Myrdalsjokull - is Katla getting ready to rumble again?

This is likely, again, not likely to amount to much, but as Jón Frímann has noted, the Katla volcano in Iceland is getting a bit antsy again. There were a couple of deep quakes nearly two weeks ago and there have been shallower and more numerous quakes since.

These might indicate that magma is making its way upwards. We saw this a few months ago, and it did not amount to anything in the end, but it is a little early to tell yet whether this pattern will be the same, or more serious.

Earthquakes in the Myrdalsjokull caldera July 7, 2014 (Icelandic Met Office)

For those who don't know, Katla has often erupted after Eyjafjallajokull, which noticeably erupted back in 2010, shutting down air traffic at the time. And Katla is often the more violent of the two - hence the current interest.

I will update this if it develops into something that may turn out to be more serious.

The Icelandic Earthquakes are a little stronger

There seems to be an increase in the strength of the quakes that are happening along the southern part of the line, initially focussed on the Reykjanes Ridge, but now apparently spreading further north. Although it looks intense, remember the scale over which the events are happening. And also remember that these indicate some movement, so that those parts next to them that don't move will build up stress that must, ultimately, be relieved.

Figure 1. Earthquakes in Iceland in the 48 hours before May 10,2014 (Icelandic Met Office)

There is no more. . . . .yet

Iceland becomes a little more active

So I just glanced at the Iceland Earthquake map again tonight, and it has become a little more active than in recent times.

Recent quakes in Iceland (Iceland Met Office)

As a reminder:The colors of the circles show the time since the earthquakes occured (the numbers below the color palette represent hours). The latest earthquakes are shown in red and the dark blue ones occurred over 24 hours ago. The earthquakes stay blue until 48 hours have elapsed since their occurrence, then they disappear. Earthquakes bigger than M3 (on the Richter scale) are represented with green stars that turn yellow after 24 hours.

No more - as yet!

Watching Iceland

Those of you who have visited this site for a while will know that I have this rather odd curiosity about Icelandic volcanoes, and more specifically the Myrdalsjokull site that sits next to Eyjafjallajokul in southern Iceland. The Icelandic site that monitors earthquakes is one I glance at fairly regularly. It has been an interesting site to visit this winter, since – more than in most years – the island has been relatively quiescent. Except that there has been this fairly consistent activity along the southern part of the rift, running roughly from Reykjavik east to Myrdalsjokull. I bring it up again today just because the sequence of earthquakes has again moved along that fault line and ended at the glacier.

Figure 1. Recent earthquake activity in Iceland (Icelandic Met Office )

Nothing big is happening immediately, just that the whole movement along this southern wing, suggests that, in time, there will be a switch to the north-south element, and at that time the stress on the corner, where we know there has been some magma movement, might prove – as they say – earth shattering!! But this is geological, where time is on a different scale than most of us.

More rumblings in Iceland

At the beginning of April there was a burst of earthquake activity just off the northern coast of Iceland, and while it occurred offshore there was no evident major disruption. Now there has been a similar flurry of activity down along the southwest corner. (Ed note This was corrected on May 13, h/t to Steve Golson for catching the error which had east and west mixed up). This contained a number of quakes that ran up to around 4, but it also now seems to be slowing down a little.


Figure 1. Earthquakes in the last 24 hours around Iceland (Icelandic Met Office )

Jón Frímann has written a post pointing out that the activity seems somewhat cyclic, and that the last significant events were in either the 18th or 19th centuries, with no indications of any imminent danger.

Both the current events and the ones at the top of the island lie on the Mid-Atlantic rift that runs through the island, and marks where two of the earth’s plates are slowly drifting apart.

NOTE: This post was updated on Sunday, May 12th.

Figure 2. Map of Iceland showing major volcanoes (The Times of London)

However, you may note on the earthquake map that there is also a little current activity around Katla, which is the volcano that I have been expecting to be the next to erupt, after the pattern of quakes that has occurred there over the past couple of years.

Over at Volcano Café the suggestion has been made that, instead of Katla, it will be the neighbor Hekla that will go next, since there was sufficient activity there to warrant a warning to the public in March. That has since been withdrawn, after the region returned to quiescence. Yet there continues to be some activity in the Katla caldera.

