The good part about volcanology is that nature will sooner or later test both your theories and your scientific models. In this case what was tested was the original model of the available pent up seismic strain in the currently active area of Reykjanes Peninsula in Iceland.
It turned out that the amount of pent-up strain was higher than the models had given at hand. Science loves when oddballs like this comes around the corner, then you get the opportunity to see if something is wrong or missing with your theories, or if there is something wrong with the models used. I love the smell of science in the morning.
As everyone has noticed by now, well at least everyone interested in Icelandic geology, the seismic activity continued well beyond what could be expected according to the model for the area. Since the start of the seismic crisis an average of 20 000 earthquakes has occurred per week, and the size of the larger earthquakes are comparatively in the same range without any sign of them abating.
During the first week of seismic activity, we mainly saw tectonic earthquakes along the portion where the Mid Atlantic Rift enters the Reykjanes Peninsula in a WSW/ENE-direction spanning most of the peninsula.
After that the activity started to concentrate in an area between Keilid and Fagradalsfjall, with a migrating trend towards the southern part of Fagradalsfjall.
At this point in time this is the largest earthquake swarm recorded instrumentally in Iceland. It is in fact so numerous in earthquakes that it stands for a large portion of all earthquakes ever measured in Iceland. It even dwarfs the famously noisy eruption of Holuhraun in 2014-2015.
Tectonic or Magmatic?
One of the most irrelevant questions in Iceland is probably if an intense earthquake swarm is tectonic or volcanic. The reason I am saying this is that sooner or later an initially purely tectonic swarm will turn into a magmatic swarm as intrusions sets in.
Here I am talking about earthquake swarms that are not directly in or around the well known large central volcanoes, they operate a bit differently. I am here keeping to areas like the Reykjanes Peninsula.
The reason for this is simple fluid dynamics. The main tectonic regiment in Iceland is caused by the rifting apart of the Eurasian and North American continents. This is by no means a constant process, instead you get increased strain locally, and that strain is later released as an earthquake swarm (or as a large earthquake, but now we are talking about swarms).
The swarm in turn is in the beginning tectonic as things are pulled apart, and here it is where fluid dynamics comes into action. I will here invoke a rule, nature abhors a vacuum. So, if a void is formed in the crust as it is pulled apart two things will happen.
Scenario number one is the purely tectonic one, and the rift will snap back together giving no dilation of the rift. In other words, the vacuum problem is solved by the sides of the rift coming back together (most often not exactly in the same way).
I am obviously simplifying things quite a bit here, but the principle is correct. In these cases, you might get lateral slip or vertical slip of one edge, but you do not see dilation. This is quite easy to see a few days later on the GPS-network.
The second option happens if the swarm is ongoing for some time and the crust is fractured enough that magma can enter from below. In this scenario you will instead be having fresh magma being sucked into the void, filling it up.
Now we have dilation that will be visible on the GPS system if we are in the happy situation that we have a GPS-station on either side of the rift as the earthquake(s) strike. It is visible since the stations will move apart, and then they stay apart.
The important part here is that it is quite common that this happens in Iceland, it usually happens a couple of times per year, or even more. After all, most intrusions will fail before an eruption occurs since the intruding magma is just enough to fill up that pesky void in the crust.
The Happy Dilating Dyke
Now things will become a tad technical, so I need to explain a few expressions as I go. The first one is obviously the word dyke. It is a form of volcanic conduit that is extending from the mantle upwards like a hanging sheet of magma, sometimes spanning a considerable horizontal distance.
It forms like I explained above as the rift is pulled apart by the continental drift, the ensuing void will be filled by magma. If the magma moves far enough upwards it will start to exert force in two ways.
The first way is through buoyancy, due to being hotter than the surrounding bedrock it will buoyant and will strive to move upwards like a hot balloon. If the magma is shallow an eruption might happen since there is not enough time for the magma to solidify in the dyke.
This is a fairly slow process, and it can be followed since the earthquakes will be slowly migrating upwards.
The other force will kick in if the magma comes close enough to the surface. All magma contains volatile compounds like water and gasses. In Iceland there is comparatively little water in the mixture, but there is quite a bit of gas that can nucleate out of the magma.
The amount of gas becomes higher the closer you come to the mantleplume that is residing near Bárdarbunga below Vatnajökull. In Reykjanes the magma is poorer in gas, so the force is less.
As the magma nears the surface the pressure from the crustal over-burden will decrease, and the gasses that are locked into the magma will start to come out.
Think of it as a warm soda can that has been shaken. If the lid is on (over-burden pressure) the carbon dioxide will stay dormant in the soda, but as soon as you pop the lid you will get covered in sticky sugary soda.
As and when this start to happen the magma will start to expand and that increases the pressure, and this in turn will push apart the dyke even more, and the gasses will push mainly upwards since they are a lot lighter than the magma it is originating from.
