El Hierro: new record at 4.9

The earthquake registered at 10:59:54 UTC (coordinates 27.7110, -18.2727) : magnitude 4.9 mbLg according to IGN. It is the largest since the beginning of the activity ... in 2011!

Computed depth is 20 km.

As usual in such cases, the website of the IGN is currently overloaded.

Fictional El Hierro...

A bit of geological fiction (or not?)

Imagine you want to push a whole volcano up. Now imagine that, instead of a typical, conical volcano, the volume is more complex and globally looks like a triangle, like, for instance, say, El Hierro.

If you mentally divide a island like that using a North/South line, it is obvious that the amount of mass in the West part is much less than the amount of mass in the East part. Take into account that the overall high is 4 km above the see bottom in the area. This is a lot to move.

If then the pressure centre is near the centre and a bit westwards from this N/S line, then it is probable that the West part will move more than the East part. Less mass in this area means it is "easier" to move. After a certain time, it is probable that a break-up will occur, around that North/South line, with lots of earthquakes, then propagating outwards. Now, if the West part continues to go up faster than the East part, a constraints field will start to build-up, and after a while this constraints field will need to be relaxed.

This new activity will start where the constraints are maximized, so near the centre of pressure. Then, this will propagate outwards. For an analogy, try to bend a plank of wood, you'll see cracks occurring. This cracks can happen everywhere, but will preferentially occur near a point of previous failures.

Now, the triangular shape is supposed to be a triple-junction set-up, and one branch extends in the North-West direction. So, cracks will propagate in that direction.

After a while, the cracks will reach a distance where the field of constraints no longer propagate outwards, but rather propagates laterally at the same distance from the pressure centre.

However, there are less pre-existing failures because now the activity is moving outside of the old failure area. The constraints needed to break the rocks are higher, and instead of frequent, but lower intensity earthquakes, you get less frequent, but more powerful ones.

By the way, magma is probably following the cracks...

Sounds familiar?

El Hierro; major earthquake: mbLg 4,7

A major earthquake was detected near El Hierro today at 17h10 UTC. More info later.

El Hierro, set of huge earthquakes today

Since 21h50 UTC, a set of powerful earthquake are occurring near El Hierro.

Short summary:

Data © IGN

Fortunately, they are located in the same area, at deep depth, so no serious danger for the people there.

Hope they will be able to sleep tonight!

El Hierro: what a day !

The activity is amazing today. For the first time since long ago, multiple earthquakes with a magnitude greater than 4 were recorded in a very short period. This activity continues so far with no end in sight.

© IGN

Other earthquakes with lower magnitude were recorded as well. They are too numerous to be displayed here.

The interesting point also, is that the evolution is rather scary, as more higher magnitude are recorded as time passes:

© IGN

At the beginning, few earthquake with a magnitude greater than 3 were recorded (see the lack of purple bars). Now even magnitude 4 and above are becoming common.

Fortunately, the activity is located far at see:

© IGN

There is also a tendency of the last earthquake to move a bit south. We'll see later if this is confirmed.

Stay tuned !

Update 22h00 UTC

The situation forced the PEVLCA to raise the alert level to yellow for the western part of the island.

El Hierro follow-up, restarted

I'll restart the follow-up concerning the seismic activity on El Hierro, and time permitting, other informations.

I'll try to translate the other posts from my French blog, when appropriate.

El Hierro update

Updated (18:00 UTC)

This value is normal again. To be continued...

Potentially important update (17:20 UTC)

The concentration of sulfur dioxide (SO₂), as measured by the mobile station located at La Restinga, rose sharply to nearly 900 micrograms per cubic meter (µg/m³). This value is more than 100 times the usual value.

It can be a technical problem. If this is not the case, the source of this increase must be determined and analyzed, as if it is of volcanic origin, the risk of eruption increases significantly (it is a known precursor).

Increased activity in El Hierro

Update (17 september, 20:00 UTC)

The activity stays at high level (more than 100 recorded earthquakes on average per day), but earthquakes are currently occuring at high depth (most deeper than 20 km), so no immediate risks for the population.

Update (20:00 UTC)

Two figures quickly:

Number of earthquakes of magnitude greater than 2 between 1 ^st September and 13 September:    7
Number of earthquakes of magnitude greater than 2 for 14 September between 00H and 20H:    15

So, twice the count of the last 13 days for this day alone ( and it is not finished yet!) .

From now, updates will be added at the beginning of the post.

Important Update (14H:00 UTC)
The activity seems to become more important, as detected on several stations.
This is to be followed carefully.