I must continue to remember that imminent in geologic terms does not necessarily mean this month.


Figure 3. Relative positions of Hekla and Katla (which is under Mydralsjokull) (Icelandic Met Office)

UPDATE (Sunday 12th May). While the overall intensity of the quakes it diminishing, I can't help note that they are now extending along the rift line, and taking a straight shot at Hekla.


Figure 4. Earthquakes in Iceland leading up to noon 12th May 2013 (Icelandic Met Office)

Fixated on Katla

It has been more than a couple of years since I started to write on the potential for an eruption of the volcano at Katla in Iceland, following the eruption of its neighbor at Eyjafjallajokull, and the resulting disruption of air traffic.

One of the reasons that I continue to do so, apart from the historic pattern that Katla often erupts following Eyjafjallajokull, is the ongoing earthquake activity around the caldera. I noticed it clearly again today where with most of the zones of Iceland quiet, there is still that steady pattern. And this activity has persisted at a relatively low, but varying level, across this time. Doesn't say it will happen soon, but does continue to suggest that it is, nevertheless, going to happen. Patience is a good word.


Earthquakes in Iceland in the last 24 hours (Icelandic Met Office) Hopefully there won't be much more news for a while.

Katla twitches

While the earthquakes in the region around Myrdalsjokull have remained concentrated in and around the Katla caldera for some time, they have been relatively small. However I see that one has just crept up above a value of 3. (Barely it is a 3.1). While in itself this is not significant, it continues to suggest that there is activity within the volcano that will, in time, create problems. There has been too little associated activity to suggest that this is, however, anything critical at this stage.

Figure 1. Recent activity at the Katla volcano in Southern Iceland. The star indicates the location of a 3.1 earthquake, and the green shade indicates that it occurred within the last 24-hours. (Icelandic Met Office.)

There is no more.

Katla draws a line

Just another interesting geometric pattern:
 
Figure 1 Earthquakes around Katla in the 24-hours to June 30, 2012 (Icelandic Met Office)
  There is no more. . . . .yet!

Katla in Iceland keeps on grumbling

It has been some while since I have posted on the happenings around the Katla volcano in Iceland. I still drop by the Icelandic Met Office site to see how the quake patterns are developing, and the region continues to be the focus of earthquakes that consistently focus around the caldera of the volcano. Something is obviously going to happen, but what and when remain somewhat of a mystery. But I was struck by the pattern of quakes over the past 24-hours. Nothing of any great significance – though I wonder why the quakes are forming the circular pattern? We shall see, no doubt.
Figure 1. Pattern of quakes on Mydralsjokull over the 24-hours up to June 27, 2012 (Icelandic Met Office).
  Jón Frímann has noted that there have been several earthquake swarms in the region in recent weeks, though the most recent did not have any harmonic tremors associated with it. But since this could be an indicator of some thermal activity under the ice cap, it may lead to a flood of melt water from the glacier in the near future. No more.

Rumblings in Iceland, (Katla sleeps restlessly)

The problem of dealing with Geological time and having events that occur in very short intervals thereof, is that for those of us on a faster life schedule, “almost instantly” can still be a period of years.

That thought has struck me quite frequently as, over the past year, I have watched the earthquakes that are happening around the Myrdalsjokyll glacier in Iceland, with the Katla volcano rumbling beneath it. I first noted how the earthquakes in the region were focusing down into the caldera of the volcano last May. Since then I reported occasionally as the intensity of the underlying earthquakes increased in intensity up to levels above 3.0, and started to align along possible fissures that might lead from the magma chambers up to the surface.

In the end the volcano did not seriously erupt, but nor did the patterns change that much, and I have kept an eye on the region over the months since. It is worth noting that the pattern of quakes continues to focus in the region of Katla, and that while they vary in intensity and frequency (some days there may only be one or two quakes) that they are still ongoing.

Pattern of Earthquakes in Iceland in the last 24-hours (Icelandic Met Office )

If one clicks on the map at the site over the clump of quakes at the bottom (those on the right at the base of the peninsula are caused by water injection from the geothermal program in Iceland) then one gets this picture:

Quakes in the Katla region of Iceland in the last 24-hours (Icelandic Met Office)

This is about as dispersed a pattern as there has been over the last few months in the region. Eyjafjallajokull was the volcano that erupted two years ago, and brought some disruption to Europe. Katla will likely go at a somewhat greater scale, and likely quite soon – in geological time. For those with shorter attention spans it remains hard to tell whether that will be in 6 months, or 6 years. We’ll just have to keep watching.

