The number of earthquakes recorded since July 17 on El Hierro, the smallest of The Canary Islands (Spain), exceeded 8,850 on Saturday.
The surge in the number and intensity of earthquakes in the past week has prompted officials from the Instituto Geografico Nacional (IGN) and The Canary Islands Government to raise the alert level for the Hierro volcano to ‘Yellow’, the highest alert status since the unprecedented earthquake swarm commenced in mid-July.
Hierro, a shield volcano, has had a single historic eruption from the Volcan de Lomo Negro vent in 1793. The eruption lasted approximately one month and produced lava flows.
The Canary Islands Government commenced an in-depth geological survey of El Hierro earlier this month in an effort to determine the source of an earthquake swarm. The Government raised the volcanic risk level to ‘Yellow’ last weekend after a surge in seismic activity
The IGN announced on Saturday that the number of earthquakes felt by the local population (approx 10,000) has reached 55. The strongest of the tremors, which have been recorded at depths between 10km and 15km, measured 3.8 magnitude on the Richter Scale.
Spain’s national seismological agency warned the local population to be prepared for any possible future increase in volcanic activity. The agency noted, however, that the majority of earthquakes recorded during recent days have been centred to the south of the island in Las Calmas Sea. Previous to this, the vast majority of the tremors were recorded in the northwest of the 278.5-square-kilometre island at El Golfo.
Over 150 earthquakes were recorded on the smallest of the Canary Islands during Tuesday prompting officials to evacuate some local residents, shut El Hierro’s main tunnel, and close local schools.
A surge in seismic activity on Tuesday prompted the Spanish Civil Guard (Guardia Civil) to advise almost 50 residents of the municipality of La Frontera to leave their homes because of landslide fears. Two units of the Spanish military’s emergency intervention unit (EMU) were also placed on standby to depart the nearby island of Tenerife to assist in the possible evacuation of hundreds of other El Hierro residents.
Meanwhile, the island’s main tunnel (Tunel del Golfo), which links Frontera to Valverde, was shut forcing motorists to travel across the 280-sq-km island via a mountain road. The Cabildo de El Hierro also ordered the closure of schools on Wednesday.
Eruption would have “low explosion value”
Speaking to the El Pais newspaper, volcanologist Juan Carlos Carracedo suggested that an eruption on El Hierro would “not be a major surprise”. He explained: “It is the youngest of the Canary Islands. There is a ball of magma which is rising to the surface and it is stationed at the limit of the earth’s crust. At the moment we do not know if that ball of magna will break the crust and cause an eruption.”
IGN Director, María José Blanco said that any eruption on El Hierro would most likely have a “low explosion value”. He added that an imminent eruption is unlikely.
Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey (USGS) points out that an event may last for days, weeks, or months.
4-D Plot Of El Hierro Earthquakes July-September 2011: youtube video.
WEBCAM: Las Puntas de Frontera, El Hierro
El Hierro’s Volcanic/Seismic Past
El Hierro is situated in the most southwestern extreme of the Canaries. The island was formed after three successive eruptions, and consequent accumulations, the island emerged from the ocean as an imposing triangular pyramid crowned by a volcano more than 2,000 metres high.
The volcanic activity, principally at the convergence of the three ridges, resulted in the continual expansion of the island. A mere 50,000 years ago, as a result of seismic tremors which produced massive landslides, a giant piece of the island cracked off, crashed down into the ocean and scattered along the seabed. This landslide of more than 300km3 gave rise to the impressive amphitheatre of the El Golfo valley and at the same time caused a tsunami that most likely rose over 100 metres high and probably reached as far as the American coast.
According to the Global Volcanism Program, the massive Hierro shield volcano is truncated by a large NW-facing escarpment, seen here from the east, which formed as a result of gravitational collapse of the volcano. The steep-sided 1500-m-high scarp towers above a low lava platform bordering 12-km-wide El Golfo Bay, which is barely visible at the extreme left. Holocene cones and flows are found both on the outer flanks and in the El Golfo depression. The last eruption, during the 18th century, produced a lava flow from a cinder cone on the NW side of El Golfo.
El Golfo, El Hierro, The Canary Islands (Spain)
According to ElHierro.com: “Although over 200 years have elapsed since the last eruption, El Hierro has the largest number of volcanoes in the Canaries with over 500 open sky cones, another 300 covered by the most recent outflows, and some 70 caves and volcanic galleries, notably the Don Justo cave whose collection of channels surpasses 6km in length.”
El Hierro is located south of Isla de la Palma (population 86,000), currently the most volcanically active of the Canary Islands. About a half a million years ago, the volcano, Taburiente, collapsed with a giant landslide, forming the Caldera de Taburiente. Since the Spanish occupation, there have been seven eruptions.