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

How rifting occurs in Iceland:

Iceland sits 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:

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.


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 and apparently only the earthquakes at 99% find entry there. That´s why here 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 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:
Thank you Irpsit for this comment. Used in a post at your request!


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

111 thoughts on “How rifting occurs in Iceland, explanations regarding the recent earthquake swarm.

    • This one was transported by …. water. Likely jökulhlaup (flash flood) or from eruption or left behind by the Glacier. This photographer took many good pictures, but there were some good photo days I think he missed 🙂

      • Ah….thank you. But ye gods, transportation by water seems even more incredible than being tossed out by a volcano!

    • You can also wait some days, perhaps better to follow the swarm still a bit.

      So there are also Hekla and Grímsfjall … 🙂

  1. Thank you to Spica for preparing this post . I am sure Irpsit’s and Chryphia’s comments will be useful for many visiting here and will put the present activity into perspective.
    Chryphia, I do hope your son is feeling better after suffering Post Easter Bunny Syndrome 😀
    I have also just commented on a wonderful photo of Hekla that Islander included in his last comment on the previous post. It’s well worth going back to look at to give a new perspective of that monsterous mountain.

      • -the interactive map for the stations is a vast improvement, it expands! No need for high precision on the glide pad.

  2. Gott kvöld. That article was great. Allways interesting to learn more about the activities on and beneat Iceland.
    It is fun that we can learn much more about quakes and volcanoes there than it we did at school when we was young (At least I have, and still will do thanks to this amazing blog 🙂
    I tried to count the numbers of green stars, but gave up 🙂 I guess it’s more than 20.

    • Hello. Green stars: Presently at 62 (00:29 hrs) and changine every hour. Green star is for quake larger than 3. Here is the present list. You can have it open in a tab, and refresh now an then and notice is there are sinificant changes in numbers, list is for preeceeding 48 hours (please bear in mind this creates server load, which we avoid on purpose in time of emergencys, so IMO system can better do its work)

    • Yes, central Iceland is a sink-hole (pile) and comes up now and then via Hekla Volcano, and, yes I had seen this. I think this swarm today(s) is MAR related, last big episodes approx. 40 years ago (the 7+ R event in 1960´s), so this was the expected they thought was coming earlier this winter. As this is writtem, some slacking seemed in the swarm, but it apparently picked up again.

    • reply to Lurking above, that’s interesting.. this is another example of plate boundaries suddenly taking a step to the right instead of progressing in steady motion. The same thing has happened in New Zealand / Havre Trough / Kermadec Trench. It’s as though the plate boundary suddenly gets stuck and starts somewhere else because its easier.

    • hi Mike, good catch on the incandescene at White Island. I have been keeping an eye out for the onset of Strombolian activity but hadn’t seen anything. It seems like it’s there, just very very small.

  3. Just when I was thinking El Hierro was going back to sleep it goes and has a 4.4 quake.
    1203736 03/04/2013 02:45:26 27.7355 -18.2838 19 Sentido 4.4 mbLg W FRONTERA.IHI

      • Hi

        Well it is always difficult to make a guess, but it makes for quite a different episode than in 2011….I’m getting more and more concerned for the population of El Hierro

      • Besides those earthquakes today there seems to have been an increase in the tremor signal over the past 12 hours. It is a little windy so it could possibly be weather related but it seems to have crept up during the night

  4. BTW, re Iceland, isn’t it great to see a large igneous province form before our eyes!
    And does anyone see any dinosaurs dying?

    (just a thought-for-the-day)

    • Despite my sometimes flippant comments, deep down I am always excited and awed by the fact I am watching those somewhat dry Geology facts learned at school that seemed something related to Dinosaurs and Eohippus and almost as extinct, happening, moving, and unfolding here in my little Office at home. If someone had told me 60 years ago as I sat watching the Young Queen Elizabeth’s coronation on this wonderful thing called television , that I would be able to press a button and see live pictures from Iceland I don’t think I would have believed it. My lifetime has seen amazing technology developments. Science fiction becoming fact! I am glad I can appreciate all this rather than take it all for granted.

      • With you on the amazement front Diana. One of my more fulfilling moments was waking one morning and finally seeing the eruption cloud from Eyjafjallajökull jetting straight up 3000 meters before billowing away to the east in the early morning light after days of bad weather and no sightings and sitting there at home watching this awesome spectacle after weeks of deliberation and quizzing over it all.

        • For the people living under the volcano it was not a spectacle. People feared for their crops, their animals and their children. Didn’t even want their children to come home from boarding school for the weekend. Little children weren not allowed to play outside in the surrounding villages so as not to inhale too much ash (their noses nearer to the earth).

          • This is not so easy, most of the people living near the Eyjafjallajökull (and Grímsvötn, also Hekla etc. ) area are farmers. This is their living. And their families have been living there for ages.

          • Let alone all the people living near volcanoes in developping countries, who are too poor and uneducated to have any choices!

