How rifting occurs in Iceland, explanations regarding the recent earthquake swarm.

How rifting occurs in Iceland:

Iceland sits in both in the rifting area between two tectonic plates.
The North American and European drift apart in opposite directions. One towards east-southeast, the other towards west-northwest.
However the rifting is not homogeneous. It occurs differently in different sections of the rift area.

Please have a look at the following picture:

image http://images.sciencedaily.com/2010/04/100416193630-large.jpg

Over the area between Hekla, Katla and Vatnajokull, the rifting occurs almost perfectly at angle perpendicular to the direction of the plate movement. The rifting opens aligning volcanic fissures from the NE to SW, while plates drift towards the NW and SE respectively. This means the earth rips easily open in those regions and this allows for large volumes of magma to erupt. Of course, earthquakes do happen in these regions, and they happen constantly (nearly every day – more than in any other region of Iceland) and therefore they are mostly small sized earthquakes. These are the daily earthquakes at Katla, Vatnajokull and Askja.

However in two sections of Iceland, the southwest seismic zones, and in the north, in Tjornes seismic zone, the plates move apart each other in a almost sideway motion. This creates notorious strain in the rocks, and creates much larger earthquakes. These are called transform seismic zones; regions where motion is sideways.

Daily, they are only very tiny earthquakes, but occasionally, every few decades, strain accumulates and then these areas experience large 6.5 earthquakes. Once they come, they occur in clusters, with every few years a larger earthquake (just like the south Iceland earthquakes of 2000 and 2008), and then reverting to calm decades again.

Moreover, these two regions, experience mostly their plate movement through large earthquakes and do not allow much for volcanic eruptions. Yes, they do occur, but more rarely and smaller. Volcanic eruptions have known to occur in Tjornes and SISZ, but last one in the SISZ was 3000 years ago, in the Grimsnes volcanic system, and it was very small.

It has been confirmed that every major tectonic episode is joined by magmatic intrusions at deep, but rarely they come near the surface. To understand why, just imagine the two plates drifting side by side; this does not create much fracture space in the rocks to allow magma to move upwards, magma would move easily laterally than upwards. However, in the rifting regions near Vatnajokull, like the dead zone, rifting is perfect, and when the crust rips open, large volumes can move upwards!

In other regions of Iceland, rifting occurs at different angles, like Reykjanes, where plates create a rift area about 45º angle from the plate movement. Like results it seems in a “middle ground way” between the two former situations. Eruptions are mostly medium size and occasionally large but rather rare, and strain does accumulate and is released in larger earthquakes, but not as large as the two transform zones of Iceland (mostly up to 5.5).

Furthermore, it seems that rifting activity in the north has been occurring in recent decades. In 1975 Krafla began several episodes of fissure rifting eruptions. This lasted until 1984. Also further north, in Tjornes, there was a large and damaging earthquake (almost up to M7) in 1976. Probably this was a consequence of the rifting process extending further north. So, now it is mostly natural that we are experience another series of major rifting episodes, mostly tectonic, even further north.

Recently, also another volcano has shown signs of uplift and more earthquakes, the Theistareykjarbunga volcano. While still unlikely to erupt, this shows how activity in the rifting regions seems to propagate further north or south over the decades, as strain is released and triggers other nearby regions.

I could almost say that the whole of this could have started back in 1874, when there was a very large rifting episode in Askja, which extended further northwards, and culminated in an eruption very large in 1875.

In Tjornes, there is still the potential for a large earthquake, up to 6.8.

Likewise, we have been experiencing what seems in recent decades an increase of rifting activity in south Iceland. Hekla has been erupting regularly, the Westman islands erupted in recent decades, and Eyjafjallajokull also, as well as two large earthquakes in the south seismic zone. It is natural to expect further movements in neighboring seismic fractures and volcanoes in south Iceland.

Irpsit
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The north of Iceland saw more than 400 quakes today. GFZ lists the strongest one as a 5.3, IMO has it as 5.4. But more than 400 quakes shook the Tjörnes fracture zone today.
Chryphia did an image and explains it:
The Tjörnes fracture zone is really busy again! This is how it looks in respect to all earthquakes from 2011:

The red dots represent (from North to South) Kolbeinsey Ridge, TFZ submarine volcano and Theistareykjarbunga. The current swarm is nicely blending in previous swarms from the past years. They are all very much contorted.

The data are from http://hraun.vedur.is/ja/viku/2013/vika_14/listi and apparently only the earthquakes at 99% find entry there. That´s why herehttp://en.vedur.is/earthquakes-and-volcanism/earthquakes/tjornes-small/#view=map they seem to be distributed widely, all qualities are included, when revised they will become more “concentrated” I guess.

This was to been seen on http://en.vedur.is/earthquakes-and-volcanism/earthquakes/tjornes-small/ this evening.

