Vulkaanuitbarsting bij Canarische Eilanden #5

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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.

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Op maandag 17 september 2012 13:16 schreef bwt het volgende:

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teideano twitterde op maandag 17-09-2012 om 12:04:41 #volcano New GPS data from El Hierro. Inflation continues with a high rate. 2.81 cm up in PINA in just 3 days... http://t.co/d67M7KpP reageer retweet

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Op maandag 17 september 2012 14:05 schreef bwt het volgende:

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teideano twitterde op maandag 17-09-2012 om 13:27:20 #volcano New GPS data from El Hierro. Inflation continues with a high rate. 3.13 cm up in REST in just 3 days... http://t.co/Vu7CeU4H reageer retweet

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Op maandag 17 september 2012 14:08 schreef bwt het volgende:

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teideano twitterde op maandag 17-09-2012 om 13:45:31 #photography Anciente lava flow at El Hierro, Canary Islands, actually suffering a big seismic swarm #volcano http://t.co/XKL1xSte reageer retweet

twitter:

teideano twitterde op maandag 17-09-2012 om 16:22:09 #volcano Some parts of El Hierro island have uplifted more than 15 cm since July 2011 and still inflating, faster than Santorini! #amazing reageer retweet

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Update 17/09 – 15:36 UTC
- Is the crust starting to show cracks ? Why are we writing this ? If IGN data are correct, we have noticed a number of shallower earthquakes the last couple of hours. Nothing to be scared off, but this is definitely a change in patterns versus earlier today and the 2 preceding days. From 12:27 until 14:39 (less than 3 hours) we have noticed 6 under 20 km earthquakes on a total of 17.
The shallowest being a 13 km, but also a 15 km and a couple of 18 km. Is the magma the cause ? IGN would probably know it but mostly keeps his information for international science conferences a couple of months later!
The shallowest one was recorded below the Sabinosa area, the area where the other 10 km were recorded last week.
- 71 earthquakes so far

Update 17/09 – 17:05 UTC
- 84 earthquakes until 16:11 UTC
- Another 2 shallow quakes at 10 and 11 km , both in the El Golfo, Sabinosa area. The phenomena described in the last update is being confirmed with these quakes. The Sabinosa – El Golfo area has become active again. We are however unsure whether IGN has deliberately omitted the quakes in this area when the strong swarm started. If this was not the case (what we should find very normal) then we can speak of a changing pattern. The following hours will give a better idea of the situation

Update 17/09 – 20:01 UTC
- 95 earthquakes, all the recent ones at 20 km or deeper

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Update 18/09 – 11:37 UTC
Flat lines again in El Hierro – The swarm has suddenly stopped which means that we are still at 10 earthquakes since midnight
- nobody will be able to predict what will happen now. The swarm started after increasing weak seismicity in the El Golfo area . The graph still shows some micro-seismicity, but this sudden halt comes also as a surprise to us.

Update 18/09 – 08:01 UTC
- Only 10 earthquakes so far today. The swarm was very short-lived this time.
- Only the last one at 06:03 was shallower than the rest (11 km) – epicenter near Sabinosa
- Except for the 11 km earthquakes all other epicenters are located in the old crater area (like the images below)
- It is clear that there are 2 magma feeders at work , a weak one below the El Golfo – Sabinosa area and a main feeder who originates below the island and is searching for an exit. During the first eruption it succeeded in finding a system to reach the Las Calmas sea. During the second swarm, the western part of the island showed to be too strong to overcome. The current third swarm does not succeed to get to upper layers (very fortunate for the population on the island, and is subsiding already now
- Deformations
Restinga : slight uplift of approx. 0.5 cm
Pinar and Frontera : slight uplift and pressure to the North (both a couple of mm)
Deformations are confirming the earthquake trend, less inflation today than yesterday

twitter:

teideano twitterde op woensdag 19-09-2012 om 10:46:25 #volcano More GPS data. REST station inflating at quick pace. 4,81 cm in 6 days. #earthqueake number up to 16185... http://t.co/bAmtkNIa reageer retweet

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2012-09-19 13:22
- Apparently nothing is happening at El Hierro (at least no more earthquakes)
- seismicity is back to pre-swarm values

2012-09-19 10:16
- 23 earthquakes in total yesterday, the last one being a M1.7 at 14 km
- 7 earthquakes so far this morning, 2 at a depth of 10 km (El Golfo and Las Calmas), the rest at approx. 20 km deep in the old crater area and El Pinar area
- GPS deformations :
Frontera : a few mm further inflated, a couple of mm to the North-East
Pinar : stabilised in horizontal and vertical axis (normal as with the decrease in number and strength of the earthquakes, the pressure below the center of the island has strongly subsided)
Restinga : comes as a surprise – Restinga is pushed a few mm to the east and has a further strong inflation of nearly 1.5 cm Ultra Rapid preliminary results)

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teideano twitterde op vrijdag 05-10-2012 om 13:34:05 @eruptionsblog @rschott @jonfr500 LP´s at La Palma? mmmmm...will upload spectra in a second... http://t.co/j9ZpZiSH reageer retweet

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teideano twitterde op vrijdag 05-10-2012 om 13:34:48 @eruptionsblog @rschott @jonfr500 so my firends...what do you think ´bout that? http://t.co/WQvtilUe reageer retweet