The Canary Islands Government says it has commenced an in-depth geological survey of El Hierro, the smallest of the islands, in an effort to determine the source of an earthquake swarm.
The unprecedented seismic activity commenced on 19 July. In excess of 6,000 earthquakes have been recorded up to 14 September 2011. More than two dozen tremors were recorded during Wednesday (14 Sept.) alone.
The vast majority of the tremors have been recorded in the northwest of the 278.5-square-kilometre island at El Golfo, the location of a massive landslide that created a 100-metre high tsunami almost 50,000 years ago. Theearth tremors have ranged between 1 and 3 magnitude, the National Geographic Institute (IGN) reported.
However, the Volcanological Institute of the Canary Islands and Actualidad Volcánica de Canarias have both reported a sizeable decrease in the number of tremors recorded during the first two weeks of September, compared to the latter half of August.
Location of tremors on El Hierro. Image Google Earth
El Instituto Volcanológico de Canarias (INVOLCAN) seismic report for August 2011
The National Geographic Institute confirmed on Wednesday that it has commenced a geological study of the epicentre of the tremors in the town of Frontera. Local officials admitted that the origin of the seismic movements could be volcanic, but further examination is required.
The Council of Hierro noted that there is no imminent threat to the residents of the sparsely populated island (10600 inhabitants), but refused to rule out an evacuation of island residents in the event of a heightened risk of a volcanic eruption.
The earthquake swarm, prompted the Canary Islands Government to convene the first ever meeting on 22 July of the Steering Committee and Volcanic Monitoring, reflected in the Specific Plan Protection Civil and Emergency for Volcanic Risk, given what it described “the significant increase in seismic activity”.
The Committee has met numerous times since then to discuss the low magnitude seismic activity. It reported on Monday that it had stepped up its seismic monitoring operations to identify the source of the earthquakes.
It remains unclear if the unprecedented seismic activity on El Hierro is a precursor to a possible future increase in earthquake or volcanic activity. However, the latest surge in recorded earthquakes and the inflation of the volcano could indicate magma rising underneath El Hierro.
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 latest eruption, during the 18th century, produced a lava flow from a cinder cone on the NW side of El Golfo.
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.
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. Volcanic activity, principally at the convergence of the three ridges, has 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 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.
Taburiente, La Palma, marked on Google Earth
Caldera de Taburiente. Image wiki
In a BBC Horizon programme broadcast on October 12, 2000, two geologists (Day and McGuire) hypothesised that during a future eruption, the western flank of the Cumbre Vieja, with a mass of approximately 1.5 x1015 kg, could slide into the ocean. This could then potentially generate a giant wave which they termed a “megatsunami” around 650–900 m high in the region of the islands. The wave would radiate out across the Atlantic and inundate the eastern seaboard of North America including the American, the Caribbean and northern coasts of South America some six to eight hours later. They estimate that the tsunami will have waves possibly 160 ft (49 m) or more high causing massive devastation along the coastlines. Modelling suggests that the tsunami could inundate up to 25 km (16 mi) inland – depending upon topography.
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