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

Abstract

We reanalyzed the seismic and deformation data corresponding to the preeruptive unrest on El Hierro (Canary Islands) in 2011. We considered new information about the internal structure of the island. We updated the seismic catalog to estimate the full evolution of the released seismic energy and demonstrate the importance of nonlocated earthquakes. Using seismic data and GPS displacements, we characterized the shear‐tensile type of the predominant fracturing and modeled 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 postinjection 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 preexisting 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 toward the surface. The final phase involved magma ascent through a prefractured crust.

Crustal Strength Variations Inferred From Earthquake Stress Drop at Axial Seamount Surrounding the 2015 Eruption

“…Rapid reductions in seismic velocity on the order of 1% have been observed during inflation weeks to days before volcanic eruptions (e.g., Brenguier et al, 2008;Carrier et al, 2015;Duputel et al, 2019;Hotovec-Ellis et al, 2015;Lamb et al, 2017;Olivier et al, 2019). Pre-eruptive reductions in seismic velocity may be attributed to damage accumulation and increased permeability in the volcanic edifice during magma ascent (e.g., Carrier et al, 2015;Lamb et al, 2017;López et al, 2017;Olivier et al, 2019). While Tan et al (2016) observed a rapid decrease in seismic velocity of 0.3% at Axial weeks before the eruption, we do not resolve a corresponding decrease in Δσ ( Figure S8a).…”

Magma Flow Rates and Temporal Evolution of the 2012–2014 Post‐Eruptive Intrusions at El Hierro, Canary Islands

“…From ~27 September onward, magmatic activity changed, with an increase in the surface deformation rate and deeper seismic hypocenters, up to 16 km depth. The deepening of seismicity with time has been interpreted to reflect the increase in depth of a strained crustal volume due to additional magma emplacement (López et al, 2017), tracking an inclined structure (Domínguez Cerdeña et al, 2018;Martí et al, 2017). On 3 October, the focal mechanism of earthquakes changed from thrust to strike-slip, probably due to a change in the stress field, with one of the nodal planes aligned with the direction of maximum horizontal compressive stress in the island (López et al, 2017).…”

“…The deepening of seismicity with time has been interpreted to reflect the increase in depth of a strained crustal volume due to additional magma emplacement (López et al, 2017), tracking an inclined structure (Domínguez Cerdeña et al, 2018;Martí et al, 2017). On 3 October, the focal mechanism of earthquakes changed from thrust to strike-slip, probably due to a change in the stress field, with one of the nodal planes aligned with the direction of maximum horizontal compressive stress in the island (López et al, 2017). A M w 4 earthquake on 8 October, the largest event during the entire pre-eruptive seismic swarm, has been interpreted to mark the beginning of the magma upward migration toward the surface (e.g., López et al, 2017).…”

A retrospective study of the pre-eruptive unrest on El Hierro (Canary Islands): implications of seismicity and deformation in the short-term volcanic hazard assessment

“…S1). The bandwidth is a free smoothing parameter included in the kernel function that we used to estimate the corresponding probability density functions and determines how probabilities are distributed in terms of the distance from the volcanic structures or vents (Martí and Felpeto, 2010;Bartolini et al, 2013).…”

“…In the evaluation of the final susceptibility, weights were assigned based on expert opinion and on previously published work (Becerril et al, 2013(Becerril et al, , 2014 and by taking into account the average depth of the seismicity during the unrest episode. In detail, the relevance and reliability values (Table 3) (Martí and Felpeto, 2010) have been assigned as follow: relevance was given through an elicitation of expert judgment procedure (Aspinall, 2006) among the members of the Group of Volcanology of Barcelona (GVB-CSIC) and external collaborators; reliability was considered as maximum in all the datasets (value of 1).…”