Drumbeats of El Hierro

Photograph by Jon Major and Dan Dzurisin, courtesy USGS. A mountain being pushed up half a centimeter every other minute

A magmatectonic phenomenon called Drumbeats was originally discovered by the USGS at Mount Saint Helens Back in 2006. At 16.15 a similar phenomenon started at El Hierro in the Canary Islands, Spain.

First of all, let me say once and for all, El Hierro will not erupt like Mount Saint Helens. They are two totally different types of volcanoes.

Image: IGN, forcefully released thanks to EU and Swedish law. Clear and present drumbeats started at 16.15

At Mount Saint Helens it was caused by pressure build up pushing a magmatic plug that had formed in the vent after the explosive eruption. That pressure than pushed the plug up 5 millimeters at every other minute. Seeing a small mountain jumping up half a centimeter every 120 seconds would be, I presume, a rather impressive thing to actually watch.

In all likelihood it is a similar process that is ongoing at El Hierro as I write this. But, it is very unlikely that there will be a plug pressed out, it is more likely that this is a widening of a fissure leading up to a new eruptions spot, or a widening process of the tube leading to Bob south of La Restinga. It could also be pulsations in the magma flow.

I guess that before the evening is over we will have a plot showing exactly from where the Drumbeats are originating from.

Physics lesson

A short lesson of physics seems to be in order here. The reason for this is that Avcan has stated something that is plain wrong regarding a ship putting out “pulses” that creates the Drumbeats.

The energy in each pulse (drumbeat) is a function of amplitude times duration. In this case the required energy needed in each pulse varies between 60MJ and 360 MJ. What is that then? Well, that would be the same as 60 to 360 mega-Watts of electricity. Or 60 to 360 cars hitting the same spot at 100 km/h each. Or 15 to 83kg of C4 explosive. To put it mildly, there is no ship and no equipment on El Hierro capable of producing that every other minute for hours. Pure, and simple physics. Remember that we can see the signal on other stations, on other islands.

Logic is good, but physics is better when dealing with nature!

CARL

http://news.nationalgeographic.com/news/2006/11/061122-volcanoes.html

http://www.isla.hawaii.edu/volcano/IWARS06/pdf_presentations/matoza_iwars06_sthelens.pdf
 Realy good Power-point, a bit big.

http://pubs.usgs.gov/pp/1769/chapters/p1769_chapter01.pdf
 This paper is explaining what I think is happening here.

Images of El Hierros magmatic system

Image: GeoLurking, rights reserved. Basic triangulation, how to find a spot or a station

First, my qualifications. None. I have been a fan of geophysical processes and phenomena for the last 35 years. More so if you count the time spent hanging out in the archeological part of the library.

Image: GeoLurking, rights reserved. Margin of error from one station within one standard deviation

When an earthquake occurs, the energy from that quake travels directly to the seismic station. Granted, there is more to it than that, and it actually takes a curved path through the earth depending on the density of the material (refraction), but for our purposes it’s a direct path.

When an earthquake appears in a seismic catalogue, you are given the latitude and longitude of the epicenter, and the depth. The addition of depth turns that position report into a hypocenter, because it locates the quake in three dimensions. In the more detailed phase portion of the reports, you can obtain the arrival times of the quakes. Take the difference in arrival times for each station with respect to the event time, and you have how long it took for the quake’s energy to reach the station. Knowing the distance from the station to the quake, and you can work out the speed of that seismic wave.

Image: GeoLurking, rights reserved. The P-wave

This presents a problem if you don’t realize what you are looking at. The distance from the station to the quake, is usually seen as surface distance if you use a map. This is not the actual path that the wave took. Remember, it takes a direct path. How do you find it?

Geologists, and geological organizations, think in terms of central angle when discussing seismic events. This is the angular distance as measured from the center of the earth that describes the arc length on the surface between two points. I’ve mentioned AK135 before, in it you can find the expected arrival times for the various phases based on our current model of the Earth’s structure. All of its data is listed by the central angle. It is used to assist in identifying what each individual squiggle in a seismic trace represents. (Based on the path that particular portion of the wave took)

Image: GeoLurking, rights reserved. The S-wave

For the simplistic technique employed here, AK135 and the more advanced concepts aren’t really needed… as long as we remember that this is a simplified approach, and prone to error.

