Insights into the 2011-2012 submarine eruption off the coast of El Hierro (Canary Islands, Spain) from statistical analyses of earthquake activity

Abstract

SUMMARY The purpose of this work is to gain insights into the 2011–2012 eruption of El Hierro (Canary Islands) by mapping the evolution of the seismic b‐value. The El Hierro seismic sequence offers a rather unique opportunity to investigate the process of reawakening of an oceanic intraplate volcano after a long period of repose. The 2011–2012 eruption is a submarine volcanic event that took place about 2 km off of the southern coast of El Hierro. The eruption was accompanied by an intense seismic swarm and surface manifestations of activity. The earthquake catalogue during the period of unrest includes over 12 000 events, the largest with magnitude 4.6. The seismic sequence can be grouped into three distinct phases, which correspond to well‐separated spatial clusters and distinct earthquake regimes. The estimated b‐value is of 1.18 ± 0.03, and a magnitude of completeness of 1.3, for the entire catalogue. B is very close to 1.0, which indicates completeness of the earthquake catalogue with only minor departures from the linearity of Gutenberg–Richter frequency–magnitude distribution. The most straightforward interpretation of this result is that the seismic swarm reached its final stages, and no additional large magnitude events should be anticipated, similarly to what one would expect for non‐volcanic earthquake sequences. The results, dividing the activity in different phases, illustrate remarkable differences in the estimate of b‐value during the early and late stages of the eruption. The early pre‐eruptive activity was characterized by a b‐value of 2.25. In contrast, the b‐value was 1.25 during the eruptive phase. Based on our analyses, and the results of other studies, we propose a scenario that may account for the observations reported in this work. We infer that the earthquakes that occurred in the first phase reflect magma migration from the upper mantle to crustal depths. The area where magma initially intruded into the crust, because of its transitional nature is characterized by high fracturing, thus favours anomalously high b‐values. The larger magnitude earthquakes recorded in the second phase may reflect relaxation around the magma reservoir that had fed the eruption and, thus, lower b‐values.

The 8 October 2011 Earthquake at El Hierro (Mw 4.0): Focal Mechanisms of the Mainshock and Its Foreshocks

“…This time coincides to when López et al (2014) reports the beginning of the upward seismic hypocenter migration, the maximum release of seismic energy, and reaching the maximum strain rate of the preeruptive swarm and the first days of the eruption, and with gravity changes reported in Sainz-Maza Aparicio et al (2014). Ibáñez et al (2012) also observed a considerable decrease in b-values in that period compared to the previous weeks of the unrest.…”

“…The 2011-2012 eruption was the first historical eruption in El Hierro and the first documented submarine eruption in the whole archipelago. The eruption was preceded by a three-month dense seismic swarm (Carracedo et al, 2012;Ibáñez et al, 2012;López et al, 2012;Martí et al, 2013). The anomalous activity started on 19 July 2011 beneath the center of the island, at a depth of ∼10-15 km (Domínguez Cerdeña et al, 2014).…”

Comment on “Evidence from acoustic imaging for submarine volcanic activity in 2012 off the west coast of El Hierro (Canary Islands, Spain)” by Pérez NM, Somoza L, Hernández PA, González de Vallejo L, León R, Sagiya T, Biain A, González FJ, Medialdea T, Barrancos J, Ibáñez J, Sumino H, Nogami K and Romero C [Bull Volcanol (2014) 76:882-896]

“…This network is able to detect a seismic tremor with amplitude of the order of 0.1 cm 2 of reduced displacement. As suggested by McNutt (1992), this value can be obtained by taking into account the location of the eruption proposed in Pérez et al (2014) and the sensitivity of the seismic network, which is able to locate many earthquakes with magnitudes lower than 1 (see, for example, Ibáñez et al 2012 or the complete online IGN seismic catalog for the Canary I s l a n d s a t h t t p : / / w w w. i g n . e s / i g n / l a y o u t I n / (2010).…”

Repeated magmatic intrusions at El Hierro Island following the 2011–2012 submarine eruption

“…On 19 July 2011, an intense seismic swarm began at El Hierro Island Ibáñez et al, 2012;López et al, 2012;Martí et al, 2013b). Initial seismic activity of low magnitude (<M2.6) started in the central part of the island and migrated 3 km towards north at depths of 9-11 km over a period of one month (Domínguez Cerdeña et al, 2014).…”