Chryphia: This 3D video shows the recent swarm in the Tjörnes Fracture zone, but only 99% earthquakes until 7:07 this morning. In the close-up view you can see how twisted (the angle Irpsit was talking about?) the fault zone is and that commonly the swarms happen along a narrow streak.

While i am publishing this post, the swarm is still ongoing.
So for our newbies some links to watch:
http://en.vedur.is/earthquakes-and-volcanism/earthquakes/
http://en.vedur.is/earthquakes-and-volcanism/earthquakes/tjornes-large/
http://en.vedur.is/earthquakes-and-volcanism/earthquakes/tjornes-small/
Thank you Irpsit for this comment. Used in a post at your request!

Spica

Update: The trend of the quakes is slowly moving to the southeast.

There have no been a little under 700 quakes 75 of them over Mag. 3.

http://www.vedur.is/skjalftar-og-eldgos/jardskjalftar#view=table

And of course neither Irpsit nor chryphia nor me are experts, just interested amateurs.
Spica

Name that Volcano Riddle!

Resident of Siglufjördur looking rather wooden eyed after yet another sleepless night with earthquakes.

The week has been calm eruptionwise, but fairly frisky earthquakewise in Iceland. The transform faultline north of Siglufjördur kicked into high gear with continous medium sized earthquake swarms.

The active faultline is one of two major faults making out two of the boundaries for a microplate that is seismically locked at its southeastern corner under Theistareykjarbunga volcano. Theistarykjarbunga is as most readers of this blog know one of the two volcanoes in Iceland and the world competing for the title of having had the largest flood basalt eruption during the last 10 000 years, the other one being Bárdarbungas Thjórsahraun eruption.

Image by IMO. The earthquake band of the fifth most powerfull earthquake swarm during the last 12 years.

The earthquake swarm has so far had a couple of hundred earthquakes in them, 13 between 3 to 4, and 5 earthquakes from 4 and upwards to 4.3M. This is a fairly vigorous, but not unheard of amount of energy released from an Icelandic earthquake swarm. It is though the 5th largest during the last 12 years. There is currently no other signs that this will lead into anything too exciting in the near future.

RIDDLE

This week we will try a new version of the friday volcanic riddle game. It is a “Name that Volcano Riddle!” by commenter Suzie. One point to be had.

The French footie fans looked on in horror as their European opposites ran riot round the capital! They asked themselves “What does this unruly orange mob mean to us?”

VOLCANOSPORTS

During the week I tried to come up with some extremely extreme sports that you can do if you have a volcano handy. My favourite was a surfing down a Hawaiian lava stream on a ceramic surfboard. The idea though was not as novel as I thought. Apparantly it is big too skate down scoria cones.

TGIF!
CARL

Theistareykjarbunga – Icelands slumbering Giant

Mudpots in the Theistareykir thermal fields. Photographer unknown.

The Five Large Volcanoes of Iceland

Iceland holds five volcanoes that in historical time shave had eruptions transcending the 10 cubic kilometer mark during post-glacial times. The most obvious are of course Grimsvötn, Bardarbunga and Katla who share the same eruptive grounds for their large scale fissure eruptions in the area of rifting running roughly from Vatnajökull down towards Katla. There we find the Eldgja/Laki/Veidivötn-fissure systems. Among those Veidivötns Thjorsarhraun is the largest lava flow after postglaciation.

These 3 volcanoes are of the rifting fissure type (with that I mean that they have massive fissure swarms that rift, not that they are driven by rifting processes per see), have massive central volcanoes with huge calderas and massive systems of magma-chambers. Normally they have frequent small scale to medium sized eruptions, and only have their large eruptions when the SIFZ (south Icelandic fissure zone) endures a rifting episode. Two of these are powered pretty much directly from the hotspot mantle plume under Iceland. The current location of the hotspot is believed to be close to Bardarbunga.

The two triple junction Behemoths of Iceland

The other two are different from the first 3 in many respects. Both of them are triple-junction volcanoes, driven almost entirely by rifting processes, are mainly fissure volcanoes with fairly unknown internal systems. Both of them are located fairly far away from the hotspot, and both are centers of unusually strong seismic activity for being located in Iceland. Both of them erupt in large scale when erupting, before going back to long periods of dormancy.

In southwestern Iceland we find Hengill, a large scale fissure volcano located next to Lake Thingvellir. The other is the northernmost of Iceland’s large sub-aerial volcanoes, just north of Krafla volcano. Theistareykjarbunga lies in the junction where the Tjörnes Fracture Zone (TFZ), the Grimsöy Oblique Rift (GOR), the Husavik Flatöy Fault (HFF), either directly intersects, or where they mechanically interact in a sense of it. Southwards from this the Northern Volcanic Zone (NVZ) comes running from Bardarbunga. Another thing to keep in mind is that Theistareykjarbunga is the northernmost of the main band central volcanoes of Iceland, and that it might have been responsible for the sub-aquatic eruptions out in Tjörnes area at the tip of its northern fissure swarm.

