Although the 2,265 volcano-tectonic earthquakes at Planchon-Peteroa as reported yesterday by Georges Vitton (Le Chaudron de Vulcain) is perhaps the more interesting news, our readers are probably more interested in the large earthquake swarms in Tjörnes Fracture Zone. What is happening? Is it an eruption, is an eruption imminent or can an eruption be expected there soon?
The TFZ lies north of Iceland and is where the MAR, Mid-Atlantic Ridge splits in two to form a microplate between the Eurasian and North American plates. Those who follow the IMO Earthquake map will be very familiar with the outlines as indicated by numerous earthquakes over the years. The Global Volcanism Program defines it as “an oblique transform zone that separates the northern volcanic zone of Iceland from the Kolbeinsey Ridge”, but there are no known or identified volcanoes there. The GVP lists one submarine eruption immediately north of Manareyjar Island in 1867-1868. If there is an eruption in the TFZ, we should expect a submarine one but as the depth of water ranges between approximately 300 to 800 metres, there is little chance of one breaching the surface (like Surtsey did in1963) or even much being evident on the surface.
So how do I find out what is or may be going on for myself? There are three things to check for immediately – tremor, drum plots and depths. First of all, go to the IMO heading “Tremor measurements”!
Had it been an eruption, tremor would have risen instantaneously and in all three bands; red, green and blue, and by several thousands of units. Since both red and green have remained virtually unaffected, there is a very good chance that this is only a matter of tectonic readjustment. Now it is time to asses which type of earthquakes we are dealing with so let’s go to the drumplots (tromlurit in Icelandic)!
The profiles are those of tectonic earthquakes with little or no hint of it being a question of anything else. Could this still be of interest to me as a volcanoholic with an unquenched thirst for eruptions or do I look elsewhere for my “fix”? Well, let’s check at what depths these earthquakes occurred!
The IMO list reveals that the quakes centred at ~8 km NNW of Gjögurtá are at depths of between 8 and 13 km whereas the swarm ~13 km NE of Grimsey ranges between approximately 10 and 15 km. Now, let’s compare those with the known thickness of the crust at the two locations!
At Gjögurtá, the thickness of the crust is approximately 25 km which places the current activity about mid-way through the crust, something that indicates that it is probably nothing more than tectonic readjustment. At Grimsey however, the thickness of the crust is no more than 15 km which places the activity right at the bottom of the crust where it meets the MOHO. IF, and it is of course a big if, it is an indication that the crust is splitting open right at the bottom, this could result in a huge fissure opening up. This in turn would lead to a dramatic lowering of pressure which can produce something known as “decompression melt”, i.e. the production of great amounts of eruptible magma.
This is something to keep an eye on if the activity continues, grows progressively shallower and begins to show evidence of volcano-tectonic or magmatic earthquakes. Will there be an eruption? Well, earlier this year the crust split open in just this manner close to Eldey on the Reykjanes Ridge SW of Reykjanes (Reykjavik). For a short period of time, there was a 50-50 chance of an eruption but in the end, nothing came of it. For whatever it is worth, it is worth keeping an eye on future developments even if this activity is not out of the ordinary for the Tjörnes Fracture Zone.