Still watching Katla

It has been a little while since I wrote about the possible eruption of the Katla volcano in Iceland. For a while it appeared that the increased frequency of earthquakes and their growing magnitude, was leading toward an eruption. Since then the mountain has become somewhat quieter; but the earthquakes remain focused around the caldera, and continue to occur at a higher than background rate. Looking at the pattern over the past 24-hours, there is still a strong linear component to the earthquake pattern, which would tie in the continue evolution of fissures along the edge of the caldera.

Earthquakes around Katla in 24-hours (Icelandic Met Office ).
UPDATED

Jón Frímann does not expect this activity to lead to an imminent eruption, nor do I. But the continued linear nature and focused activity around the caldera is indicative of an ongoing preparation for an eruption. However we are talking about geological events, where time has a somewhat different meaning. Imminence is more likely therefore to be months, rather than days. Yet, as Jon notes, normally winter is a period of quiescence and this year we have not seen that. So I continue to expect an eruption of some significant size, it is just not clear when.

UPDATE
Revisiting the site a day later, there is still a lot of ongoing focused activity, though not yet strong enough to show any immediate large scale activity.

The Icelandic volcano at Katla is becoming yet more active

I realize that any claims I have as a prophet have faded as the ash that lies on the Myrdalsjokull glacier from the Eyjafjallajokull eruption last year, failed to be blown away under the summer sun. The underlying volcano, that is Katla, while focusing the earthquake activity in the region has done little more than have a relatively small jokulhlaup (or glacial flood) or two. But I can’t help stirring that old pot at least one more time. Because the earthquake activity at the site remains focused in the caldera, and there has been a recent increase in the frequency with which magnitude 3+ quakes have occurred at the site. I was checking on the one that happened last week, and noticed that there are two new ones since last I looked.

Earthquakes in the last 24 hours around the Katla volcano in Iceland (Icelandic Met Office)

UPDATE: The Icelandic Met Office has another page, which shows the quake map in a different mode, and also gives the frequency over the past day. It currently shows an interesting pattern for the quakes of the last 24 hours.

Earthquakes around Katla in the last 24 hours (Icelandic Met Office ) The line follows that of the perimeter of the caldera (the black hatched line).

Note the site also shows the corrected size of the earthquakes, while the earlier site gives an automated assessment. and, as an update, there are consequent tremors that are falling along the same boundary.

The larger earthquakes are also increasing in size, these two being a 3.8 and then a 3.3, occurring about 3 km apart, which with the intervening activity (the red dots) which included a 2.4 suggests that there just might be a fissure opening. (The now corrected values are given in the lower figure).

Three days ago Jón Frímann noticed signs from his geophones indicating that a new dike had intruded into the caldera. The current activity is a little north of that event, it is also very shallow, with the first and larger quake being at 2.5 km depth while the subsequent ones are down around 1.1 km.

Two stars for Katla (a volcanic intensity scale)

Having just finished the post on Eastern Canada, I glanced at the Icelandic Activity Map, on my way to shutting things down for the evening. Katla, the volcano that lies under the Myrdalsjokull glacier has seen a steadily increasing level of earthquake activity over the summer, as I have been noting. This now seems to be picking up a little more with two quakes in the caldera region that have reached a level of 3 or more. (Signified by a star rather than a dot). The younger (green) of the two shown was a 3.3, while the older is reported at 3.0.

Earthquakes in the region of the Katla volcano over the past 24 hours (Icelandic Met Office)

UPDATE: And just as I finish the post on Western US temperatures, and check with Katla, the activity seems to be picking up even more with a 3.9 earthquake sitting on top of the earlier 3.3 (you can see it as a yellow star in the background). But there is also a considerable amount more activity in the caldera, which has to be worrying.

Activity on the evening of the 5-6th October (Icelandic Met Office).

Jon is posting some of the seismic information on the quakes at the Iceland Volcano and Earthquake site, and the Icelandic Met Office has now issued a statement which also shows the unusual nature of the current activity, in that it includes a number of earthquakes above a level 3, and that the activity still appears to be increasing. A more distributed graph of the recent activity can be found here.

