How to read the Icelandic borehole strain and seismicity plots and NtV Riddle

In this post I will elaborate on how to understand the Icelandic borehole strain and seismicity graphs. For the experts I might just be stating the obvious, but for the more general public (like myself) this might be a guide on how to understand all these enigmatic waves and ripples.

This map shows the locations of three kinds of instrument that monitor earthquake and volcanic activity around Hekla volcano. SIL stations (of the South Iceland Lowland automatic earthquake data acquisition and evaluation system; black triangles), GPS stations (yellow) and volumetric borehole strainmeters (green squares).

Location of the SIL and GPS stations and borehole strainmeters.
Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/Stadsetning_stodva_31052011.jpg

Strainmeters can be of various design. In Iceland we are dealing with Sacks-Everton volumetric strainmeters. Wikipedia reveals: “a design that uses specially shaped volumes to measure the strain tensor.” In other words, changes to the volume of a fluid filled chamber anchored in the borehole.

Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/thensla_burfell.html

The sample rate of the volumetric strainmeter data is one second (1 sps = samples per second, i.e. 1 Hz). The unit “strain counts” on the vertical axis is arbitrary, because a gain is manually set to determine what amount of relative change in strain or stress is one count. Strainmeters indicate ground velocity (displacement per time). Positive strain values mean volume increase in the bedrock (extension due to tension force, i.e. strain), negative values decrease of volume (contraction due to compressive force, i.e. stress). If you think of driving a vehicle, this plot shows your velocity relative to the starting velocity, since the start of the trace is always set to zero. A massive drop or rise might for example indicate you came to full stop at a tree or reached escape velocity for space travel.

Whether a strainmeter shows extension or contraction during an eruption depends on its relative position to the conduit/rift, see the opposite reactions during the Hekla 2000 eruption.

Búrfell darkblue,
Saurbær blue, Skálholt red, Geldingaá yellow, Stórólfhvoll violet, Hella light blue. Image courtesy of IMO
http://hraun.vedur.is/ja/englishweb/heklafigure1.html

Besides the Hekla strainmeter Búrfell is the second closest to Hekla, roughly 15 km at a perpendicular angle to the rift direction. The huge strain drop (i.e. massive stress increase) at Búrfell was interpreted as magma forcing it’s way up, opening a conduit. On the other hand, the simultaneous strain increase (decreased stress) at the other stations was due to emptying of the magma chamber. Here is further (paywalled) read on the strain during the 1991 Hekla eruption. The unit nanostrain indicates a change by a billionth part of the volume, i.e. 10^-9. Earthtides are known to have an amplitude of about 50 nanostrains. The 2000 eruption caused a sudden drop about an order of magnitude larger.

A seismometer literally measures shaking, i.e. motion of the ground, which can be recorded as a seismogram.
The seismometers of the SIL array can both measure ground displacement (unit is meters per second, m s^-1) or be used as accelerometers (unit meters per square second, m s^-2).
Most Icelandic seismometers are 5 sec (0.2 Hz resonant frequency, limiting the frequency range) Lennarz seismometers. The sampling frequency is 100 Hz. The Haukadalur seismometer (63°58´08.4´´ N / 19°57´54.0´´ W, appr. 10 km West of Hekla) is a LE-3D/5s, measures oscillations in three dimensions (“transverse”, North-South; “radial”, East-West; “vertical”, Up-Down).

Tremor amplitude time series with different frequency bands. Vertical axis: One-minute averages of the vertical component of the tremor amplitude, x micro meters s^-1. Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/oroi_hau.html

First of all, this graph does not show the raw seismogram, but is a spectral analysis. You remember the colorful spectrograms from the El Hierro stations? A spectral analysis is performed on the waves of the seismogram to extract oscillations of different frequencies. Several algorithms can be used to create a spectrogram, for example STFT, short-time Fourier transformation, or CWT, continuous wavelet transform. For El Hierro the amplitudes are given over the whole frequency range while in Iceland they show averages of three frequency bands.

This example is a tremor amplitude time series showing averages of the frequency bands 0.5–1.0 Hz (red line), 1.0–2.0 Hz (green line) and 2.0–4.0 Hz (blue line), of the vertical component (Z) for the station HAU. Unfortunately the vertical axis is not labelled, but is presumably representing the amount of bedrock displacement in micro meters per second multiplied by a variable scaling factor (x). The values are presumably one-minute averages. An example for this analysis is described e.g. in this thesis, see p. 564 ff.

The blue trace (high frequency band, fast shaking) mainly represents earthquakes and the green and red traces (low frequency bands, slow shaking, harmonic tremor) tremor from magma movement, which for Hekla is usually in a well-defined spectral band at 0.5–1.5 Hz (see the thesis).
Based on previous observations, the  following scenario might occur when the next eruption is about to happen: First there will be more earthquakes opening a fissure, showing as an increase of the blue earthquake trace amplitude by an order of magnitude. When the fissure is opened earthquake activity seizes and the blue trace will go back to normal. Meanwhile the magma starts spilling out and a sudden increase in the red and green tremor trace amplitude by at least an order or magnitude will be seen, which gradually decays with decreasing pressure. What we should actually look for in this graph is not the width of the traces, which only indicates how much the shaking amplitudes vary, but a really really strong rise of the curves as seen in 2000:

Tremor amplitude time series with different frequency bands. Vertical axis:  One-minute averages of the vertical component of the tremor amplitude, x micro meters s^-1. Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/hau20000226.gif

Lastly, the following graph is a composite of data derived from the volumetric borehole strainmeters and from the Haukadalur seismometer, plus information on local earthquakes determined by the SIL system.

