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

Abstract

We have studied the focal mechanism of an M w 4.0 earthquake that oc-

Stress barriers controlling lateral migration of magma revealed by seismic tomography

“…Magma only ascended to the surface when it found a suitable path for it, which corresponded to an east-west oriented strike slip fault that opened due to a combination of tectonic and magmatic stresses5152. This is in agreement with the regional stress field assumed for El Hierro in which the maximum compressional stress is oriented approximately north-south and the minimum compressional stress is east-west25153 and with the fact that the rift zones do not correspond to deep structures inside the volcanic edifice. An additional evidence of the shallow role of the rift structures is that it is now clear that magma ascended mostly aseismically for nearly 15 km - in about 30 h through a E-W regional fault.…”

“…An additional evidence of the shallow role of the rift structures is that it is now clear that magma ascended mostly aseismically for nearly 15 km - in about 30 h through a E-W regional fault. This fault has an orientation normal to that of the southern rift and was opened by an earthquake of magnitude 4.3 with a transcompressional focal mechanism51. When magma arrived at very shallow depths it was captured by a N-S fissure of the southern rift zone starting the eruption along that fissure2.…”

Seismic hydraulic fracture migration originated by successive deep magma pulses: The 2011–2013 seismic series associated to the volcanic activity of El Hierro Island

“…The configuration and density of the seismic network on El Hierro only allowed estimating focal mechanisms for a limited number of earthquakes. Del Fresno et al [] calculated the focal mechanism of the largest preeruption earthquake ( M w 4.0, occurred on 8 October 2011) and of 34 of its aftershocks. This earthquake was located at 12 km depth.…”

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

“…At least 90 felt earthquakes were reported by the population and a five day-long cycle of deflation/reinflation was observed (Domínguez Cerdeña et al, 2014;López et al, 2012;Martí et al, 2013b). On 8 October 2011, the largest earthquake of the unrest swarm, a Mw 4.0 event, occurred at a focal depth of 12-13 km (del Fresno et al, 2015). This was followed swiftly by the onset of a swarm of low magnitude, shallow earthquakes (>80 events in 30 hours, with depths ≤4 km and magnitudes <M2.5) clustered at a distance of ~13 km south from the previous site of activity (Fig.…”