Activity picks up in the Katla calderas

We spent the last four days traveling back home and then taking a day in bringing the place back into respectability after our several-month absence.

Thus I haven't been able to respond very much to outside events, nor to make many comments or updates showing the evolving quake pattern at Katla, but the increasing activity centered around the calderas, is unusual, with the increased density being such that they could shatter the cap overlying any magma intrusion. I have been incautiously concerned about an imminent eruption in the past, but this suggests I was just getting a little ahead of the actual events. The frequency of quakes has also been increasing in the last few days, and I will revisit that earlier post to update those figures as soon as I have a little time.

Earthquakes around Katla in the last 24-hours (Iceland Met Office.

Jón Frímann has noted that there is some sign of glacier melting, suggesting that something hot is approaching the bottom of the glacier, and he has also suggested that there may be some harmonic evidence for magma movement.

UPDATE: Jon has also just posted a video that summarizes the situation very well.

Katla reminds us of her presence

Unfortunately, as I mentioned earlier, the effects of the storm that moved through New England after Hurricane Irene moved ashore extended longer than I would have thought, given that in the immediate neighborhood the branches were cleared and the streets and local area back in shape very quickly. Unfortunately the size of the electrical problems, across a number of states, meant that the power and Internet services were not as easily restored.

I have been maintaining the summary maps of the Katla earthquakes, and the quakes have continued to align along a couple of planes. The activity did not seem enough to write about, however, particularly given other distractions. That might, however, be changing again. I will add earlier summaries of the quakes recently to show that the current alignment has been consistent for a short while, but these may be added sporadically as I am also trying to draft last weekend's OGPSS post that I have needed the internet to complete.

But in the meantime, you might want to consider this:

Earthquakes in the last 24-hours under Myrdalsjokull (Icelandic Met Office )

Jón Frímann has posted the plots from his geophone in the region and he notes that they show that there has been some magma migration into the caldera.
Earlier records will be added below as I can find time - sorry.

Katla - aligning earthquakes

It is worth commenting that the activity at Katla is beginning to increase again. But it is doing so along a couple of discernible lines at the moment.

Earthquakes in the last 24- hours around Katla (Icelandic Met Office)

I first noticed this yesterday as the small increase in earthquakes and that they seemed to fall along two planes.

Earthquakes on the 9th (Icelandic Met Office)

Given that this was an increase from a period of relative quiescence, albeit with the quakes that were occurring were still in the region of previous activity, it has started to become interesting again.

Earthquakes on the 7th (Icelandic Met Office)

And from the 5th:

(Icelandic Met Office)

So, we’ll see how this progresses.

Katla continues to develop toward an upcoming volcanic eruption

The Icelandic press is reporting that there are ponds collecting as the ice in the Myrdalsjokull glacier continues to melt from the increasing heat coming from the underlying volcano, Katla, in Iceland. (h/t Jón Frímann ).

Collapsing ice cap above Katla (Iceland Review – English, and Icelandic ) (Remember that the black cover is because of the ash from Eyjafjallajokull last year)

As the fractures from the earthquakes continue to fracture to the surface, so the paths that they create allow water to migrate down to the underlying hotter rock. This then converts to steam, and flows back up helping to further melt the underside of the ice sheet, with some water possibly escaping down the mountain, under the ice. These floods can be sudden, as was the one that took out the ring road around Iceland. That has now been repaired, a week after the flood took it out.

The narrowness of the fracture paths, at least initially, will slow magma migration and this will likely allow more additional activity at the surface, although Katla does not give a lot of warning before it erupts, apparently.

One of the misfortunes of having retired and given away many of my books is that I no longer have the references that relate quake size to rock damage. A lot of this work was done in the South African gold mines, which go down over 3 km (2 miles) and are thus at the depths of many of the current quakes around Katla. As the mines extended the workings so the weight of the surrounding rock shifted, and the speed of this (usually following a mine blast with explosives that broke out some rock) would cause rock fracture in the area.

Quakes could be created that were up to a magnitude 4, similar to some of those around Katla recently, and resulted in significant rock movement and fracture. Underground this leads to problems with keeping the tunnels open and safe for the workers (since the rock bursts can be violent and hurl rock fragments a long way, as well as creating air blasts that can also be dangerous). There have also been larger ones.

