Iceland seismicity – monthly review (July 2017 edition)

Ever since I began plotting earthquake data for Iceland and generally for the world (where data is available), I was planning to do a monthly review of the seismicity in Iceland, so we can keep track of it on a monthly basis. Of course, if there is any stronger activity or something unusual happens, usually a new post is written within hours or even minutes of the activity.

All the earthquake data that I use is from the Iceland Met Office earthquake catalogue dataset, and contains only manually verified earthquake data (99% quality).
It is very important to note, that while the data is very accurate, there are still some uncertainties in the dataset. Some earthquakes that happen are not added into the catalogue right away or in some cases are not even added (lower magnitudes with too weak signals).
Horizontal locations (epicentres) are very good, especially for M1+ quakes, but they can still have a possible minimum margin of error of about 100-200m in all horizontal directions, up to 300m+ in regions with lower seismometer coverage.
The depth (hypocenter) however, is harder to accurately determine. And the accuracy depends first and foremost on the density of the monitoring network and the distance to the closest seismometer. It also depends on the depth (the deeper we go, the harder it is to accurately locate the hypocenetre of the earthquake), and the magnitude of the earthquake, since stronger earthquakes have stronger signals and are easier to accurately locate. Therefore we have to assume an average minimum margin depth error of 500m-2km (depending also on the seismometer
coverage of the area and signal strength).

The count begins….

Now that we have the disclaimers out of the way, we can start counting. The system I am planning to use is to look at Iceland as a whole first, and then pick out any regions of interest, like Bardarbunga, Katla or any stronger swarm areas like the Fagradalsfjall unrest in the last week of July. I will create the graphics and leave a basic comment where needed, and then the more knowledgeable “Dragons” will occasionally add their view/interpretation. These posts are not intended to be long, because it is just a regular update on the activity since Iceland has always seemed to be a fan-favourite place to monitor and talk about. Likely also because of its relatively frequent eruptions and constant activity, compared to some other places around the world.

In July 2017, there were around 2.590 earthquakes recorded around the area of Iceland. Besides the Fagradalsfjall swarm and some Katla unrest, there was no unusual activity. The strongest two earthquakes were an M4.5 at Katla on 26th, and an M4 on the same day in the Fagradalsfjall unrest. 

July 2017 Data by IMO       Andrej Flis


As mentioned before, Fagradalsfjall area saw a spike in earthquake activity. Two articles were written about it on the blog by Carl: and

In July 2017, there were 654 earthquakes recorded at the Fagradalsfjall area, of which 639 happened from 26th onward when the swarm began. As of writing this post, the activity is still ongoing but diminished and in waves, similar to Herdubreid.
The graphics show the Fagradalsfjall area earthquakes for July 2017 top down and looking from the south.

Fagradalsfjall area swarm. Andrej Flis

Fagradalsfjall area swarm. Andrej Flis View from south

Katla has also had another stronger seismic episode this month. Such episodes are happening since late summer last year, in separate swarms, and usually, feature earthquakes of magnitude 3 and above. It is though normal for Katla to have increased activity in summer months, together with an occasional minor to moderate jökulhlaup (glacial outburst flood).
Katla has counted 246 recorded earthquakes in July (elevated activity), with the strongest earthquake being an M4.5 on 26th. It coincided with the onset of the Fagradalsfjall swarm (not-related) and preceding the later minor jökulhlaup episode when possible harmonic tremor was detected.  A possibility of a small sub-glacial eruption is not completely excluded.
Out of the 246 earthquakes recorded, 6 were M3+. Also worthy of note is some deep activity, around 26-28km deep. 

Katla caldera July 2017 seismicity. Top down view and a view from south.

