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.

Computing earthquake magnitudes for El Hierro

Introduction

Since the onset of many earthquakes on El Hierro, questions are posed in the media (and by some blog authors) on the quality of information provided by various official bodies. It is understandable that the authorities seek to avoid panic or rumors, but this implies a good communication policy leaving no room for improvisation.

Given the diversity of knowledge of the general public, the relevance of disclosing raw data is not obvious. However, for individual researchers or even curious amateurs, access to these data is crucial. The scientific method is based on the disclosure of the work done and the data used (to communicate new theories and results, but also to allow independent verification).

El Hierro

Seismograms and spectrograms are available on the IGN website, but can only be used qualitatively. In fact,

• There is no mention of procedure used to obtain them and, in particular, filters (e.g. frequency) applied

• For seismograms, there is no mention of measurement unit, it is not possible to know if the graph is displaying a raw recording (in general, the speed of the ground) or the actual displacement (calculated by integrating the speed).

• Regarding the spectrograms, the same problem exists.

By comparison, the USGS indicates that information. If we consult http://earthquake.usgs.gov/monitoring/helicorders/nca/51/latest/ for instance, we can see the unit indication

We clearly recognize a speed (in centimeters per second).

Fortunately, IGN provides bulletins in IMS format. These bulletins are caused by the need for monitoring seismic waves related to detection of nuclear tests, under the "Comprehensive Nuclear-Test-Ban Treaty" and their use in scientific research is an indirect consequence, but it is very important.

One of the entities set up by the treaty is the International Monitoring System (IMS), and this organization has defined the content of the bulletins that are used to report the detection of seismic events and waves.

In the case of earthquakes on El Hierro, the only source of information for serious analysis in this context is the availability of the IMS bulletins.

Using IMS bulletins

It is beyond the scope of this article to describe the format of a IMS bulletin, and it suffices to note that the following information is available:

• The depth of the epicenter (km)
• The distance between a station and the epicenter (in degrees)
• The amplitude of the waves reported (in nanometers)
• The wave period (in seconds)
• The local magnitude calculated by IGN

The local magnitude is calculated by IGN based on a formula of its own:

mbLg(L) = log (A/T) + 1.17 log R + 0.0012R + 0.67

With:

• A: amplitude (micrometers)
• T: period (second)
• R: hypocentre distance (kilometers)

Important point: in everything that follows the assumption is that the information found in these reports are correct.

Can we recalculate these magnitudes? The answer is yes. As an example, for event 1165013:

Station	Distance (degree)	Amplitude (nm)	Period (s)	MbLg(IGN)	MbLg Computed.	Difference (IGN-Calc.)
CTAN	0.06	15.4	0.06	-	1.3
CTAB	0.07	85.8	0.08	2.0	2.0	0.0
CCUM	0.08	14.0	0.22	-	0.8
CJUL	0.09	132.6	0.18	-	1.9
CMCL	0.11	21.9	0.16	-	1.2
CHIE	0.14	3.1	0.16	0.4	0.4	0.0
CTIG	0.14	3.2	0.28	0.2	0.2	0.0

Table 1: Data and results for event 1165013

Surprisingly, IGN does not calculate the magnitude for all stations, even if the data are available. In the table above, only three values are calculated (for stations CTAB, CTIG and CHIE). We can immediately make other comments:

• We can see that the recalculated magnitudes correspond to those calculated by IGN. The procedure seems to be in good agreement with that of IGN.
• The period is always a multiple of two. It is therefore likely that the period is the result of a calculation.
• The dispersion of the values calculated for the magnitude is large (0.2 to 2).

Now we visualize the magnitude (depending on the distance from the hypocentre).

Fig 1. Magnitude vs distance.

We note that:

• The more the distance grows, the less the magnitude calculated is high
• The correlation coefficient R ² is intermediary (it can vary between 0 and 1, and here it is about 0.5), which indicates a misalignment with a straight line (if the points are perfectly aligned, the value is 1). Warning: this coefficient is not the "R" value used in the calculation of the magnitude, it is just the same symbol.

The first point is surprising: although it is known that the effect of earthquakes fades with distance (attenuation), the formula used by IGN has two terms (1.17 log R + 0.0012 R) dependent on the distance and it might therefore be expected that the damping effect is taken into account. This point will be discussed further in a future article.

We can still do other analyses. So ...

to be continued ...

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).