But I was trying to get some sense of displacement and damage as a function of seismic strength, since when the rock fractures there is often some crushing of the rock along the interface of the fracture, and this would then be removable by the upward pressure of magma, and the fractures released the overlying confinement. The initial crushed zone probably measures just a few inches, but will be eroded out by water/steam passage and the later passage of magma, since fractured rock is easily removed. This then gives the open passages for magma to move. If the fractures intersect then the intervening rock will likely also be removed since the magma has a higher density and thus more power than mere water or steam flows. There have now been several dozen quakes within the region of the Katla caldera, and knowing the damage zone from each would allow a better estimate of how damaged the rock is. Bear in mind that the re-healed fractures (healed by cooling magma) from earlier eruptions are likely the weakest links that are now failing and opening, and in the process, therefore rebuilding the network of passages that are needed for flow. The levels of permeability generated are orders of magnitude greater than that from a typical oil well.

Where the rock is attached to the overlying ice, then cracks in the rock can also occur in the overlying ice. And you may note that in the above picture apart from the circular rings of fractures, there are three well defined line cracks that cross the circular fractures. If you also look at the web cam you can see (in daylight) some of the lines where magma has flowed up to the surface in earlier eruptions (the picture shown in the last post).

So we now watch as the process continues to unfold.

Katla in Iceland is getting closer to a volcanic eruption

So what is the definition of “hours” that I should have used in anticipating the Katla eruption in my last post on this subject? I must confess that I had anticipated about 72 hours being the likely envelope within which we would see an eruption following the magnitude 3 quake that happened there on Wednesday. But here we are just into Monday, and there has been no eruption yet. But there has been another magnitude 3.8 quake, in this case:

07-18-2011 63.660 -19.116 3.8 magnitude (1.1 km deep 7.0 km ENE of Godabunga)

There have been a flurry of small quakes, that had both preceded the current larger one, and which have now followed it (at least a dozen).

Katla earthquakes of the last 24-hours. The green star denotes the larger earthquake referred to above (Icelandic Met Office)

In the 24 hours before this last there had been over a dozen other earthquakes within the caldera region. The stress is inducing more and more fractures into the rock (hence the earthquakes) and as these coalesce and the rock becomes fragmented in the region of previous magma flows, this creates the weakened channel along which the magma can force its way. Jón Frímann is not reporting any signs of the harmonics that signal significant magma movement at the moment though he suspects that the location of the quakes, now indicate that magma is making its way to the surface. Looking at the seismograph plot that he has there is no question that activity seems to be rising.

Just as a reminder there are two webcams on the site, one is here and the other is here. Neither is showing much yet.

Katla from second webcam at 12:33 am Eastern US time.

We shall see if this is just another step along the way, or if this is now at the point of erupting. But I suspect that it won't be before I get up in the morning, so goodnight!

UPDATE
Well looking at the cloud patterns this morning, if I wanted to stretch my imagination, they do suggest there may be some steam





But as I continue watching, it is purely an artifact of the light. And the scenes return to normal.

UPDATE 2 At the end of the day the sky is clear and there is nothing visible above the surfaces on eith webcam. And so we can relax for a little. The Icelandic Met Office is also saying that an eruption is not likely. However there continue to be a significant number of small quakes that are not just at the surface. Bear in mind that after fractures are created, if the magma is to get through them they have to be held open by the fluid pressure, and that has to work its way up as the magma migrates upwards, and this all takes time.

Katla, the volcano in Iceland, seems about ready

Just before the eruption at Grimsvotn on 21st May, the relative quiescence of earthquakes above a level 3 along the volcanic rift that runs through Iceland was broken by a sequence along the line.

24 hours of earthquake activity in Iceland 22 May 2011 (Icelandic Met Office) Green stars indicate quakes above a level 3.0

with the more specific details
100) 05-21-2011 64,369 -17,201 3,6 (1,1 km deep 5,3 km SE of Grímsfjall)

101) 05-21-2011 64,154 -17,642 3,3 (1,1 km deep 30,2 km ENE of Laki)

102) 05-21-2011 64,366 -17,266 3,0 (1,1 km deep 4,3 km S of Grímsfjall)

103) 05-21-2011 63,905 -18,523 4,6 (1,1 km deep 23,1 km SW of Laki)

104) 05-21-2011 64,049 -17,770 3,1 (3,3 km deep 22,7 km E of Laki)

So now, following the events of the past few days at Katla, there has now been a larger earthquake at that site.