Here below is also a graphic I made for Katla caldera (since it is my favourite), showing earthquakes over time by depth, filtered at 0.5 magnitudes. The area used is the same as on the graphic above.
Notice the increased trend towards deeper quakes lately, as the feeder root slowly reactivated over time. This is the safest sign that Katla is nearing its next eruption (but aren’t all active volcanoes always nearing their next eruption?). 🙂
There was likely more deep activity over the past, but perhaps it was not recorded well enough due to the lower density/sensitivity of the monitoring equipment. It is though worthy of note that there were two earthquakes recorded below 30km, back in 1999 (M2.7 – 33km) and 2000 (M1.6 – 35km), so it’s likely that the sensitivity was good enough back then, but only for stronger activity.
There is also some noise from Godabunga swarm around the 2001-2003 period, as the activity also affected the western parts of the Katla caldera. And the line of earthquakes at 1,1km depth is artificial because some of the earthquakes were likely assigned the default depth of 1,1km (were likely not completely re-checked or fully corrected). Filtering by magnitude does not remove this “artefact” since the magnitudes vary a lot in that line. I have also coloured 2017 quakes, for a comparison with historic data.
There are just over 16.000 earthquakes in the database for Katla caldera since 1995 until present time. Around 11.200 are M0.5+, and 
around 4.900 earthquakes recorded are below M0.5. The number of lower magnitudes has increased over time, as the monitoring network got (and still gets) more sensitive. There are also 102 earthquakes of magnitude 3+ in the database for Katla caldera, of which 5 are M4+, two in 2016 and two in 2017 so far. One (M4.6) is listed in March 1998.

Katla earthquakes by depth and time.           By Andrej Flis           Data-IMO

At or around Vatnajokull, 373 earthquakes were recorded, of which 143 were at Bardarbunga. Other earthquakes were spread out across all the other areas like Grimsvotn, Hamarinn, the cooling Holuhraun dyke and also Oraefajokull.
8 earthquakes were recorded at Greip. No earthquake was of magnitude 3 or above (a star on IMO plots).
Activity at Oraefajokull might eventually be worthy of a separate post down the line. 

July 2017 Vatnajokull seismicity.

Another area which features daily activity is the Tjörnes fracture zone. In July, around 566 earthquakes were detected in this region, with depths ranging (as usually) in the entire profile from the surface all the way down to the MOHO boundary.

Tjörnes fracture zone  July 2017 seismicity. Data by IMO     Andrej Flis

Final numbers :

Some areas overlap, like Bardarbunga and Vatnajokull (for obvious reasons), so if you were to summarise the numbers, you will obviously always get a higher number than the total number for Iceland.

July 2017 EQ Count

2017 (Jan-July)

2016 (Jan-July)





Askja + Herdubreid 196 2200 1520
Bardarbunga 143 1039 1151
Hekla + Vatnafjoll 19 109 106
Katla 246 1467 581
Tjornes fracture zone 566 3388 3304
Torfajokull 27 314 245
Vatnajokull 373 2616 2790
South Iceland seismic zone 231 1703 1273
Reykjanes peninsula 860 1824 1541
Oraefajokull 31 175 214
Grimsvotn 15 170 161

This concludes this first “prototype” post of Iceland monthly seismic activity review. I am not sure if I will actually continue this on a regular monthly scale since there is quite some manual work required and many data revisions, which takes time to make it as high-quality as I want to.

But I do hope and believe there will be more, and I do plan to add more graphics of different types, like cross-sections or cumulative energy plots or earthquake depth-by-time plots for any other specific region when needed, looking back with up to 22 years worth of data.

Comments are welcome for suggestions and to point out any Icelandic activity in the current month (August) that would be worthy of adding into its monthly edition.


Andrej Flis – (Down Under @Recretos)

59 thoughts on “Iceland seismicity – monthly review (July 2017 edition)

  1. There is a noticeable gap in earthquakes between Katla and Bardarbunga any reason for this? Softer ground, locked plates? The plates have already adjusted away from each other?

  2. There has been a significant increase in EQ activity in Iceland between 2016 and 2017. Especially, in Katla and Askja. Also at Reykjanes. I expect eruptions in these 3 regions in the next few years.

    • Katla does seem like a good bet at the moment, although with no previous data record prior to an eruption we can’t do a ‘Grimsvotn’. We could be speculating for some time or enjoining an eruption next week…

      I agree about Reykjanes, although where exactly would be a good idea for a VC sweepstake.