Very important update (19H00 UTC)

According to the seismographs on various stations, it seems that the magma is now moving.
An example from CJUL:

Fig. 1. CJUL: tremor?

 

If the graph shows indeed a tremor, then the magma is moving, and it remains to be seen where it goes. Three hypotheses:

• or it migrates from Frontera (at 20 km depth) to El Golfo (at 10 km depth)
• or it migrates from El Golgo (at 10 km depth) to the surface
• or it migrates ... elsewhere.

The next hours will allow us to decide. At least I hope.

GPS accuracy... or lack thereof

It is increasingly common to have access to GPS measurements (Global Positioning S ystem). Initially, the goal is to be able to locate any place on the planet. Practical ways to get the position are automatic stations and portable instruments. The general public has become familiar with this technology, for example, car navigation systems.

Technically, GPS tracking consist on receiving signals broadcast by satellites in low orbit. The satellites transmit position and clock (time information) signals. To ensure a good location, simultaneous reception from several satellites is necessary, and it requires a direct line of sight to the satellites (which is why the GPS is not directly usable in tunnels or underground parking).

Location usually requires only rough accuracy, a decimeter is sufficient in most cases (for example, a car or a plane). This means that the exact value is not known, the value calculated and displayed is an approximation.

With a precision of a decimeter, for example, if the height displayed is 203.12 meters, this means that in reality the exact value is between 203.02 and 203.22 meters.

To monitor the evolution of a volcano-tectonic or tectonic phenomenon, GPS is a useful tool. But greater accuracy is required. The specific methods for this increased accuracy are beyond the scope of this article, but you can quickly note several ways:

• comparison of the information received from nearby stations (with the assumption that some of the errors is due to the path of the waves and therefore, different from stations to stations)
• comparison of the location for the same station as a function of time (the assumption being that a station moves linearly or is "fixed", the moving speed is low compared to the positional error)
• corrections (often retrospectively) taking into account several other parameters

Some advanced software like BERNESE (University of Bern), GIPSY-OASIS (NASA JPL) ... use these methods to obtain more accurate values, and one can commonly get the following error margins:

• horizontal (latitude and longitude): a few millimeters
• vertically (height): about 2 centimeters

Let us now consider some concrete examples based on information available from El Hierro.

The following two graphs are derived from analyzes of prof. Sagiya (Nagoya University). We know that BERNESE is used in this case, so the precision specified above applies.

Let us first look at the horizontal evolution for a GPS station (of Tenerife):

Fig 1.Horizontal of station TEIT (Tenerife)

The first graph corresponds to the east-west trend (east is positive) and the second the north-south trend (north is positive). And now compare with the vertical plane:

Fig 2. Vertical evolution of station TEIT (Tenerife)

It is clear that the dispersion of the measuring points is greater in the vertical plane, but overall there is no clear trend. By adding the error margin (red lines), we get:

Fig 3. Vertical evolution of station TEIT (Tenerife), with error margin.

Now we examine the Frontera station:

Fig 4. Horizontal evolution of Fontera station

We compare with the vertical evolution:

Fig 5. Vertical evolution of Fontera station.

We can see that the trend is similar (several movements) and dispersion is again much greater is the vertical plane. By adding a visualization of the error margin (red curves), we obtain:

Fig 6. Vertical evolution of the Fontera station (with error margin).

It seems that the margin of error is larger for Frontera than for Tenerife.
If we compare with the CVCB station (La Palma):

Fig 7. Vertical evolution of the CVCB station

The dispersion of the measurement points is even more important here (maybe 3 cm). It may be a local effect. Too often, non - informed personsdo not take into account the error margin and, looking at the changes from day to day and "zooming in" the last days, conclude that a station "jumps" two centimeters per day (sometimes in one direction and sometimes in the other). In fact, the rate of change is generally much lower, except for sudden movement of a tectonic fault. It is possible to obtain millimeter accuracy in the vertical direction, but it is extremely complex (for instance, you must take into account the effect of tidal height variation due to the attraction of the sun and moon on the ground. Yup, there is not only the sea that undergoes this!)

Volcanic art / science

When the data are incomplete, the results of the automatic analysis can be surprising ...

Fig 1. CHIE Spectrogram © IGN.

Here it is almost a textbook case ... in art. The colored graph is supposed to indicate the frequency content (spectrogram) recordings. It goes without saying that such regularity is not natural.

Update for El Hierro

The number of earthquakes recorded remains low; the maximum magnitude reaches barely 2 (Mblg) and they are located in the same area (El Golfo).