07-13-2011 63,651 -19,098 3,1 (1,1 km deep 7,6 km E of Goðabunga )

Quakes in the last 24-hours around Katla, the star denotes a quake larger than 3 (Icelandic Met Office)

So if you follow the logic from my previous post then Katla is about to erupt, since the larger quake denotes the creation or opening of a larger crack/ flaw/ chamber and now that the path is made, the magma should not be that far behind. We have moved through counting from months, to weeks, to days, and now I suspect we are into hours.

But at 9:38 pm all is still quiet:

Katla Webcam

Second Katla webcam
York Harbor Inn,
Maine
July 13, 2011

Why Katla is neither over nor dormant

There is a suggestion at Ice News that the volcano at Katla in Iceland is, and remains dormant. There is a similar article at the Volcanism blog noting that all is quiet. There are others including the Iceland Volcano and Earthquake blog and those who comment there, who believe, as I do, that the current events continue to presage a relatively strong eruption.

Let me step through some of the reasons for this, which involves me first going back to explaining the basic geology that is happening in Iceland, and then a little on the events that occur when rocks break, and what I conjecture then occurs post eruption, prior to the next time that one comes around. First I should add the caveat that one of the first things that one learns as a rock mechanic is that most rocks are different, i.e. there are very few cases where happened at one site is exactly repeated somewhere else, or the same the second time around, and rock properties change very rapidly over short distances, so that what I am about to write has a lot of generality to it, rather than being specific to this site. And I ask my colleagues to bear with me as I write what I recognize is a very simplified version of what happens, since I don't want to write a book chapter.

There is a certain amount of repetition in this post, of information that I have written over the past fifteen months, and for this I crave your indulgence. Nevertheless, looking at the evolution of the earthquakes around Katla since January of this year, it is clear that a magma passage has been created, which is still being developed, and will likely continue in that mode until an eruption occurs, with the likelihood that the longer it is now delayed, the larger that it will be, as I will explain below.

The volcanic complex at Eyjafjallajokull/Mrydalsjokull in Iceland is located where the Earth’s plates are moving relative to one another, and as they do so and create passages, magma from deeper within the Earth can make its way to the surface, giving the eruptions that occur from time to time. These occur through the volcanoes that line the two rifts that join in the middle of the island, Eyjafjallajokull and Katla, which lies under the Myrdalsjokull glacier, lying along the Eastern rift and on the Eurasian plate.

Illustration of the rifts through Iceland, and the location of the volcanoes (London Times)

Let me try a very simple analogy to try and explain what happens. Think of a sheet of plywood, floating in a bowl of hot glue. As I pull the sheet apart it will start to tear at some point within the sheet. The tear won’t be straight or initially continuous, and as it happens the tear will open a path for the underlying glue to flow up through the split to the surface, where it spills out, and then cools and hardens, re-attaching the two sides together and sealing over the underlying, still molten glue. The movement of the wood as it tears, lowers the forces (stresses) held within the sheet, and so, initially the wood becomes stable again. But as the sheet continues to be pulled apart, the stresses start to build again and that initial tear is now a weaker spot in the surface, and so when the pull again becomes too strong, the sheet will fail again in about the same place (the weakest link failing), opening a tear, glue will rise, sealing and reattaching the two surfaces, and the process repeats. And so, over time these points where the weakest links are located become, in the real equivalent, where volcanoes erupt at intervals. This is only partially valid as an analogy, since rock and wood fail in different ways, as I will explain, but it hopefully allows you to see a volcanic eruption as a part of a sequence of events, rather than just looking at the eruption as a single point in time.