      I’m not convinced about Askja proper, Andrej might have to do another plot 🤔. Herdubreid could rumble away for years with no show as well.
      I’m on team Katla, but Hekla is always there to ruin the data party 😉

      • I can’t remember where I originally cast my vote, but now that I’ve started actively looking at the latest quakes myself?
        Yeh, for sure. My money is on Katla.
        Andrej, I really appreciate the work you’ve done here. It makes all that raw data so much easier to digest and put into context.

  3. I like this and look forward for more in the future. I appreciate the time spent in gathering info and putting it together for us to read, That goes for others too who have written articles on here. It helps assuage the volcanholic addiction until the next eruption. Thx 😀

  4. The sensitivity of the network to earthquakes varies with location and time. During stormy weather, small earthquakes may be missed. When comparing earthquakes from different months, it may be better to set a minimum magnitude, say M1+, to make sure you are not just measuring weather. Perhaps add total earthquake moment as well?

  5. Thanks Andrej, appreciate the time and effort that you put in! 🙂

    I too am in the Hekla / Katla band camp and thinking we’ll also see a surprise hello from an unusual suspect. Question is will we see it before the year is out?

    One thing for sure is it won’t be this weekend, I have a few days off but can’t go anywhere as the other half is on jury duty. Volcano watch readiness = no activity.

  6. Great job Andrej! Thanks a lot!
    A monthly review would be nice, the longer the series, the more interesting it will be.
    Maybe a small team can enlighten your effort? Just ask! 🤗 🙂

  7. At I read that there are drilling operations ongoing on Surtsey.

    “New drill cores will show how mineral and microbial changes have progressed in still hot basalt 50 years after the final eruption in 1967. This blogspace hosts photographs and videos taken by the Surtsey team. Explore the menu bar to experience the progress of scientific research as if you were here!”

  8. In regards to Antarctic volcanism, this makes me once again wonder…. What the heck is causing all the volcanism in Antarctica? From what we see, it looks almost as if there is subduction style volcanism here, yet we have no known subduction zones in the area.

    Is there an african style rift here that we’re completely unaware of? I can’t think of what else it may be here.

    • According to the article at mjf’s link; “These active peaks are concentrated in a region known as the west Antarctic rift system, which stretches 3,500km from Antarctica’s Ross ice shelf to the Antarctic peninsula.”

      But, nothing shows up in the USGS plate boundary KML file for the region. Just a spreading ridge somewhere between Australia and Antarctica, complete with an odd triple junction out there quite a ways. If there is a rifting area in Antarctica, you would think that it connects back in with the rest of the boundaries somewhere. 4-way junctions are a no no and imediately move to a dual triple junction arrangement. (per Global Tectonics, 3rd edition.) Such as when the Farallon/Pacific ridge subjected. (became the Mendocino and Rivera triple junctions which then migrated away from each other, leaving the San Andreas and Imperial fault systems behind… along with the Gulf of California fault system.

      • Interesting to read about this (after doing a google search). Considering this is a rift system that is larger than the East African Rift, that seems pretty significant to me. Can’t help but think this is woefully under-studied simply due to being on Antarctica. With that said, this rift system may not be as active or prolific as the African rift – it seems it has been on and off over millions of years, so may not be quite the same as the African rift system, which is actively shearing apart the African continent.

    • There will be surprises underneath the ice. They do not distinguish between extinct and active/dormant peaks, by the way (how could they?), so 91 peaks may not be as unusual as they say. Iceland probably has that many, in a far smaller region. But some seem impressive in height.
      It may be an extinct subduction region. It it is an extension of the Andes, and subduction melt should continue for ten or more of millions of years after subduction itself has stopped. But that is a guess.

      • I would somewhat assume if these were ancient subduction volcanoes, they would have been mostly eroded by glaciation by now, wouldn’t they?

        • The volcanoes would be young(ish), driven by subduction which has gone extinct upstream but that information hasn’t made it down yet..