Upper panel: Volumetric strain rate.
Lower panel: earthquake magnitude (left), horizontal components of tremor amplitudes (right)
Image courtesy of http://hraun.vedur.is/ja/hekla/borholu_thensla.html

The upper part shows the “two-minute median from one-second data” of borehole strain rate (strain counts per second) measured by the four stations Búrfell, Hekla, Hella and Stórólfhvoll. See the green squares on the map. A change of the strain rate means the bedrock is compressed or extended faster or slower than before. The cause of this is a change in the pushing or pulling forces. Think of it as your vehicle being accelerated or decelerated when pushing your gas or brake pedal. This graph shows what your feet do. When Hekla erupted in 2000 the strain rate looked like this:

The rate of strain changes in Búrfell (blue, 15 km from Hekla) and Skálholt (red, 45 km from Hekla) (nanostrain per hour)
Image courtesy of IMO http://hraun.vedur.is/ja/englishweb/heklafigure3.html

The minimum in the strain rate indicates the time of the surface breakout of the magma, along with the visual observation of the eruption at 18:17.

Because the ground is moved by several variable sources, mainly earth tides (very slow change in strain counts rate) and microseismicity (very fast change in strain counts rate) the above mentioned two-minute time range is chosen by which these events are filtered out. Then the median, the mean value separating the higher half of a data sample from the lower half, is plotted.

The left axis in the lower part shows the magnitude (in Ml) of local earthquakes. Since most of the time there are no earthquakes (counted in the lower right corner) no trace appears.

The right vertical axis in the lower part indicates the bedrock displacement, i.e. velocity in micro meters per second. The data is derived from the horizontal components (North and East) of the Haukadalur tremor amplitude time series data, which are 60-sec averages. Short-lasting shaking, for example caused by single earthquakes or a sledge hammer, are averaged out by plotting the the three-minute median. When an eruption is imminent, the blue (high frequency) trace will rise first indicating fissure opening and the green and red traces will follow when the eruption starts.

Standard VolcanoCafé disclaimer: I am not an expert on this topic, just read a few papers while researching for the post. Please excuse me if I jumped to false conclusions and feel free to post corrections!

chryphia

Many thanks to the dragons who read the draft and special thanks to Geolurking for helpful comments!

Other links:
-The SIL seismological data acquisition system – As operated in Iceland and in Sweden. Abstract only (2003)
-How a seismometer works from Sep 25, 2012 by Geolurking
-Summary about long and short period and broadband seismometers in this blog post
-ON THE USE OF VOLUMETRIC STRAIN METERS TO INFER ADDITIONAL CHARACTERISTICS OF SHORT-PERIOD SEISMIC RADIATION
-Seismometers of the SIL used as accelerometers
- Earthquake engineering research center, University of Iceland operating the Icelandic strong-motion network since 1984.
-Sturkell et al., 2005, Volcano geodesy and magma dynamics in Iceland
-Description by IMO of the Hekla 2000 eruption.
-Visualizing Stress is a good site, even if you are not into the math aspects of it, it has some really good narative data in the tutorials.

Name those Volcanoes Riddle

1 point for each volcano … enjoy!

No 1 - Did it crash in the Gobi Desert during CE3K? SOLVED COTOPAXI

No 2 - Volcanic group associated with siblings and satelites. SOLVED LES PLEIADES

No 3 - In English it can be added to seal, crow and mantis. SOLVED HEKLA

No 4 - Bruce and Nigel’s buddy studies this one. SOLVED Axial Seamount

KILGHARRAH

Katla and volcanic weekend Riddles!

Today commenter Islander brought to my attention some news that the Icelandic volcano Katla seems to be picking up activity. But what is actually all the hoopla about really?

Katla is as most of you know one of the larger volcanoes on Iceland, but not the largest. The largest volcano on Iceland is the far less known Bárdarbunga volcano. Katla has been dormant since 1918, with the possible exceptions of small hydro magmatic events during 1955, 1999 and 2011. The hydro magmatic events are though not entirely proven to even have happened.

Katla has a 10 by 14km caldera that seems to have been formed during a minimum of two large VEI-6 eruptions. The volcano is covered by Myrdalsjökul glacier. During eruptions the volcano emits copious Jökulhlaups (water discharged as the volcano melts the glacier) and they are the single greatest threat from this volcano. Normally Katla has eruptions that range between VEI-3 to VEI-4, with larger eruptions happening after longer times of dormancy. Katla has had two large flood basalts after the ice age ended, the 5 500BC Hólmsá Fires and the Eldgjá eruption, both of them where following NW rifts trending roughly towards the larger volcano of Grimsvötn.

