We rarely, if ever write articles about earthquake swarms at the Tjörnes Fracture Zone. The reason for that is glaringly obvious, those swarms are incredibly common. Pretty much, a day without fairly intense activity is a rare beast indeed. So, this is a bit outside of the ordinary programming.
The area is one of the least studied in Iceland from a volcanic standpoint, but the general dynamics of the faultlines in the area is well known. So, let us start there.
Tjörnes Fracture Zone
It is no secret that the Mid Atlantic Rift (MAR) is running through Iceland. In southern to mid Iceland it is running through several different faultlines, this is though of no consequence for this article. But, from the triple-junction volcano of Bárdarbunga the MAR is running in a uniform line northwards through Iceland to Theistareykjarbunga, via Katla, Fremri-Namur, Heidarspordar and Krafla.
All rifting of the MAR in this area occur along the NNE-trending fissure swarms of these central volcanoes. At Theistareykjarbunga things start to become interesting again. It is yet another triple-junction volcano.
Just south of the volcano you have the WNW-trending Dalvik Line, an older more sedate major faultline that seems to be declining in activity compared to its two other compatriots. Or, it may be locked and could be re-opened in the future in a larger seismic event. No large earthquake swarms are known on this fault.
Directly from Theistareykjarbunga the Husavik-Flatöy Fault runs towards WNW. It is a strike-slip with dextral spread faulting, which is a fancy way of saying that the edges of the continents are both grinding against each other as they slip by each other, and that the fault is also spreading apart.
The third faultline is the Grimsey lineament, and it is here that things start to become interesting, both from a seismic standpoint and a volcanic one.
The fissure swarm of Theistareykjarbunga continues out into the waters of Öxarfjördur where we find a volcano that is unnamed. I usually call it Öxar from the water it resides in. Almost nothing is known about this volcano, but it is believed to have been active in the Holocene.
It is from Öxar that the Grimsey lineament takes off on a WNW line running north of Grimsey proper until it joins the Kolbeinsey Ridge. Unlike the Husavik-Flatöy Fault the Grimsey Lineament is not a true strike-slip fault, instead it is a fracture zone where the spreading occurs in discrete N-fracture swarms. The fractures are either created through large earthquakes ranging up towards M7, or through intense earthquake swarms.
East and North of Grimsey you find two more unnamed volcanoes, the one to the east and northeast I call Skjalfandi, and the one to the northwest I call Grimsey. The one we must contend with today is Skjalfandi.
A little more than two weeks ago an earthquake swarm started at Skjalfandi volcano. During the first ten days it was a pretty ordinary sequence of earthquakes for the area consisting of tectonic earthquakes.
The swarm during this period produced episodes lasting from hours of seismic activity up to a day or so with calmer interludes. During this period, we saw a select few earthquakes just above M3 in strength.
At the same time a couple of weak signals was detected that could be interpreted as hydrothermal activity, but that is not something out of the ordinary since Skjalfandi is intensively hydrothermally active to begin with.
Skjalfandi volcano itself is either a large subsidence graben due to spreading, or a large subsidence caldera. Both what little literature there is about the volcano, and the general geometry, is unclear on this part. Probably it is a little bit of both, so one could call it a grabera just to annoy people.
It is a sizeable volcano with lots of parasitic vents and cones littering the ocean floor, especially on the north-west side. The caldera (it is easier to call it that instead of a grabera) floor is intensely hydrothermally active, with water-temperatures of up to 160 degrees. The caldera sediment has been transformed from volcanoclasts into kaolinite and smegmatite and ranges from slightly alkaline to acidic. This transformation has occurred due to the hydrothermal activity.
The sheer volume of hydrothermal activity indicate that this volcano has been repeatedly active during the Holocene, even though no historical eruption is known to have occurred.
About five days ago the earthquake swarm intensified quite a bit after an earthquake of M4.1 happened. Both the number of earthquakes, and their relative strength increased significantly. So far, this swarm is the strongest recorded since the run-up to the Bárdarbunga eruption. The strength of the largest earthquakes is comparable, but the numbers of earthquakes appear to be less, but the numbers are lying a bit.
The network covering Skjalfandi is not able to reliably pick up earthquakes below M1, so all those small earthquakes are lost to the monitoring network. If the network could have picked them up, we would see roughly the same numbers as we did at Bardarbunga in 2014.
It is here good to remember that the earthquakes are hard to locate here, the stronger the better the positioning is. Even the M2.0 to M2.9 earthquakes are not perfectly located. But, with the number of M3+ earthquakes we have at hand we can still get a good picture of things.
Up until Saturday all of them occurred between 11 to 15 kilometres depth, but on Saturday we started to see a sprinkle of shallower M3+ earthquakes ranging all the way up to 4.5 kilometres depth.
This means that the main activity was located just above the mantle/crust-boundary, and that the activity now has started to move upwards.
Another thing that we can see on our plots are that there are 3 discrete fractures operating that have formed three individual sheet dykes running at an angle upwards.
I should here point out that no volcanic signals have been detected, but when we see these dyke-like formations occurring in Iceland magma is rarely far behind if the seismic activity is running long enough.
These dyke formations occur quite often without causing any eruption, it is just magma filling in deep ruptures in the crust as Iceland is being pulled apart.
Another interesting thing is that the seismic activity has spread across the length of the fault system all the way from Theistareykjarbunga all the way up to and beyond the Kolbeinsey Ridge.
What I find intriguing here is the patterns and shapes that these dykes form, and that there are three of them forming at the same time. To me this looks more like the start of a rifting episode than anything else.
So, if this continues for a longer period and an eruption occurs we might see a strange thing indeed since the depth is not that great. We might see Skjalfandi go from a caldera to Skjalfandey in front of our eyes. Or, perhaps that is just me wishing for a tad too much.
Another thing that I find intriguing is that the GPS at Grimsey is not picking up a lot of movement. During this activity the station has moved 7 millimetres to the southwest and about 10 millimetres down.
That is not a lot of movement for neither an intrusion, or a seismic event on this scale. I think one reason that we do not see more motion is that so far, the bulk of the larger earthquakes have been comparatively deep, and that the movement will pick up if the earthquake activity centre keep on moving up.
So, to sum it up. No eruption yet, but if the activity continues for long enough and the earthquakes continue to move upwards we might get something in the end, but do not book your tickets yet
S.Magnúsdóttir, et al (2015)