, University of London, UK, 3Department of Geography, University of Cambridge, UK, 4Department of Earth Sciences, Eritrea Institute of Technology, Asmara, Eritrea

Nabro Volcano, situated in the Afar depression, erupted on 12 June 2011. This first recorded eruption was one of the most unexpected large volcanic events of the last decade. Nabro Volcano is considered to be one of the most promising geothermal reservoirs in Eritrea, and fracture characterization provides a mean to help investigate its potential. A few weeks after the eruption, a temporary network of eight three-component seismometers was deployed around the volcano. We base our study on a dataset containing events manually located below the caldera. We use shear wave splitting (SWS) to investigate the fracture network lying in the shallow brittle zone above the magmatic chamber. Fracture preferred orientations are inferred from the orientation of the fast shear wave polarization. We also calculate high b-values, suggesting the presence of a dense fracture network with fluid content.Current focus is on the inversion of the SWS parameters (Φ polarization of the fast shear wave and δt delay time between the fast and the slow shear waves) for fracture compliance. Fracture compliance has been shown to be a good indicator of fracture saturation in reservoir settings. Overall fracture characteristics as well as spatial and temporal variability will be investigated.