This will just be a short update with the information that has come at hand.
There seems to be a full on war between Pevolca, Involcan and José Luis Barrera VP of the ICOG (Spannish Association of Geologists). Pevolca has stated that there are some reneval of tectonic earthquakes, with no risk for the population or any need for any measurments to be taken. They also point out that there is no increase in gases. They do though mention that they will be watching things. They have also stated that there is no inflation at El Hierro (GPS).
Involcan has stated there is inflation after studying the GPS system of Professor Sagiya from Nagoya. They also point out that the reason for Pevolca not being able to see any heightened gas levels is that they have not measured the gas since April 5.
José Luis Barrera and the ICOG have issued a statement that the more than 350 earthquakes are tectonic, but that they might be a run-up phase for renewed volcanic eruption. They also note that the activity is unusual.
While the Spannish authorities and organisations are involved in their usuall pissing contest there are some things worthy of comment.
A little tidbit on the earthquakes during the 48 last hours…
Yesterday had the fifth highest recorded number of earthquakes (241), the highest recorded number is close to 454, and that was in August as the former wad of magma had it’s peak of arrival.
But what is really interesting is that if one take a look at how those quakes break down into size…
Yesterday first, then August number-record.
0-2M 55 (448)
2-3M 180 (6)
3<M 8 (0)
And that would have made yesterday into releasing 5 times as much energy and destroying about 32 times as much rock. It was the record of all time energywhise. This leads me to believe that the amound and speed of arrival is higher this time around. So, the last two days have had the largest accumulated seismic energy release since onset of activity at El Hierro. Energy record in short. And that is note noteworthy according to Pevolca.
LP Earthquakes
The earthquakes today has long amplitude component to them normally associated with magmatic movement into the cracks. These LP earthquakes is considered to be magmatic. There has during the last 8 hours been at least 3 of them. The 17.16 is one of them.
I believe this is onset of movement upwards of magma into the actual system of Tanganasoga.
First LP
Image by IGN. The First LP Earthquake.
Second LP
Image by IGN. The Secong LP Earthquake.
Third LP
Image by IGN. Third LP Earthquake, and what is most likely onset of a heavy magmatic intrusion upwards.
The third LP seems to have opened a conduit somewhere, most likely from the crustal boundary (MOHO) up to the chamber under Tanganasoga, or directly towards Bob. The Long Periodicity Earthquake is caused by an initial earthquake that opens up a fissure, sill or dyke, after that magma moves in to fill the opening, and that creates and unusual type of earthquakes.
Conclusion
There is no reason that I should recant on what I wrote yesterday. I still believe that there is a rather high risk of a new eruption at El Hierro. I still see no reason to not believe it will be in the southern part of the island, or out in the ocean south of La Restinga. I still feel that La Restinga is not entirely safe for it’s population.
Update
While I was writing this post the signal changed sufficiently for me to believe that there is risk that the eruption is either about to start, or has already started. We are all waiting for news, and think about the unprotected civilians in El Hierro.
Photograph by Zanthia. On the picture one can see Mount Herðubreið. Herðubreið (Broad Sholder) is a tabletop mountain, or in another word, a Thuya.
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.
Image by Icelandic Met Office (IMO). Askja to the left of the green star, the black “circle” above is Frémrinamúr. Kverkfjöll is due south and not showing here.
Image by Icelandic Met Office (IMO).
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.
Image by Icelandic Met Office (IMO). There seems to be magmatic components to the earthquake swarm when looking at a higher resolution.
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.
Image by Icelandic Met Office (IMO). The earthquake swarm shows well also at the Dyngjufjöll SIL-station.
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.
Image by University of Iceland and Professor Sigrún Hréinsdottir. Inflation showing at Askja. The inflation at Herðubreið started 2 years before.
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.
Picture from sandandreas.org Picture shows the perhaps most famous faul line on the planet, the San Andreas Fault. It is the cause of large earthquakes in California.
What is an earthquake?
Perhaps the question should have been; when does an earthquake start? For me as a physicist what we normally perceive as an earthquake is just the boring business end of a rather fascinating process, because the earthquake actually starts far back in time.
What we normally see and feel as an earthquake is nothing else than rock breaking. During a small earthquake it is not a larger piece than what you might find in your back yard, and when it is a large earthquake it can be a breakage running for hundreds of kilometers in a fault-line. To simplify it we say that the amount of energy released is the same as the area of the rock breaking in a fault line. There are other factors of course, but this is not the time for that.
Back to when an earthquake starts. To answer that well we have to understand that there are 3 types of earthquakes (that we need to mind today, there are more types of course).
Tectonic earthquakes
This kind of earthquake is driven by the movement of the continental plates; the motion is rather slow from a human standpoint. The speed of the continental plates is normally ranging from 1 to 5 centimeters per year. This type comes in 3 common subtypes; rifting (Mid Atlantic Rift) is when two plates are being pulled apart, clearly visible as the MAR slowly pulls Iceland apart. The second is subduction earthquakes as one continent plate is pushed down under another. Japan is a good example of this. Then we have shearing rifts, which is when two continental plates are sliding past each other.
