Yesterday an earthquake swarm started with a 3.4M earthquake at Herðubreið. So far the swarm has had 15 earthquakes above 2M, among those 3 where at Askja proper. This swarm as well as the previous ones at Herðubreið have been to the west of the volcano. And those earthquake swarms have been deemed to be lateral intrusions from Askja.
Only problem here is the 3 earthquakes that happened within Dyngjufjöll (Askja). Having 3 earthquakes above 2M at the same time as a medium sized earthquake swarm takes place rather beggers coincidence. I think when the hubbub of this is over the area will be removed from Kverkfjölls fissure swarm. One should also remember that Kverkfjöll is the smallest volcano on the riftline.
On the other hand, this is as far as known not anywhere near any part of Askjas fissure system. We should remember that. Personally I thought up untill now that Herðubreið itself belonged to the Frémrinamúr volcanic fissure swarm. Apparantly I was as wrong about that as the ones who thought it belonged to Kverkfjöll.
So, now we are back to a long dormant volcano that had it’s last eruption before deglaciation. And that put it as having erupted at 6000BC latest (time when the glacier withdrew). How do we know that? Thuyas only form under glaciers that are big enough to contain the erupted lava thusly forming the tell-tale tabletop look of a thuya. So, we are talking about a long dormant volcano here.
If we look at the 12 +2M earthquakes we find that 9 of those are between 2.2 and 7.9 kilometres deep. 2 of them are 1.1km deep, and that is a dummy value when an actuall depth has not been set, then we have the original 3.4M quake that has a suspiciously undeep figure. The current given depth is almost certainly around 5 to 7km and will be revised sooner or later. What does this then tell us? That the figures point towards a magmatic intrusion into an old chamber. Remember, this is my interpretation.
So, back to Herðubreið. What is Herðubreið? In my eyes Herðubreið is starting to look like a volcano on it’s own. One of the reasons is that it started to inflate just to the east before Askja started to inflate. It in fact started inflating and having earthquake swarms to the east before Askja stoped deflating. So, I am actually contemplating that Herðubreið and Askja had a common origin and has been rifted apart by the EISZ part of the MAR over the course of millenia. What I am trying to say is that they might actually share a deep root found in the current EISZ. We could think of them as two non-twins sharing the same womb and umbilical cord.
Untill we have new data from the area this is a bit speculative, but I do not think it is that much way off.