Laura García-Cañada

Driving magma to the surface: The 2011–2012 El Hierro Volcanic Eruption

We reanalyzed the seismic and deformation data corresponding to the pre-eruptive unrest on El Hierro (Canary Islands) in 2011. We considered new information about the internal structure of the island. We updated the seismic catalogue to estimate the full evolution of the released seismic energy and demonstrate the importance of non-located earthquakes. Using seismic data and GPS displacements, we characterized the shear-tensile type of the predominant fracturing and modelled the strain and stress fields for different time periods. This enabled us to identify a prolonged first phase characterized by hydraulic tensile fracturing, which we interpret as being related to the emplacement of new magma below the volcanic edifice on El Hierro. This was followed by post-injection unidirectional migration, probably controlled by the stress field and the distribution of the structural discontinuities. We identified the effects of energetic magmatic pulses occurring a few days before the eruption that induced shear seismicity on pre-existing faults within the volcano and raised the Coulomb stress over the whole crust. We suggest that these magmatic pulses reflect the crossing of the Moho discontinuity, as well as changes in the path geometry of the dyke migration towards the surface. The final phase involved magma ascent through a pre-fractured crust.

Large landslide stress states calculated during extreme climatic and tectonic events on El Hierro, Canary Islands

Composed volcanic edifices are particularly prone to large-scale failures—these often result from the acceleration of preexisting deep-seated gravitational slope deformations. Consequently, a complete understanding of the kinematic behaviour of such slope deformations would represent an important step towards mitigating against human casualties or fatalities and damage to critical infrastructure. In this manuscript, a 9-month time series of three-dimensional fault displacement measurements has been used to determine the stress states of the San Andrés Landslide on El Hierro in the Canary Islands. These stress states have been calculated on the basis of single-displacement events using a novel approach which only requires information about the magnitude of the movement vector and its orientation. The analysis focused on four specific periods: a reference period in November 2013; an extreme rainfall event at the beginning of December 2013; and two endogenous impulses at the end of December 2013 and during the middle of March 2014. On the basis that the direction of principal stress represents a marker for the direction of landslide mass movement, it has been possible to define six landslide activity modes which correspond to specific stress states. The response of the landslide to the extreme rainfall event was immediate and reflected increasing saturation of the porous landslide mass. The response of the landslide to the endogenous impulses was more complicated as compressional pulses often alternated with gravitational relaxation. In this study, it is demonstrated that the landslide stress state can be determined on the basis of a single-displacement event whenever fault displacements are monitored in three dimensions. This innovative approach may represent a valuable step towards a complete understanding of the kinematic behaviour of potentially catastrophic slope deformations, particularly those which are in a critical stability state.