          Glaciers only erode if they move. I don’t know whether these do.

          BTW, I noted the different versions of CBUS. Which one is the update?

  9. Yes – while Kilauea has gone to sleep (no recognizable inflation at the moment), Mauna Loa has begun a growth spurt. Will be interesting to see whether this continues.

    • I am the furthest thing from an expert there is so would need input from the usual crowd but whatever it is it does seem to be localised at HAU; I can’t see the same type of activity at FED which sits on top of Hekla. The anomalous readings also start just after 9am UTC so I wonder if it’s man made.

      In saying this it has been a bit ‘quakey’ at Hekla of late …

  10. I hope this isn’t too obvious a question, but did lava “eventually” drain out of the Bardarbunga magma chamber during the Holuhraun eruption via a lava tube? I can see where magma would flow within a dike or tectonically-generated extensional voids nearer the surface, but what about further upstream towards Bard…especially before the “elbow”?? Despite the elbow and the tortuous path it created, we saw what appeared to be a very tight and smooth flowing hydraulic system was in place by observing how earthquakes and changes in eruptive behavior in Holuhraun were so closely time spaced…sometimes on the order of hours. Once Holuhraun really got going, the flows were so prodigious that I have a problem with visualizing magma pushing through crooks and crannies upstream so efficiently….but magma moving in a more laminar fashion within a smooth flowing tube might.
    Anyway, thanks if anyone can share any articles on the mode and mechanism of magma movement during the Bard-Holuhraun event.

    • A lava tube is round. A dyke is vertical, deep, with rock sides pushed apart by the pressure from the flowing magma. It may have been a few meters wide, but hundreds of meters high (deep?), and of course several kilometers below ground. The image shows one that solidified. This image and others can be seen on

      • What I find interesting in the photo, is tat you can see what appears to be 3 separate dike emplacements. 2 either side of the original dike. Apparently the best path was,alongside the interface of the original dike and the country rock.

        GL Edit: Fixed the phone typos that made me look like a babbling idiot.

        • You would probably find something similar if you had a cross section of the Holuhraun dike. Remember that before Holuhraun there was Holuhraun – a lava field from a previous eruption. The 2014-2015 event reused the same old crater row from the previous eruption, so probably the dike was reused as well.

      • Possibly a silly question, but looking at the offset in the (presumably) sedimentary layers either side of the dyke there, would the uplift on the right (as we look at it) have occurred before or during the intrusion of the dyke? Basically I’m wondering whether the dyke is likely to have forced a path upwards, or whether it exploited a fault that was already there?

        • Good catch! Other people know more here than I do. It looks indeed like uplift of the higher levels (but not of the lowermost level). My guess would be that the dyke fed a horizontal sill to the right which lifted the layers above it. But it is very much a guess.

    • Probably not. I dont think lava tubes exist that can handle the pressure and volume of the initial eruption. They also dont connect with magma chambers or other deep underground structures.

  11. Thanks for the great graphics as usual Andrej. They make it a lot easier for the verbally and mathematically challenged like me to understand what is happening.

  12. Riddle no. 2 – Montagu Island, Antarctica. Saint Montague was the patron saint of railways (he was actually struck by a locomotive). Lee Montague is the only survivor of the 1952 Moulin Rouge movie cast.

    For some reason all your comments end up in the dungeon. Hereby released.

  13. I always wanted to go to Iceland to see their volcanoes but I’ve had a stroke recently that I recovered full from, with their stance on abortion of Down’s syndrome babies I’m afraid they’d pull the plug on me if I suffer another stroke… to risky to my humanity as a person to go over there now.

    • I don’t think they terminate tourists though when they get ill, but as an alternative I recommend Stromboli in autumn or spring when things are cheaper. There’s usually some activity, and you can sit in a bar on a warm evening watching the bursts of ‘smoke’ from the summit craters. If you are fit enough for a 3,000 ft/1,000 metre climb you can be guided to the summit and look down on the three(?) active craters.

      And much as I love Iceland, Italy is a lot cheaper for eating out!

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