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.
As mentioned before, Fagradalsfjall area saw a spike in earthquake activity. Two articles were written about it on the blog by Carl: http://www.volcanocafe.org/unrest-at-fagradalsfjall and http://www.volcanocafe.org/volcanic-unrest-in-iceland.
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.
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.
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.
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.
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.
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
|Askja + Herdubreid||196||2200||1520|
|Hekla + Vatnafjoll||19||109||106|
|Tjornes fracture zone||566||3388||3304|
|South Iceland seismic zone||231||1703||1273|
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)