Lately there have been a few earthquakes with a magnitude larger than M3 at the ring fault around the Bárdarbunga caldera. This has led to a lot of question about that particular volcano, and that is to be expected after a medium sized Icelandic eruption. They do tend to attract attention.
This has been coupled with an erroneous factoid that Bárdarbunga is inflating at a rate of 10 centimeters per year. In reality this factoid was caused by selective data-picking, GFUM-station (Grimsvötn) was omitted from the dataset. If you omit that station the data indicates 10 millimeters inflation per year at Bárdarbunga.
After reviewing earthquake and GPS data I think it is more furtive to take a broader look at what is going on under the Vatnajökull, there are after all more volcanoes there.
This volcano is a steady supply of Jökulhlaups caused by the geothermal energy release into its twin caldera. Sometimes it tends to have brief eruptions or suffers from phreatic detonations. It has been relatively quiescent after the medium sized eruption from Bárdarbunga. One interpretation would be that the eruption lowered the magmatic pressure in the volcano. If that is true we can expect a quiet period from this volcano.
On the other hand, there were a few deep earthquakes indicating fresh magma entering the system prior to the Bárdarbunga eruption, so there could be some small scale activity upcoming in the future, but nothing seems to be imminent.
If we look at the proven data at hand, the earthquakes, we get a different picture of what is going on. But, let us start with looking back at the events prior to the onset of the eruption. First we saw an intrusion coming up from depth north of the Bárdarbunga volcano, that intrusion in turn changed direction and headed almost dead south and entered the magmatic reservoir of Bárdarbunga. That reservoir was already teetering on the brink of an eruption and the fresh magma caused the reservoir to rupture in the ENE side and a dyke started to propagate.
The ensuing eruption caused a massive drop in pressure and the caldera floor fell like a gigantic plug as magma moved out. As the eruption dwindled the subsidence dropped accordingly until it stopped.
Now it would be easy to believe that the volcano would start to inflate immediately. But that is not how it works. Before any inflation can occur the pressure must first build, and that takes a long time since the magmatic inflow is not consistent, instead it happens in fits and bursts, almost always shown as earthquakes at depth or around the magma chamber. So far, there is very little evidence of this taking place.
So what then are those M3+ earthquakes? Well, it is just a bout of resettlement earthquakes after the medium sized eruption. The very shallow depths seem to indicate that it is the edges of the caldera plug that is settling downwards a little. This might be caused by seasonal ice pressure, it is after all winter in Iceland.
Well then, where did then all the inflation hoopla come from? Well, it is absolutely no GPS-data to support any 10 centimeter per year uplift at Bárdarbunga. But if we look at the usual suspect Grimsvötn we find that uplift. And that is not unexpected, after all Grimsvötn has the highest and most even rate of magmatic intrusion on Iceland.
When Bárdarbunga put in an appearance Grimsvötn was already busy inflating after the VEI-4 eruption of 2011. As the Bárdarbunga eruption happened it momentarily released tension in the magmatic reservoir of Grimsvötn, but as soon as the eruption ended the pressure continued the build up towards the next eruption.
Grimsvötn is normally fairly quiescent at the beginning after a big eruption, but the inflation is almost linear. This seems to imply that the inflation itself is almost aseismic, but as the pressure increases the magma reservoirs start to “creek and groan” as the reservoirs surrounding bedrock start to fracture with the mounting pressure.
The inflation-rate and a slight uptick in seismic release seem to imply that we are closing in on the next Grimsvötn eruption. GPS data points to it being close, 6 months to 2 years away. On the other hand cumulative seismic release data points to an eruption occurring 3 to 5 years from now. It is though a good idea to remember that the cumulative seismic release plot can increase very rapidly, and I think that it soon will do so.
If you are waiting for an Icelandic eruption happening, the best bet is Grimsvötn. As we get closer to the eruption I will be able to give a better prediction about when it will go off.
P.S. During the weekend the Whizzards of all Things Technical will do some changes and move the place to it’s brand spanking new permanent home. Hopefully you will not notice any difference at all, except a faster better Volcanocafé. D.S.