[breaking] Vulkaanuitbarsting bij Canarische Eilanden #2

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Recordaantal aardbevingen op Canarisch eiland

Nog nooit eerder werden er zoveel aardbevingen achter elkaar geregistreerd op het kleine vulkanische eilandje El Hierro, het kleinste van de Canarische Eilanden. De reeks van maar liefst 720 bevinkjes heeft de overheid van deze eilandengroep voor de kust van West Afrika ertoe doen besluiten om voor het eerst in de geschiedenis een vergadering bijeen te roepen van het Stuurcomité van Vulkanische Monitoring. Dit comité werd opgericht om specifieke plannen te realiseren, die de burgerbevolking dienen te beschermen en noodplannen voor vulkanische risico’s moeten ontwikkelen. Inmiddels wordt gesproken over een ‘significante dreiging van seismische activiteit’.

Het Nationaal Geografisch Instituut (IGN) en het Volcanologische Instituut van de Canarische eilanden blijft nauwlettend de aardbevingen in de gaten houden, waarvan de kracht van de bevingen (elke dag!) ligt tussen de 1 en 3 op de Richterschaal. De meerderheid van de bevingen vindt plaats tussen 5 en 15 km diepte.

Volgens de autoriteiten betekent deze seismische activiteit nog niet direct dat een grotere, significante aardbeving zal plaatsvinden. Het merendeel van de bevingen die zijn geregistreerd, vonden plaats in het Noord Westen van het 280 vierkante kilometer kleine eilandje, bij het plaatsje El Golfo (zie foto hieronder). Een aardbeving op deze plaats zorgde er zo’n 50.000 jaar geleden voor, dat er een enorme landverschuiving plaatsvond, waarbij de helft van de vulkaan in zee gleed. Hierdoor ontstond een ca. 100 meter hoge tsunami die naar de overzijde van de Atlantische oceaan zou zijn gegleden. Schuin tegenover dit eilandje, in het verlengde van de resterende halve kratermond, ligt het Noord Amerikaanse continent..

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El Hierro Earthquake Swarm Enters Third Week
The intensity of earthquakes recorded on the smallest of the Canary Islands, El Hierro, has increased slightly during the last 24 hours. However, the total number of earthquakes recorded daily has lessened since the earthquake swarm initially began on 17 July.

The total number of tremors recorded during the past two weeks has now exceeded 1,050, according to the Instituto Geografico Nacional (IGN).

The majority of the earth tremors ranged between 1-2 magnitude, but during the past 24 hours there have been least five tremors measuring in excess of 2.0 magnitude on the Richter Scale. A 2.4 magnitude earthquake was recorded at 4:56 p.m. local time on Sunday (31 July 2011).

Approximately 50 earth tremors were recorded on Sunday, according to local earthquake monitoring agency Actualidad Volcánica de Canarias (AVCAN). The number is slightly down on the number of tremors recorded on Saturday and significantly less than the 195 earthquakes recorded on 22 July.

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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.

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 met again on Friday 29 July to discuss the low magnitude seismic activity. It reported that it had stepped up its seismic monitoring operations to identify the source of the earthquakes.

Following an evaluation of seismic and volcanological information gathered by the IGN and volcanological Institute of the Canary Islands, the Committee agreed to activate “monitoring and surveillance mechanisms necessary to coordinate the actions of civilian self-protection and information to people listed in the Plan” (translated from Spanish press release). It remains unclear, however, if the unprecedented seismic activitiy on El Hierro is a precursor to a possible future increase in earthquake or volcanic activity.

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, 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.

Nuevas imágenes de la mancha de emisiones volcánicas en la costa de El Hierro

Material piroclástico en superficie tras la erupción de El Hierro. Día 15-10-2011

Hierrovolcan 17102011

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Op woensdag 19 oktober 2011 14:40 schreef Scrummie het volgende:
Kan deze gekoppeld worden aan de vorige in de reeks?

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Op woensdag 19 oktober 2011 14:53 schreef bwt het volgende:
Dan zal ik gelijk een paar handige links er neer zetten die veel info bieden over de uitbarsting.

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Op woensdag 19 oktober 2011 15:14 schreef Scrummie het volgende:
Raar, hij stond niet in mijn AT lijst, vandaar de vraag. Nu wel uiteraard omdat ik erin gereageerd heb.

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El Hierro - update 6
Little seems to have changed in the eruption of El Hierro since my last update on it. That was on 19 October, 2011. Harmonic tremor is constant, but appears to have dropped a little in the past few days. But with the fluctuation that is often increases again before it drops down again. This means that the eruption is ongoing, but is loosing power to continue at current eruption vent. This was not unexpected. This has however not slowed down inflation in El Hierro from what I can gather on GPS data on El Hierro.

As the current eruption vents close down it seems that earthquakes have started again under El Hierro. But the earthquakes have the depth from 28 km and up to around 5 km. This strongly suggests that new magma is entering under El Hierro and is again increasing the pressure inside the El Hierro magma sill (or chamber, but note that El Hierro does not have a stable magma chamber it seems). This increases earthquakes while the magma does not have any good path up to the surface. When a new path for the magma starts to form again, a large earthquake swarm is going to take place in El Hierro. It is not going to be anything bigger then already has taken place in El Hierro already. With the largest earthquakes going up to Mb5.0 in size. That is at least my opinion.

The major risk in the current eruption cycle of El Hierro is the risk that new eruption vent is going to open up nearby a human population without warning. But El Hierro has a lot of cinder cones from earlier eruption cycles. That creates the risk that a new eruption vent is going to open up on dry land without warning. But it is impossible to know when and where that might happen.

Claims that this eruption in El Hierro is going to create tsunami due to landslides are false and have no basic in fact or reality. As landslides are unlikely to happen in this eruption episode or if a eruption happens on dry land.