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).

Volcanic news (17/08/2012)

News of the day :

• A new eruption of the Soputan (Sulawesi, Indonesia) occurred at 15:00 UTC, the ash column reached an altitude of 10 km ( source: VAAC Darwin ).
• The ongoing eruption of the Lokon (Sulawesi, Indonesia) continues, phreatic activity ( source: PVMBG ).
• The Sheveluch (Kamchatka) has exploded at 23:20 UTC yesterday, the ash column reached an altitude of about 8 kilometers ( source: VAAC Anchorage ).

For the follow-up of El Hierro, see the dedicated page.

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.

Fuego is erupting

Volcán de Fuego (Guatemala) has entered a very active eruption phase in the last hours. Ash emissions are important, lava flows are present, and at least 30,000 people were evacuated. Fuego means fire in Spanish.

Fig. 1 Fuego Eruption (©) INSIVUMEH

More information later.

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 ...

Mixed news from Yellowstone

The Yellowstone caldera, in the United States, is a well-known volcanic complex, and in the media associated with the superlative " supervolcano". The site is physically very active and has many interesting phenomena, both from scientific point of view, and also… tourism. The geysers are perhaps the most known, but there are also hot springs, gas emissions, and it also houses a very diverse flora and fauna. The site is protected as a nature reserve.

The caldera has witnessed colossal eruptions, the last one 640,000 years ago. Earthquakes are frequent. Their analysis revealed changes in rock density below the park, and this is interpreted as a partially melted area, so a magma chamber. The stored heat there is also the origin of geothermal activity.

It is not the purpose of this post to make a full description of the complex, and it will be done later in another article, but interesting information about the behavior of the giant was recently published, in particular the eruption dynamics.

Indeed, smaller eruptions have occurred more recently, but they are of lower intensity. Determining their probable location is complex. The presence of fractures may influence the location of future eruptions. Those seem to remain unlikely in the short term, the amount of molten magma seem low (less than 30%). Note also that the lavas are rhyolites with an explosive potential due to their generally high viscosity.

The presence of smaller-scale eruption shows that the adjective "supervolcano" should be replaced by "super-eruption" limited to some of them.

However, examination of the composition of rejected lavas indicate a rapid rise of magma from the main chamber at a depth of 10 kilometers, without intermediate storage at a shallower depth. This indicates that, from the moment the eruption process is triggered, the time of apparition of the lava at the surface may be small and it could be more difficult to detect this process.

This makes monitoring even more important.

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Sources: http://www.geosociety.org/gsatoday/archive/22/9/article/i1052-5173-22-9-4.htm
USGS (various pages)

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).

Pressurization of Mount Fuji?

There is news on the Internet about an increase in the pressure in the magma chamber of Mount Fuji. All this started from a single press note from the Kyodo News agency, but the information is fragmentary and imprecise. I am not aware of the scientific report (and it is not for lack of trying). I refrain from further comment due to lack of clarity (for the moment).

The "volcanic" gases

The "volcanic" gases are volatile compounds released by volcanic processes. They are improperly called so because they are usually a mixture of different gases that are not specific to volcanoes (they can occur naturally due to other processes). The main components are as follows (in descending order of average concentration):

• water (H₂O) is the most abundant component.
• carbon dioxide (CO₂), is a colorless, odorless gas (at standard temperature and pressure). This is one of the gases responsible for the "greenhouse effect" that keeps the temperature of the planet to an acceptable level for living organisms. CO₂ if dissolved in water, forms carbonic acid (H₂CO₃), making it more acidic. This is the component used to make fizzy drinks. It is used by plants for growth (photosynthesis). CO₂ is dangerous when the concentration is too high (over 5%). Denser than air, it can create pockets of gas that are death traps (see e.g. the Lwi lake, better known as the "Nyos" lake, Cameroon).
• sulfur dioxide (SO₂) is a colorless, dense and highly toxic gaz. At low concentrations, it is used as a disinfectant and preservative in food under the code E220 (in wine, dried fruits, meat ...). Oxidation gives sulfuric acid (H₂SO₄) and sulfurous acid (H₂SO₃ ). Before regulations to reduce its concentration in the air, he was responsible for "acid rain". In the air, it is irritating. The alert threshold is set at 300 mg / m ³ (300 micrograms per cubic meter). WHO recommends the 24-hour average concentration to be less than 0.5 ppm (parts per million). If the gas is injected into the stratosphere, it can lead to a decrease (reversible) of the temperature of the Earth.
• carbon monoxide (CO) is a colorless, odorless gas (at standard temperature and pressure) and very toxic. It is well known as responsible of domestic accidents (due to poor combustion).
• hydrogen sulfide (H₂S) is a highly toxic gas, flammable, with strong odor of rotten eggs. At low concentrations, it irritates the eyes and at higher concentration the upper airway. The attack of the olfactory nerve is insidious because it makes gas detection impossible. Other effects are headaches leading to unconsciousness. It may cause pulmonary edema after prolonged exposure.
• hydrogen chloride (HCl) is an irritant gas. It can cause acid rain when mixing with water (HCl is a strong acid).
• hydrogen fluoride (HF) is a colorless toxic gaz. In the presence of water, it forms hydrofluoric acid, highly corrosive. The exposure can cause conjunctivitis with corneal destruction. It also causes skin irritation. Although a small amount is beneficial, a high concentration of fluoride causes bone degeneration. Fluorine (F₂) is it a pale yellow gas.

Other gases are hydrogen (H₂) and helium (He). Gas is one of the main engines of eruptions. The example that follows is adapted from the USGS (and based onSparks et al ^[1]). Suppose we follow a block of one cubic meter of rhyolite at the temperature of 900 º C under high pressure (due to depth) containing only 5% dissolved water (by weight). No we bring it abruptly to the surface, so at atmospheric pressure. The magma cannot contain the water anymore, which forms bubbles, and this leads to a strong increase of the block size. The block then occupies a volume of 670 m³ . The length of the edges of the cube is multiplied by 8.75. In extreme cases, it forms a "foam", which, even when cooled, has a lower density than water and therefore floats . A typical example is pumice . This mechanism is - relatively speaking - similar to what happens when opening some bottles of soda or Champagne. The pressure drop also causes the expansion of bubbles, escaping violently.

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[1] Sparks, R.S.J., Bursik, M.I., Carey, S.N., Gilbert, J.S., Glaze, L.S., Sigurdsson, H., and Woods, A.W., 1997, Volcanic Plumes: John Wiley & Sons, Inc., England, 574 p.