Treating the orientation of the seismic station and the quake as being two points on a slice of a sphere, the problem reduces to being a series of calculations on a circle. You have the chord, which is point from the seismic station to a point equidistant from the quake on the other side of the quake. That chord will have a height which is the distance from it to the part directly below the epicenter. Calculate that and you can then find the parts of a right triangle when you determine the depth of the quake in relation to the midpoint of the chord. Find the hypotenuse and you have the direct patch distance to the quake. If your eyes have glassed over by now, don’t sweat it. It took me three days to get the spreadsheet formulas down. Sometimes you just have to get up and walk away for a while.

Image: GeoLurking, all rights reserved. P and S wave paths

Now that we have the direct path distance, we can calculate the actual P-wave and S-wave speeds. We can do something with that.

Before we do, I want to point out (yet again) that this IS NOT seismic tomography. That involves far more than our simple juggling of the geometry and data.

Image: GeoLurking, rights reserved. Preliminary image

P-waves are compression waves, much like sounds are a compression waves. The molecules move toward and away from the direction of travel. S-waves are transverse waves, they move side to side with respect to the direction of travel. The density of the medium that the waves travel through affects the speed of propagation, or how fast the waves moves. S-waves cannot travel through a pure liquid as S-waves.

When the rock density goes up, the difference in the speed betweed P and S waves grows smaller. With lower rock density, the speed difference grows larger.

Image: GeoLurking, rights reserved.

About the Plots

I’m not going to interpret what the plots mean. I’ll leave that to Carl and the others. But I will tell you what they are (it took me a few days to figure this one out). They are a plot of the speed field of quakes in a particular area. The quadratic surface interpolates the regions between the individual quakes, and shows you what a quake originating there should look like if one occurred there… based on the speed of the ones that did occur.

Like I said, it’s not tomography, but failing that, it’s about as good a representation of what is going down there that we can obtain as non geologists, based on available data.

Image: GeoLurking, rights reserved.

Since quakes occur over a period of time, these plots are a summary of what was observed over that period of time. In other words, they have very poor temporal resolution. Keep that in mind.

Other aspects of the plots show what would be expected from rising pockets of magma… compression of the overlying material, and a subsequent decrease in the speed difference of P and S waves.

Image: GeoLurking, rights reserved.

The unnerving part of these plots, and why I was apprehensive about releasing them, is that they dovetail well with anecdotal information about the dynamics in play.

Image: GeoLurking, rights reserved. The original image that started the discussion. Here we can see what might be a magma chamber, and the feeder tube that goes down into the deep. Location of the inferred magma chamber is below Tanganasoga volcano.

Again, my standard caveat: I am not a geologist and am not trained in that field. I could be very wrong in my methods. Take it with questioning view, which is sane way to look at it.

GeoLurking

Afterword: Peer review

In science when you wish to publish something new, you send the paper in and then a review-board reads it through, and either it pass and becomes published (often after revisions), or not.

Since this is something that can affect a lot of peoples lives the author wanted the post checked. So, here is how the peer review was done.

First I picked a part the reasoning, and then I looked if I would get the same result. I also went through  the physics of it to see that it checked out.

Then the paper was sent to an engineer in the oil industry that works with tomography on a daily basis. And he found it to be correct.

After that I started to think about what other ways one could “see” what the author had found. So I deduced that the quakes that are smaller then 2M are just hiding the real action. So I tricked KarenZ and Ursula into plotting that, and guess what, the same structures showed up again. For those who follow the comments in here, those plots are there to be viewed.

I guess there are more ways we could have checked it, but in the end, I wished to publish this before the volcano became demented with old age.

CARL

Infernal Sheepy Dalek Bar

The Dalek magma-fireplace of the Sheepy Dalek Bar

Midway upon the journey of our life I found myself within a forest dark,
For the straightforward pathway had been lost.
Ah me! how hard a thing it is to say What was this forest savage, rough, and stern, Which in the very thought renews the fear.
So bitter is it, death is little more; But of the good to treat, which there I found, Speak will I of the other things I saw there.
I cannot well repeat how there I entered,
So full was I of slumber at the moment In which I had abandoned the true way.
But after I had reached a mountain’s foot, At that point where the valley terminated, Which had with consternation pierced my heart,
Upward I looked, and I beheld its shoulders, Vested already with that planet’s rays Which leadeth others right by every road.
Then was the fear a little quieted That in my heart’s lake had endured throughout
The night, which I had passed so piteously.
And even as he, who, with distressful breath, Forth issued from the sea upon the shore, Turns to the water perilous and gazes;
So did my soul, that still was fleeing onward, Turn itself back to re-behold the pass Which never yet a living person left.

DANTE ALIGIERI

Some activity in Iceland

As most of you have noticed, Iceland have been really quiet lately. Well, now there seems to be a bit of activity in Iceland. So, here is the first activity report of Iceland on this blog.