By now we know that a dyke has formed at the old (formerly) dormant Fagradalsfjall volcanic system. From GPS measurements and InSAR we know that the dyke is 7km long (horizontal extension) and that it is 5km deep (vertical extension), and that it is 1.2 to 1.5 metres wide (dilation).
We know that in volcanic rift systems dykes prefer to extend horizontally so that they become longer as magma continues to pour into the expanding rift. This is due to the mass of the magma creating a disproportionate pressure lengthwise.
This will continue until the expanding rift encounters harder colder bedrock at the ends. When this happens, the systemic pressure will jump up a notch.
In a system like Fagradalsfjall there is a lot of pent-up strain, up to 22 meters worth of it. This means that the lava can push the sides of the dyke apart until all strain has been accounted for.
What now, 22 meters? Well, that is the maximum figure of the accumulated motion of the MAR at Fagradalsfjall since the last eruption in the region. From this we obviously need to deduct all swarms that have formed dykes since the last eruption in the region.
To this question we do not have an answer. So, the available pent-up strain might be another half a metre, or several meters.
And this is the interesting part, because as soon as that is done the pressure will notch up again, and the dyke will start to propagate upwards, plus the buoyancy effect, plus the expansion of the magma as it nucleates out volatiles… By this point there is literally no return and an eruption will occur.
Now we just need two more figures of interest. The first is how much magma is entering the system, and that is roughly 20 cubic metres per second on average. This in turn equates to 50 cubic meters per second of lava in its fully nucleated fizzy state if an eruption would occur. At least if we assume a steady state of magma arrival to eruption rate.
Now we just need to know how much magma has been emplaced at this moment. It is so far a measly 0.05 cubic kilometres and counting.
This equates to what is called a tourist eruption in Iceland, complete with nice lava fountains that may come from one or several vents along the rift. There will obviously be little, or no ash if the eruption does not end up in the ocean. I will soon return to the ocean issue.
This section could also be called: When nature kicked Carl’s model in the teeth. I can though comfort myself with being in good company on this.
My initial modelling was based on historic instrumental data. And this gave at hand that the pent-up strain was significantly less than it was.
This made me assume that the maximum possible seismic release would be akin to the famously noisy eruption at Holuhraun, and that when about as much seismic energy had been expended an eruption would occur. Oh boy was I wrong.
Back when I did my research on the Lakí eruption I modelled that it had by necessity had a very high seismic activity, including several earthquakes above M6. This also seemed to be evidenced by written sources and collapsed houses.
Back then nobody agreed with my estimates, but after seeing this I think that it was correct after seeing this utter melee of earthquakes. After all, the strain potential in the area where Lakí happened is much higher compared to Reykjanes.
I do think that Reykjanes is proving my original point. Alas, I digress…
What I had done was to take the historic instrumental data and just extend it backwards for Reykjanes all the way back to the last eruption.
In hindsight it is easy to see where this model was wrong, and I should really have caught my mistake. After an eruption, the strain build-up will be aseismic. This means that for quite some time there will be no earthquakes releasing the strain, and this creates a bias in that you would assume that there is more released strain compared to the reality of nature.
Second mistake was that I assumed that the crust was more plastic compared to what it turned out to be. This in turn meant that you get more, and larger, earthquakes compared to the size of the dyke since it takes more power to crack the rock.
Therefore, my original estimate of the needed time until an eruption would start was off. Originally my original estimate was 5 to 12 days (starting the clock from the beginning of week two of the swarm).
I am still convinced that an eruption will occur, and for the same reasons as back when I started to see this as a runup phase.
That being said, pinpointing exactly when and where is something completely different. Let us start with guesstimating the when (well, at least if nature does not throw something new into the works that I have not yet figured out).
Let us start with the length part. I cannot for the life of me see that the dyke will continue much further south, at least not longer than to near the coast. There is just no evidence that Fagradalsfjall has erupted previously that far south.
This means that the crust is becoming ever harder and more resistant the further south the dyke extends. As and when this happens the dyke can extend a bit more to the north, and then the same cold and hard crust will happen again.
After that, the question is how much the dyke can dilate as the magma pushes it apart. This is obviously the big unknown part. We do know that it is less than 22 meters, and significantly less so, but how much? I have a problem seeing more than another 1.5 to 3 meters, but here is where I was very wrong previously.
Let us say that we have another 0-5 days of horizontal extension, and 0 to 20 days of dilation. After that there is less than 2 days of vertical extension until the magma reaches the surface.
Now, is there any sign that the pressure is increasing that we can look for? Well, yes there is.
Currently the magmatic intrusion has been mostly evidenced as horizontal displacement and only locally have upwards motion been detected. As and when the horizontal displacement dies off and the vertical motion picks up pace an eruption could happen anytime.
So, what is the verdict on that? Yes, there is increasing vertical uplift, not by much but enough to take notice of.
So, the guesstimate is that an eruption is probable to occur within 2 to 25 days from now, unless something happens that is stopping things up, and I see that as ever less likely. I also believe that the eruption will occur in the southern part of the rift.