Earthquake Activity Intensifies On El Hierro (Canary Islands)

El Hierro, The Canary Islands, visible from space in this

MODIS satellite image

The Instituto Geografico Nacional (IGN) has reported an increase in the intensity of earthquakes recorded on El Hierro, the smallest of The Canary Islands, during the last 24 to 36 hours. The number of earthquakes recorded since July 17 , 2011 on El Hierros has now reached 10,000, figures from the IGN confirm.

The IGN also confirmed surface deformations exceeding 35mm on the 280-sqkm island, where residents have been put on alert for a possible volcanic eruption. However, seismologists have moved to reassure the local population that a volcanic eruption is not imminent.

The agency confirmed on Friday that 890 earthquakes, most of them located in the sea to the SW of the island, have been recorded in the 7 days since 01 October, 2011. During this period, 76 earthquakes were felt by the island’s estimated 10,000 residents.

Since Friday morning, there have been 14 earthquakes exceeding 3.0 on the Richter Scale with epicentres both North and South of the NW Ridge and depths between 10 and 15 km have been recorded. The strongest of the tremors measured 3.8 magnitude on the Richter Scale. A total of 17 earthquakes were felt by the local population.

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 recent surge in the number and intensity of earthquakes prompted officials from the IGN and The Canary Islands Government to raise the alert level for the Hierro volcano to ‘Yellow’ late last month. The alert remained in place on Saturday.

Seismologists say the majority of the earthquake activity has shifted from El Golfo in the island’s northwest to beneath the Las Calmas Sea in the south.

However, magma is now on the move upwards while the depth of earthquakes has become increasingly shallow in recent days with most being recorded at a depth of 9 to 14 kilometres. Movement of magma towards the surface signifies that a volcanic eruption is likely to happen, but the timing of such remains unclear.

Volcanologist Juan Carlos Carracedo last week 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”.

Evacuations

A dramatic rise in recorded earthquakes on El Hierro last week prompted officials to evacuate some local residents, shut El Hierro’s main tunnel, and close local schools.

The Spanish Civil Guard (Guardia Civil) advised 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.

Earthquake Swarms

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.

Latest seismic activity on El Hierro

El Hierro’s Volcanic/Seismic Past

El Golfo, El Hierro, The Canary Islands (Spain) where the majority of earthquakes have been recorded up to recently

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.

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.