Katla, or more to the point, Godabunga put in a short appearance;

Thursday 24.11.2011 15:39:13 63.630 -19.190 1.7 km 2.9 99.0 3.2 km ESE of Goðabunga

Image: IMO, rights reserved. Almost a star at Godabunga

There also seems to have been a small tremor-spike (for being Godabunga);

Image: IMO, rights reserved. A small tremorspike

This seems to a rather small thing, I would say that this is just an ordinary day in the life of Godabunga. But, sooner or later there will be a re-start of the runup for the real eruption of Katla. For the record, this is not the day for Katla.

Tjörnes Fracture zone has an ongoing quake swarm, there is no ongoing tremoring there so it seems to be tectonic only. So no signs of an eruption there. This behaviour is quite normal for TFZ, and can run for days. For the record, back in the 1870s there where at least 2 earthquakes of magnitude 7. So, this is on the smaller scale of what can happen in the area.

Image: IMO, rights reserved. A good looking quake-swarm

The odd activity at Svartárkot continues. Svartarkot is located next to the Ódáðahraun lava field, the massive Dyngjufjöll shield volcano and the Sveinagja Graben. All three of these features are part of Askja volcano. Ódáðahraun lava field from the Holocene is Icelands largest with more than 6000 square kilometres of lava. Dyngjufjöll is the largest shield volcano in Iceland, and the Sveinagja Graben, a part of the Askja arcuate fissure-swarm, is the site where the 1874-1875 eruption started. Anything happening here, even if it is small, is worthy of attention. Askja has after all had inflation happening in both the caldera and at the Herðubreið Tuyja. And during the last couple of months there have been earthquakes and harmonic tremoring that seems to have originated from the Sveinagja Graben and continued down to the southern parts of the Askja fissure swarm.

Image: IMO, rights reserved. Odd patterning of Svártarkot SIL, origin is probably activity as Sveinagja Graben belonging to Askja Volcano reactivates

The new patterning is diffuse, but it is rather unusual for Askja and the Svártarkot SIL.

CARL

Tanganasoga Vulcano Inflation

Photographer: Unknown, all rights reserved. The baren volcano of Tanganasoga.

This will be a rather short post. During the last day I have received some information regarding Tanganasoga. Apparently it has been known for some time, but held back from the general population of El Hierro, that Tanganasoga is suffering from rapid inflation due to magma injection.

The Inflation

Even though I had two individual sources stating this, I still wanted to have additional confirmation before I wrote about it. After all, now we are talking about a volcanic feature that possibly can cause fatalities. So to get a bit of confirmation I asked GeoLurking to make a plot of the current uplift. It seems to indicate that there is an ongoing inflation in the Tanganasoga volcano.

Image: GeoLurking. Inflation hypocenter of Tanganasoga

Carbon dioxide map

CO2 is one of the premier gasses when you try to judge a volcanoes state. If you have an increased emission from a volcano you have yet another precursor to a possible eruption. As can be seen on the image below the area of Tanganasoga is leaking CO2 quite badly right now. The northwestern red area is related to the Roques de Salmor gas-vent.

Image: Unknown creator. Several zones of high CO2 release in and around the Tanganasoga volcano

Tanganasoga inflation

The inflation of the Tanganasoga Volcano puts strain on the rock face pointing down towards the village of Los Llanillos. As the rock face is put under strain it will sooner or later start to shear and parts of the rock face will start to come loose and fall down.

There is also a small risk of a lateral collapse of the mountain side if an eruption occurs at Tanganasoga. I am not saying that it would be like Mount Saint Helens, the pressure is just not there. But, between 0.5 and 2 cubic kilometer of rock going down into the bay is not good news. And then on top of that, magma flowing down after the falling rocks. Well, I for one would not like to be in the way of it all.

Let us be clear on one thing, I am not saying that there will be an eruption at Tanganasoga. But the pressure build up on the rock alone requires diligent monitoring. And the political will to evacuate Los Llanillos if there is an increased risk of a larger rock fall due to mountain tension. If the risk increases even more, which I think is likely after seeing the CO2 map. They need to make serious preparations for evacuation of everything from Frontera to Sabinosa. Because if there is an eruption at Tanganasoga there will be rockslides, magma flows, and yes even the small risk of pyroclastic flows. And it will all most likely be going into the town of Frontera, so there is not being any leeway for risking a non-evacuation. It is time for the small boys of IGN, Pevolca, Involca and the rest to grow up, and grow a pair.

CARL