In general the rifting earthquakes are the weakest, followed by the medium of shearing rift quakes, and then the subduction earthquakes as the strongest (Japan and Chile). The reason for this being the relative crust thickness, a rift zone has to thin crust to create the largest earthquakes, a shearing quake only affects crustal parts that are rubbing together, and the old crusts that are involved in subduction earthquakes has had the time to grow thick enough.
Math is simple. If the fracture is ten km long in a rift zone with ten km thick average crust, then a full faulting would affect 100 square kilometer. The same for 50 km thick subduction earthquake would be 500 square kilometer. Also we have the fact that the old rock in a subduction zone is generally more brittle and have a longer faulting.
Tingvellir on Iceland, the site of many fault lines after earthquakes. The entire area has sunk down due to all rifts.
Magmatectonic
A magmatectonic earthquake is when magma pushes upwards and releases pent up energy that already exists in a rift, shear or subduction zone. Here the movement of magma works as a trigger mechanism, but is rarely the cause of the earthquake. It works more like the small stone that topples the wagon.
Magmatic earthquakes
These are caused by inflow of magma into dykes or magma chambers. The pressure of the magma pushes the rock until it flexes and finally breaks. I will come back to this one.
Age of earthquakes
Here comes what I find to be the interesting part of earthquakes. The age and the processes that make some places able to contain more energy (the last part of which won’t be covered here). I am going to use MAR earthquakes as an example here. As the MAR is spreading with an average of 5cm a year it starts to stretch an area of newly formed crust, our part of the crust is when it starts to stretch 10km thick (just an example). It is rather ductile, so it can take quite a bit of stretching, how much of it we know fairly well.
How do we know it? Well, as an earthquake happens it makes a crack, these cracks normally ranges from 1 to 10 meters or sometimes even more. Our earthquake will give a 5 meter crack. Simple mathematics would give that it would take about a hundred years for our earthquake to form. Problem is just that normally only half of the energy pent up is released in one event. So, it would be more like two hundred years since our earthquake got under way.
The size of our quake will now be determined by the area affected, and effected via the length and depth of the faulting line.
I have here made enormous simplifications, so anybody who wants to explain more in detail in a comment is most welcome to do so.
A magmatectonic earthquakes age is harder to determine, it can be a small amount of magma that hits an old pre-earthquake fault line that is close to faulting. Or it can be a lot of magma hitting a juvenile fault line. The effect will be pretty much the same. With one difference, the first one will most likely not cause an eruption; the second is likely to do so. And, if you have a mature fault line hit by a lot of magma, well that would be your basic Laki eruption.
Magmatic earthquakes are normally fast, anything from a few years, to a few days. In the case of Theistareykjarbunga it took about a year from inflation started to the first earthquake, at El Hierro it took weeks. This is normally governed by the size of the magma chamber, which in Theistareykjarbungas case is huge, compared to El Hierros chambers.
Picture copyright by Magnus Lundgren. Ever been dreaming about diving from one continent to another? Thanks to Magnus stunning picture we can see how it is can be done on Iceland. On one side we see America, on the other Europe. Only in Icelands Thingvellir Fault Zone is it this easy.
Earthquakes and Volcanoes
Magmatic earthquakes are caused solely by pressure from magma and the gases released by the magma. The exception is of course rift zone volcanoes like Krafla. It can either be pressure from below as new magma comes up through the mantle, it can be dyke intrusions, feeder tubes opening up, or inflation of magma chambers, and of course as the vent is opening up as the last stage of an eruption.
Here comes the thing, size does matter. A small 1M earthquake is cracking your average garden stone (well, a large one). A 3M quake is producing a fault that is roughly 100 times larger, and releasing 1000 more times the energy counted in destructive force. I am not going into the math of that here, it is fairly complex. But if someone wants to do it in a comment, please feel free.
So it takes 100 1M earthquakes to fault as large an area as one 3M earthquake, and it takes a thousand of them to crush as much stone (this would be the important number when talking about magma chamber formation). You should think mining blasts here. It is one thing to crack a fault line the length of the detonation on the video below, it is something completely else to blow all that rock to smithereens.
So please, if you see one earthquake of 1M strength, it is not important. If you see a hundred it might be interesting as a sign of something. But to actually have any significance at all, you need a lot more than that. Before a normal eruption you have hundreds or thousands of earthquakes ranging from M1 to M3 or more. And the 1Ms are not even important really. Only exception to this rule that I know of is of course Hekla. That one just needs a few below 1M, and then one around 2M, and then it goes off. But that is the known exception.
And let me reiterate this, I am not a geologist, nor a geophysicist, I am just a normal physicist trying to explain something really hard, and by doing so I have simplified things a lot. If you find that it is too simplified, or simplified so much that it is useless, feel free to compliment and explain further in the comment field. I will only appreciate it.
And once again, small earthquakes are really small, don’t be fooled.
And now over to a huge detonation courtesy of New Boliden Mining AB, I just like big detonations. Sorry, it is probably genetic among Swedes, blame Nobel. 1100 ton explosives used, 2Kton destructive nuclear device equivalent. And now to the real candy, this a 4M earthquake equivalent.
