There are several factors that are monitored in an active volcano. For most Icelandic volcanoes the things that are monitored and are publicly available are tremor, earthquakes and GPS. In the case of Hekla there is also available mountain strain data from borehole strainmeters.
For some volcanoes there are also periodic gas measurements taken and measurements of the conductivity in the water. These are though normally not made publicly available. But, the available data is enough to give us a lot of information on any given volcano.
A brief background on Katla
Katla is one of Iceland’s larger central volcanoes and is capable of having eruptions that are VEI-5. It is not one of the more prolific volcanoes in Iceland, at least compared to Bárdarbunga, Hekla and Grimsvötn. Including the 934 Éldgja eruption Katla has only erupted 15 times since settlement of Iceland. That would make an average of one eruption about every 72 years. The last real eruption at Katla was a large VEI-4 in 1918. After that there has been at least one episode of what could have been a hydrothermal event or a phreatic detonation in 2011.
Katla has a large and shallow magmatic chamber situated between what is believed to be 2 to 6 km depth. The eruptions have a tendency to happen in different parts of the caldera. In 2002 a period of increased seismic unrest began under the volcano. It is though important to differentiate the seismic activity going on in the Gódabunga cryptodome and the earthquakes inside the caldera since they originate from different magmatic emplacements that most likely are not related.
At the current moment there is not much pointing towards an eruption in Katla, let us be very clear on this.
The first one has to remember is that one has to read all the data put together and not obsess with just one data set, and then one has to compare it with known historical data that is available. If one does not do this one will be lead astray time and again and start jabbering about Katla being close to erupting.
Let us start with the earthquakes at Katla. These can be divided into 3 categories, shallow earthquakes between 0 to 2 kilometers depth, medium depth earthquakes at 2 to 12 kilometers depth, and the deep earthquakes that take place between 12 to 30 kilometers depth.
If we look at the shallow earthquakes they are cyclical in pattern and happen far more often in the summer. This is most likely due to them being caused by an increase of water due to glacial melt affecting the hydrothermal system in the caldera. There is also blocking and fracturing occurring at the caldera floor. These earthquakes are mainly uninteresting unless they are large and accompanied by a sharp rise in tremor. If that occurs there is most likely a hydrothermal event or a phreatic event taking place. A real eruption would give more data that would show well in advance, mostly as large and fast uplift of the GPS stations.
The medium depth earthquakes take place due to motion of magma from different parts in the system or by cooling and contraction of a part of the magmatic system. These earthquakes are normal for a volcano that is active and are not normally a sign of an impending eruption. During a magmatic emplacement there will be an increase in these earthquakes, but then you will also see uplift at the GPS stations. There is currently no uplift being recorded at the GPS stations of Katla.
3D-plot of the current earthquake activity at Katla made by Cryphia
The most interesting earthquakes are the deep earthquakes. They are often taking place as new magma is forming, or as new magma moves up into the system. One can see these earthquakes as a potential for a new magmatic emplacement, or as an early warning for increased activity further up in the system. Since 1999 the amount of deep earthquakes have slowly increased, they normally consist of small earthquakes, but there has been a few that have been about 2M. From 1999 up until now the trend have been towards ever deeper earthquakes, as well as an increase in numbers. This is so far the only real evidence that Katla might be nearing an eruption.
If we then read the GPS data we can see that Katla has had at least two minor episodes of magmatic infusion into the system. These have not been large and are normal for an active volcano. If we compare with the uplift of other volcanoes in Iceland we believe that we will see a much grander amount of uplift before a real eruption.
If we put it all together we see an increasing amount of deep earthquakes that might be a sign of future unrest, but when we take into account that pretty much every other sign is lacking we get a picture that is rather different than what people think. In reality Katla is at least 1 to 10 years away from being able to have an eruption. This could of course change at any time, and rest assured that you will read about it here if that happens.
So, now that we know that the world press is wrong about her being ready to blow, how about the idea that she is a doomsday volcano? That is not true either. If we look at historical eruption data for Katla we see that a likely eruption would be between 10 times smaller to about ten times larger than the 2010 Eyjafjallajökull eruption. If we compare with the Grimsvötn 2011 eruption a Katla eruption would most likely be 50 times smaller to twice the size. And that is far from a doomsday volcano. One should though note that the ash from Katla could cause problems for aviation. So, if an eruption occurred there would most likely be widespread problems during the first week or two of the eruption, but that is about it.