          • Then again, no volcanos, no Iceland. 😉

            Speaking of which, Etna’s looking pretty good at the mo.

        • Same with me: one moment I will never forget in my life! I was entirely unsuspecting, looking for some information on Fimmförduhals, and came across a webcam that was set up for it. At that time I barely knew what a webcam was, let alone a volcano, and the name Eyjafjallajökull wasn’t in the news yet. Looking over a high snowy plain, and behind it a small black thing rose… and rose… and got big… and bigger… until weather clouds covered the view. It took several hours before the first reports about Eyjafjallajökull erupting appeared. That must have been the first explosion, and I was almost scared watching it, not knowing what it was.

  5. Good morning all. I have been reading this paper produced by the Iceland Met Office Development and Implementation of Seismic Early Warning Processes in South-West Iceland
    I highlights how relatively new the SIL stations are that we now take for granted and also gives an insight into the work behind those red dots!! This paper and others worth reading can be found here.
    II am pretty sure the rattling that is happening up North will mask the quakes under Myrdalsjokull and elsewhere unless they are very large. The migration of quakes towards the South east is interesting. I wonder if it will prod Theyr….Thei….Theistareykjarbunga awake. Oh how I would love to see how the News media cope with that name. 😀

  6. I was wondering about the strong contortion of the eq swarms in that region and found this: Décollement, or basal detachment fault. They can occur both in extensional and compressional settings.
    Rock bodies with differing lithologies have different characteristics of tectonic deformation. They can behave in a brittle manner above the décollement surface, with intense ductile deformation below the décollement surface.

  7. Morning

    This is the El Hierro earthquake animation update up to April 2nd 23h57.

    We still see no extension of the main swarm. There seems still be a sort of “reflux” to the previously active zones (Is this to be linked to the GPS graphs where the island upward deformation seems to lessen ?).
    An original triplet of quakes under the center of the island on April 1st around 9h and one quake under the Ocrhilla zone very late on the 2nd.

    The data has been “trimmed”, which mean that the quakes with the higher error margin have been removed from the list. also the quakes are shown only for April 1st and second. All the older quakes since March 18th are small black dots. All the remaining quakes (up to July 2011) are in small cyan dots.

    Color of the circles is proportional to their position in the list. (see left of colorbar).
    Right scale of colorbar is related to terrain elevation.
    Size of the event is shown in the diameter of the circle.
    The “current” event is shown in red with a multiplicating factor of 8.
    The bathymetry is also shown.

    The video is divided in 4 parts. First an event by event timelapse, then a rotational phase followed by a 0-90 ° rotation to show a top view of the swarm. Finally a zoom is made focused on the last event.

    Data from IGN, NOAA, made on Gnu Octave

      • Frankly it is very difficult ot say anything. As for GPS, the upfilt has not stopped, the graphs did. They have stopped at the end of March (that’s what I see on the ign site) so in fact…we do no know. There are less quakes, but there are still some and they are still quite stronger that what we were used to until recently. If I refer to the paper from Mc Nutt about volcano monitoring, (see fig 18) he show a generic model where there is first a rise in earthquake numbers then a quieter phase before picking up again this time with tremor before the eruption. He states also that you first have HF events (there were) and some LF events (we had also with some tornillos…). So we did not and don’t have tremor for the time being.

    • I have been watching this too, and I always wonder, why it doesn’t show up on the crater rim webcam, which surely is looking into the same crater and nearer to the action, and has the two main sources of heat in focus. Maybe it is true, as has been said elsewhere, that it is only heat being picked up by the too sensitive IR camera?

      • That’s correct. I asked GNS, and they told me they had seen it too. There’s not significant visible incandescence; they say it’s caused by the particular type of camera they use on the crater floor seeing a little into the near infrared. Still something quite hot there, of course.

  8. Totally OT, but I recall that there is one regular on here (is it Talla?) that is in to Grand Prix racing? if so then this is for her, it’s amusing

    • Speaking of Grand Prix…. my dad was quite amused at a TV station’s voice actor that he overheard rehearsing a commercial several years ago. Pontiac had just come out with their “Grand Prix” model, and the line went ‘you’ve never ridden until you’ve ridden in a Grand Prix.’ The problem was that the voice actor was using the “stick” rhyming pronunciation of prix.

    • Thanks Kelda! I am into Grand Prix racing (well – watching it on TV – it’s a rich person’s sport). The cyclist is no slouch to get round in 10 minutes. It took me ages to walk round Silverstone track here in UK – I thought it wouldn’t take long as it only takes a GP car a couple of minutes. Of course it is 3.6 miles long so it took me about 90 minutes! 😀

      • Into Grands Prix, then you’ll like this, from Oz:

        And another Darracq, the 1905 200hp land-speed record car at Loton Park hill-climb nr Shrewsbury In UK:

        and when he’s finished racing he drives it home:

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