I get this question often about Iceland, especially when there is an earthquake swarm. The problem is that it is rarely clear cut if it is magmatic or tectonic when it happens in Iceland. First of all you have all of the rift zones being tectonic, but they are also subject to rift and hotspot volcanism, so if they are at the right depth, or adjacent to a magma reservoir, there can quite often be a magmatic component to them.
First of all, there are few completely magmatic earthquakes except when you have a run up for an eruption in one of the central volcanoes. And even then there is quite often tectonic components to them since the moving magma releases pent up tectonic tension. One of the few magma only earthquakes I know of is the odd double-couple earthquake that struck Bárdarbunga and that seemed to have triggered the Gjálp eruption close to Grimsvötn.
A couple of weeks ago we had a persistant earthquake swarm up in northern Iceland at Eyjafjarðar at Gjögurtá. That earthquake swarm was interesting since it started as a tectonic swarm, but quickly changed into having more and more magmatic components. As the swarm went on it caused lowered pressure at the MOHO boundary and decompression melt occurred and magma started to move upwards. In the end a classic wedge shaped magmatic intrusion formed. Here the swarm went from purely tectonic to almost purely magmatic.
A few days ago we had medium sized earthquake swarm that topped off with a M4.8 earthquake. One of my pet peeves is that it is hard to interpret what is going on before you can compare different sets of data.
Before it started we had anomalous tremor readings at the two closest SIL-stations. The tremor was in 2 – 4 Hz range. After about two days of low level tremor the earthquake swarm started. This lead me to suspect that the swarm was caused by a magmatic intrusion that in turned caused a tectonic stress release. But, without better data it was impossible to prove anything. IMO declared that the largest earthquakes was of tectonic nature after they had looked closer at the earthquake signatures.
You need GPS data collected over several days or preferably weeks to be able to analyze what has happened, and it is not until now we have that data. So let us analyze what happened from the combined data of the tremor, the earthquakes and the GPS-data. It is quite interesting.
From the GPS data from Syrfell (almost straight ontop of the action) we get a pattern that is fairly different from what one could expect from a purely tectonic event. The green line is when the M4.8 earthquake happened. The earthquake within an instant pushed the GPS 1 full centimeter westwards and a couple of millimeters to the north. This is what we would expect from a pure tectonic event, a sudden jolt. Problem is that it is not all that we are seeing.
Roughly two weeks (red line) before the M4.8 we start to see uplift at Syrfell and just a few days before a local bradyseism starts and the GPS accelerates to the north. So, we have up component with associated north motion. This is happening at the same time as we have the anomalous 2 – 4 Hz tremor. This means that we have magma moving into the system causing inflation prior to the earthquake swarm, this in turn shifted the faultline enough to cause a release of pent up tectonic energy.
What is really interesting is what is happening after the earthquake swarm ended. Most often the trajectories will return to the same type of pattern observed before the tectonic earthquake. But that does not seem to be the case here. Instead we get a slight down-component while the north and west movement continues unabated. What now is going on?
My interpretation is that a magmatic intrusion occurred into a known and active volcanic field (Svartsengi) with a known magmatic reservoir and an associated hydrothermal field (with a power-plant). The system seems to be open into depth so no earthquake activity associated with the magma motion occurred, or was drowned out by the tectonic activity. As the tectonic swarm happened it opened up a void which was filled with magma, this relieved the pressure built up in the magma reservoir and deflation occurred, but this did not decrease the pressure in the NW direction since the rifting and magma reservoir is south-east of the Syrfell GPS.
So, we have magmatic intrusion that releases a tectonic event that in turn enables the magma to move further on. Since the motion in the NW direction continues my guess is that we will soon see inflation resume.
The ground rule for Iceland is that it very rarely is purely tectonic or purely magmatic earthquakes that happen, it is a combination of both and the question is more how the magmatic and tectonic components interact.