So, if one begins with that gross simplification of events, consider now what has been happening in the region around Katla. When one applies a load to a rock sample it will start to fail normally somewhere at about 50% of the load at which it will totally fracture into pieces. The onset of that failure is detected because the cracks that are naturally found within the rock start to grow, and as they do they make sounds, and release small amounts of energy. At first the cracks that grow are somewhat randomly distributed through the sample, as any flaws that are favorably aligned to the growing stresses in the rock fail. Within a small 2-inch diameter core normally tested in the lab, this initial fracturing can only be detected with instruments and it is only when the rock gets to perhaps 80% of final load that cracking can be audibly detected. However, in the larger scale which is occurring in Iceland, these initial failure cracks are large enough that they release enough energy to be considered as small earthquakes when they occur. Initially, as the movement of the plates acts on the rock at their edges, in the same way as with with the rock core tested in the lab, these local failures and quakes are distributed throughout the region that is being stressed. And if one looks at the pattern of quakes along the rift region near Katla last February, for example, then one saw that the quakes were scattered around the region.

The scattered distribution of earthquakes in the Katla region of Iceland in February (source Icelandic Met Office)

At this time the quakes are occurring in the same way as we see the initial cracks starting to grow when we test a rock sample in the lab. There are some pre-existing weaknesses in the rock that are set so that as the load on the rock grows (the two plates are moving relatively steadily one to the other) that fail, and at these loads the distribution tends to be more random (although it also includes the weakness planes created during the last eruption).

As the cracks grow they relieve the surrounding rock of some stress, so that the load redistributes, but as the load continues to grow, so the longer weaker planes created in previous eruptions come to be more the weakest links in the surrounding rock structure, and the crack development (being simplistic) which generates the quakes begins to concentrate in location on the pre-weakened and damaged zones created in the run-up to previous earthquakes, the weakest links. So that if we look at the quake locations in April and May, one can see the regions where the quakes are occurring beginning to shrink down and focus around what turns out to be three regions.

Quakes around the region in April

Quakes in the same area in May

By last month, June, it was clear that the quakes were concentrating more on these regions and so I color coded the quakes for periods of ten days at a time.

Volcanic activity at the Katla volcano site in Iceland in June 2011. Red were in the first ten days, green in the second, and blue in the final eleven days of the month. Katla lies under the glacier Myrdalsjokull. (info from Icelandic Met Office)

The zones of concentration are in the caldera region of Katla (the bowl in the middle of the volcano), about half way between Eyjafjallajokull and Katla, and an odd grouping down near the sea.

So now we come to the first ten days of this month, and the flood of melt water from Katla. And this is where I want to explain why I am not convinced by the expert opinions suggesting there is no problem.

If one looks at the quake activity in the 24 hours before the flood, (which occurred at about 4 am Saturday the 9th) there was a flurry of small quakes in the caldera of the volcano.

Katla quakes in the last 24 hours (Icelandic Met Office )

The tear in the rock is focusing on the weak zone and beginning to open the crack up. Bear in mind that the magma under the surface rock (like the hot glue I mentioned in the beginning) can only get out through a weakened zone in the rock, and this has been developing over the past few days as the quakes show the development of a broadly damaged done under the caldera (as well as the one half-way to Eyjafjallajokull, though that is not as pronounced). Then, as Jon has noted at the Iceland Volcano and Earthquake blog, harmonics develop within the ground as the magma starts to move and the cycle are moving toward an eruption.

So now the question is, was the relatively small melt of the glacier ice that flooded the road indicative that the magma had flowed through the broken zone, and then cooled to seal off the cracks, even as the cracks had locally reduced the stress in the surrounding rock so that the pressure was relieved and the ground would become stable for another few decades.

I believe we already have the answer to that. If we look at the pattern of quakes after the event, looking at the Icelandic Met Office site as I write this at 9:30 pm Sunday, the pattern of quakes continues in the same places as it did prior to the flood.

Quakes in the last 24-hours (Icelandic Met Office)

For the fissures to be sealed and the stress distributed I expect that there will now have to be an eruption of magma to permeate the fractured zones of rock so that they no longer become susceptible to continued growth, as the overall stress on the rock is relieved. (The flow of hot glue that I mentioned at the top of the post).

As long as the quakes remain focused in the regions that they now are, those regions will continue to act as zones of stress concentration and thus further failure and weakening, until they are sealed by a larger eruption. And I expect that this will come sooner rather than later.

And, since the stress continues to build, vide the quakes continuing to happen, and the ground becomes more fractured as evidenced by those quakes, then the larger the passage will be through the ground and the greater the eruption.