This of course creates tremendous tectonic strain that from time to time is released as massive earthquakes in the region. The last regional massive earthquake episode was in 1872 when 2 earthquakes larger than M 6.5 hit the Tjörnes Fracture Zone. During the last decades small but persistant quake-swarms have plagues the area.

Due to this a separate dense network of GPS stations where put up in 2006 for continuous measurement to enhance ability of calculating the pent up strain in TFZ and adjacent fault zones. It was emplaced and booted up in 2006. As a point of reference a GPS was placed on the believed to be dormant, or even dead, Theistareykjarbunga Central Shield Volcano, since this should be fairly stationary since it is almost dead center on the triple junction.

And for the first year that actually worked well. Then a continuous uplift started at Theistareykjarbunga with a maximum uplift of 30mm per year. This uplift caused concern, and the researchers then added additional GPS equipment directed at the volcano to get a better picture. Also other observational techniques were employed. Premier among the additional methods was the Envisat Interferograms that confirmed a circular uplift directly under Theistareykjarbunga Shield volcano. It confirmed inflation in a massive magma-reservoir with a depth of about 6.5 kilometers down, and covering an area of more than 70 kilometers making it into one of the largest on Iceland. And it is only logical that a Volcano of this eruptive ability should have a magma-reservoir on a large scale. Data and information taken from the article referenced in the end of the article.

Theistareykjarbunga Thermal Field. Photographer: Lara Stefansdottir

Theistareykjarbunga as we know it came into existence as a shield volcano in one majestic continuous eruption of Hawaiian type. The amount of magma ejected is sourced by Global Volcanism Program to be the largest in the history of Iceland. But it is actually so that GVP also states that Thjorsahraun in Veidivötn (Bardarbunga) is the largest effusive eruption in Iceland after de-glaciation. I understand their confusion. They are so close in size that a competition is rather unnecessary, but on shear effusive volume nothing on Iceland can compete with Theistareykjarbunga. But then one should remember that it was not an explosive event, and Thjorsahraun had explosive components, and was a lava flow, not a central volcano building event.

According to GVP there have been only 3 eruptions at Theistareykjarbunga, but that is most likely not true, there are more large lava-fields belonging to both the southern and northern fissure swarms than they give credit to. But, it is understandable that they miss some, this is a very poorly researched volcano. A moderate number would be five large lava-field producing eruptions scaling in around 5 – 10 cubic kilometers, and the initial eruption scaling in on a total ejecta volume of 35 to 40 cubic kilometer if one combines the fissure eruption and the lava fields that was produced, and the part of the eruption that created the 30 cubic kilometer shield volcano. The last large eruption is well known, it was the 2700 year old Theistarekjahraun eruption.

Likelihood of Eruption

Theistareykjarbunga will most likely erupt. Big surprise. This is after all Iceland. Almost all of the active volcanoes will erupt many times in the coming geological timescale. So, just saying that it will erupt doesn’t mean that much really.

Image from the Krafla Fires. An eruption at Theistareykjarbunga would most likely be like a much larger version of the Krafla fires.

So what points towards an eruption? First of all the inflation points to an increase in eruptive risk, but there is also tectonic activity under the shield volcano that has a magmatic signature. And of course the sharp harmonic tremoring episodes that happened in October and November.

If one take into account these 3 signs it will give a certain relevance to assuming that the volcano is waking up from the 2700 year old slumber. It might of course still go back to sleep for a thousand years or more, one should always remember that.

But, if it continues to show signs, the inflation keeps on being steady, when would it then go? Well, at a minimum it would need another decade. But that is probably way to short time span. We would need long periods of uplift, ever increasing quake swarms, continuing movement of the adjacent micro-plate. Regarding the uplift, we would most likely need to wait for meters of uplift to happen due to the immense size of the magma-reservoir before it achieves critical pressure. Yes, we could have missed earlier large inflation periods down through the last 2700 years. But we should remember that Theistareykjarbunga deflated during the Krafla-fires, and that we still do not see enough quake activity for there being high pressure in the system. So I still would say that we need between a meter and five meters before anything happens. And that gives a time frame spanning between 30 to 150 years and current rate of inflation. But, the inflation could pick up speed at any time too.

Time will tell.

How would an eruption look like?

Here we are leaving science totally. I admit being on skimpy ground when guessing when Theistareykjarbunga could erupt, but here I am putting on my psychic hat full on.

