We are minutely studying any hiccup from the Icelandic Usual Suspects. In the case of earthquakes we are quite used in getting all sorts of useful stuff out of them. One of my favorite ways of studying earthquakes in volcanoes is the cumulative seismic release (CSR). In the case of Grimsvötn it might even be the perfect way of predicting how close he is to erupting. Except for Hekla the Icelandic volcanoes are rather noisy, but how noisy is a volcano in the making?
I had this question ringing in my head yesterday so I tallied up the sums for Iceland’s only known proto-volcano, the Gódabunga cryptodome. A cryptodome is a magmatic emplacement that is unerupted; it is a magmatic system in the making. Normally it suffers several magmatic emplacements before erupting, but most often a cryptodome never erupts. The reason for them not erupting is that it takes a tremendous amount of energy and pressure for a volcanic opening to form. These virgin territory emplacements are hideously noisy compared to a regular volcano.
I had handy data for Eyjafjallajökull, Gódabunga, Grimsvötn, Hekla, Katla and Torfajökull. I discarded Hekla since it would not even show as a speed bump compared to the others. In the end the results got interesting.
The data is from July 1991 up to February 2013 except for Grimsvötn (1996). Energy is in Joules, as a comparison one could use a Big Mac; at 540 kilocalories it has 2.2 million Joules of energy (roughly equivalent to a 1.1M earthquake).
Eyjafjallajökull was rather active before the eruption in 2010, the total CSR was 6.8e+9 and otherwise Eyjafjallajökull was rather quiet during the period. The total tally including the VEI-4 eruption during the period was 1.06e+10. As you will notice this is surprisingly little. Let us now walk over to the quiet neighbor to the north. After the eruption this volcano has been very quiet.
This volcano latest erupted in 1477 and has since then not erupted, but it is still active. It suffers from two opposite forms of earthquakes. It simultaneously has earthquakes associated with cooling magma, and earthquakes associated with magmatic emplacement. It is probably safe to assume that this large volcano has more than one chamber, and that the most likely spot for an eruption will not be at the same place as the last one.
It would therefore have been interesting to have these earthquakes separated, so we could compare. But alas, we do not have that. The total sum during the same time period is 4.8e+9. Except for 2010 Torfajökull had higher CSR then Eyjafjallajökull during the entire period.
For being a very large volcano Grimsvötn is surprisingly quiet. The earthquakes during the period from 1996 are rather even over the years, with the only exception being
the period after the 2011 eruption where the volcano has been very quiet.
Grimsvötn seems to erupt as soon as it has reached a certain value of CSR. At least it did that the last two times in 2004 and 2011. That value is slightly more than 3.8e+9. The total strain release between 1996 and now is 7.7+9e, or slightly more than Eyjafjalla released in the 3 months before erupting up to the end of the eruption. Who thought Grimsvötn would be the shy quiet one?
I guess nobody will be surprised that Katla is a noisy volcano. It suffers from numerous small to medium sized earthquake swarms. Many of those are signs of magma entering the system. The noisiest year was 2011-2012 when the volcano suffered from a CSR of 8.9e+9. During the entire time the CSR was 3.1e+10. Now, let us go and visit the noisy unborn baby volcano next door.
Gódabunga was a quiet unassuming little Núnatak (cliff protruding above a glacier) just on the outside of the caldera wall of the massive Katla volcano when, to the surprise of everyone, it started kicking around. In 96-97 it had a CSR of 1.8e+9 and most thought it was Katla having a side emplacement. The next year it got a CSR of 6.5e+9 and it was noticed that the hypocenter of the earthquake ball was outside the Katla system. It caused a bit of a stir; remember that all of a sudden out of the blue there was more CSR than Grimsvötn produces during the entire run up to an eruption.
For two years it was showing signs of calming down after the initial emplacement. Gódabunga seemed destined to dwindle into the mist of magmatic emplacement history, the same way 99 percent of all emplacements go.
After the two years Gódabunga went on a massive spree of large emplacements that really knocked on the roof of the teenage room. During the next five years it had at least two more even larger emplacements than the initial one. The CSR for the period was 6.03e+10 with the record year (2002-2003) blasting an impressive CSR of 1.83e+10.
After those five years Gódabunga has calmed down, but the CSR is still 100 times higher than before the commotion started. The total release during the period is a whopping 9.02e+10.
What will happen to Gódabunga is written in the stars so far. Remember that science have never seen a large new volcano blast into life, especially never with instrumentation like this. The ones science has seen have been rather small, and they have been un-instrumented. If Gódabunga ever is born we only know one thing, it will be very noisy.
For those who are surprised over how quiet Grimsvötn is should remember that this volcano is erupting often. The system is permanently heated and filled with magma, so the roof over the chambers cannot withstand a lot of pressure. One should also note that the earthquakes in a volcanic system are a sign of pressure increase (or in some cases of pressure decrease as magma cools down and contracts). So, Grimsvötn will show comparatively few earthquakes before erupting since it cannot take a lot of pounding.
The fact that Grimsvötn has a weak top is what makes CSR into a good predictive tool. The amount of pressure increase the roof can take will be about the same between the eruptions. At least for as long as the eruptive cycles have roughly the same time spans.
As the roofs above volcanoes cool down it will take more and more pressure for a volcano to break through. Also size and depth of the magma chambers are factors that affect the amount of pressure needed for an eruption to occur.
Eyjafjallajökull had a rather small magmatic system that was fairly close to the surface (after an emplacement just before the period my data covers), so it had a surprisingly small conflagration of earthquakes before erupting.
Katla has not erupted since 1918, and has a very large magma system. So the roof has had time to solidify and can due to the size flex quite a lot over time. So, it is not that surprising that it can withstand a lot of CSR over time.
Gódabunga on the other hand has an unknown size of the magmatic system, but one thing is clear, it is fairly deep down, and the roof is (no pun) rock hard. Here the magma has to quite literally pound its way through layer after layer of hard old rock so naturally there is a lot of music being played. How much more pressure will it take before an eruption occurs? The answer is that nobody knows. All we can do is waiting for a new emplacement and then try to track the progress of the earthquake ball hypocenter upwards. For all we know it could withstand anywhere up to ten times as much CSR, especially if it is temporally well spaced.
If the CSR is a representation of the systemic pressure inside an unborn volcano, the Gódabunga is potentially a bad one. If we compare with El Hierro that had an eruption after a long repose time and had a very noisy eruption the energy released there was still 1 000 times less. In the end we are faced with the small thing that the CSR are caused by a magma emplacement, and the amount of activity gives a hint of the amount of magma emplaced.
Name those Volcanoes Riddle
1 point for each Volcano …
No 1 – Legendary lost home of the hairy eared dwarves? SOLVED Mount Shasta 1 point Sa’ke
No 2 – 4295 eponymous deposit. SOLVED Katla / Vedde Ash
No 3 – Thoroughly well bred parent of the US Ambassador? SOLVED Acatenango 1 point KarenZ
No 4 – Slightly under 50 miles north of a pillar of salt. SOLVED Aukland volcanic field 1 point KarenZ
No 5 – Its location and summit hold a global distance record. SOLVED Chimborazo 1 point Sa’ke
No 6 – Site of the earth’s nastiest outside loo. SOLVED Mount Elbrus 1 point Alison