The known eruption was effusive only, or almost effusive only. There could have been a bit of explosivity due to local hydrodynamic circumstances, but that would have been it. So if nothing has changed in the chemical composition in the magma-reservoir, hydrodynamics, or in the chemical composition of the new magma that has been entering the magma reservoir, then it should be a Hawaiian type of eruption with lava fountaining, probably a fissure eruption either on the flank of the shield volcano, or out into the southern fissure swarm (signs from the harmonic tremoring make the southern part a bigger risk), it would be ranging from the 5 cubic kilometer range and upwards.

Sadly we do not know the amount of gas that would be released during an eruption. We quite simply just do not know enough today to guess about that.

Is there a risk that the system has changed? Could it be explosive? Let us start with the hydrodynamics. I do not think that the amount of water in the system have increased a lot since the last eruption, my guess would actually be that it has decreased due to land uplift after de-glaciation, and a general drying out of that part of Iceland. On that reason I would say no. Could the chemical composition of the magma in the magma chamber have changed? Most likely. As time goes by magma changes chemically as it cools off and mixes with newly infused magma, and cools off again. Often this intermingling of evolved and unevolved magmas can produce magma types that can be fairly explosive. The magmas that was erupted before was low in Rare Earth Minerals, so they were not of the same hotspot origin as the Bardarbunga type that is massive in REM-content. And since the hotspot has not moved that much since the last eruption there is not much talking for it being another type of magma entering now, but this is just guess-work.

So yes, there is a very slight increase in risk that it will be a bit of explosive component during an eruption. But the average would be mainly effusive only. The scale of eruption though means that parts of the eruption probably would go as a VEI-2, or even a small VEI-3. Not much really if one count the scale of a likely eruption. But, the risk would be high gas content, and of course that so little is known about the volcano. Icelandic Met Office (IMO) will most likely keep an eye on things and expand the network around the volcano well in advance.

Edit: The end of Bob?

The end of Bob? Image gloriously liberated from Spanish authorities.

This could very well be the end of Bob as we learned to love her. The tremoring just went away earlier today. And the earthquake list for the last ten days also speak of diminshing pressure. It is still a bit to early to say that the eruption is over, but it looks like there is a possibility. If it is just a blockage of the conduit, we should during the coming days start to see a marked increase in earthquakes, both in number and intensity. If we do not see that it is probably over for this time. Sleep well little Bob.

Edit 2:

As noted, it might be to fast to bump of poor Bob. Gas emissions are keaping steady, or even showing a slight increase during the last hours. Still the big one to keep an eye on is the earthquake list. But as noted by some of the ever vigilant readers and writers on this blog, the tremoring on La Gomera did not lower when CHIE stoped showing tremoring. So, little Bob might still be able to surprise IGN.

http://www.gobiernodecanarias.org/cmayot/calidadaire/datosOnLineEstacion.do?ides=124

CARL

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2011.05176.x/abstract

Some activity in Iceland

As most of you have noticed, Iceland have been really quiet lately. Well, now there seems to be a bit of activity in Iceland. So, here is the first activity report of Iceland on this blog.

Katla, or more to the point, Godabunga put in a short appearance;

Thursday 24.11.2011 15:39:13 63.630 -19.190 1.7 km 2.9 99.0 3.2 km ESE of Goðabunga

Image: IMO, rights reserved. Almost a star at Godabunga

There also seems to have been a small tremor-spike (for being Godabunga);

Image: IMO, rights reserved. A small tremorspike

This seems to a rather small thing, I would say that this is just an ordinary day in the life of Godabunga. But, sooner or later there will be a re-start of the runup for the real eruption of Katla. For the record, this is not the day for Katla.

Tjörnes Fracture zone has an ongoing quake swarm, there is no ongoing tremoring there so it seems to be tectonic only. So no signs of an eruption there. This behaviour is quite normal for TFZ, and can run for days. For the record, back in the 1870s there where at least 2 earthquakes of magnitude 7. So, this is on the smaller scale of what can happen in the area.

Image: IMO, rights reserved. A good looking quake-swarm

The odd activity at Svartárkot continues. Svartarkot is located next to the Ódáðahraun lava field, the massive Dyngjufjöll shield volcano and the Sveinagja Graben. All three of these features are part of Askja volcano. Ódáðahraun lava field from the Holocene is Icelands largest with more than 6000 square kilometres of lava. Dyngjufjöll is the largest shield volcano in Iceland, and the Sveinagja Graben, a part of the Askja arcuate fissure-swarm, is the site where the 1874-1875 eruption started. Anything happening here, even if it is small, is worthy of attention. Askja has after all had inflation happening in both the caldera and at the Herðubreið Tuyja. And during the last couple of months there have been earthquakes and harmonic tremoring that seems to have originated from the Sveinagja Graben and continued down to the southern parts of the Askja fissure swarm.

Image: IMO, rights reserved. Odd patterning of Svártarkot SIL, origin is probably activity as Sveinagja Graben belonging to Askja Volcano reactivates

The new patterning is diffuse, but it is rather unusual for Askja and the Svártarkot SIL.

CARL