Earlier today (2012-09-29) the geologist Kristin Vogfjörð appeared on Icelandic Radio reporting that during the 24^th of September a set of deep earthquakes had happened under Katla and that is considered as a sign of an increase of the risk for an eruption. She carefully avoided making any predictions.

So what happened then? Late on the 23^rd 6 earthquakes ranging between 0 and 0.5M happened at depths ranging from 30 to 20 kilometers. On the 24^th two more earthquakes occurred at 26 kilometers depth at the same location, those two where 0.4 and 0.7M respectively. The area is in the eastern part of the caldera.

Normally I have a deep respect for Icelandic geologists, but this is just so much hot air blowing. First of all the earthquakes are so minute that the amount of in-fluxed magma can comfortably be counted in wheelbarrows. It is like taking a piss in the ocean.

There was no associated tremor pointing towards magmatic movement of any significant scale. There is also absolutely no visible motion on the GPS stations surrounding Katla. Even at worst and these earthquakes where just precursors of a later influx we are still talking about years before an eruption taking place, and one should remember that Katla has had no less than two significant influxes of magma during the last 13 years without erupting. This so called “news” will only bring out the Katla fearmongerers, without anything having happened really.

http://strokkur.raunvis.hi.is/~sigrun/KATLA11.html

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Katla with trap formation in mid-ground. (behind the lake)

Image Copyright Eggert Norddahl

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The weekend volcanic Riddles

GeoLurking posted a riddle yesterday. I am reposting it as the official volcanic riddle of the week so it will not get lost.

A Bald Hiker disappeared, the other ran away.

Who or what am I talking about? The answer should be one volcano, and one volcano-related name. Two points to be had.

Return of the Evil One’s Riddle

This time Alan has outdone himself to create a brain-wrecking geological riddle for us. I must admit that it is beyond my limited capacity.

Ek jou gesonde-of-tuin se gesig-gesentreerde kubieke “, maar my hoppers sou wees van geen nut in ‘n stope

What am I? What do I look like? What is the mining jargon? 3 points to be had.

Have a nice weekend everybody!

CARL

El Hierro – What did Elvis have to do with the Island?

Photograph by unknown. Orchilla Lighthouse, famous for being “The end of the World”.

During the night life got fairly interesting in El Hierro as the residents woke up from no less than two earthquakes above 4M. The larger of these two reached 4.4M.

Otherwise it seems like the magma continues to move in a general southwesterly direction. This has had as an effect that many of the GPS stations on the island is registering a downfall. It is at the moment uncertain where the magma is going.

The activity that we have seen during the last weak and a half is a sill intrusion, or a horizontal oblong shaped flat intrusion between different layers of rock. The intrusion has happened at great depth, around 20 kilometers deep. The activity started roughly under the old volcano of Tanganasoga, and then progressed towards the south east, passing the famous lighthouse of Las Orchillas on the way. The lighthouse is famous for being called “The End of the World”, a strangely fitting denominator when taking the current activity into account.

If we look closer at the 4.4M earthquake it had some rather intriguing features. It had a very weak transient in the beginning. The transient is the initial earthquake that is starting the rift of the fault plane, it was around 1.9M in this initial phase. After that a heavy magmatic component started as magma gushed in to fill the void left by the earthquake, and as the magma entered it further delaminated the layers in the rock, and more magma flooded in and so on and so forth. Let us just say that it was a lot of magma moving very rapidly.

Image by IGN. The earthquake starts at roughly 41 minutes, and it continued for 6 minutes before ending.

The angle was also rather interesting. The fault plane was tilted at 134 degrees, so the propagation of the earthquakes fault plane was going upwards at an angle. This gave the earthquake a fairly hard profile to plot, and both location and depth was changed due to this. Initial depth was set at 12 kilometers, as that was the end point of the faulting, but the beginning of the faulting was at 21 kilometers. And at an angle of 134 degrees this makes the start and finish line of the earthquake being somewhat different in location.

The earthquake was both Long Period, and a of the Broadband type. What I like to call a wet earthquake as oposing the dry earthquake that is associated with tectonic movement. It also look quite a bit like the sonic signature of water moving unexpectedly in a tube.

Worried in El Hierro?

The activity is right now inconclusive. The activity has started to show higher up, but the bulk of the earthquakes is still deep down. So it is fairly impossible to say where an eruption might occur. Some believe that it will happen in the western part of the island, or out at sea south to southwest of the lighthouse. Some believe it will in the end be an eruption closer to land from the vent called Bob, or even on land close to La Restinga.

In the end it is more likely that we will have another Effusive Lava Vent In the Sea, or to put it in short, another ELVIS.

If you are feeling really worried, or things get worse you should read the splendid article on volcano preparedness written by our resident Icelander. More volcano wisdom in that article than anywhere else, a wisdom coming out of extended experience of volcanic eruptions.

http://volcanocafe.wordpress.com/2012/06/29/icelanders-do-prepare-for-eruptions-a-personal-observation/

I would also recommend that if you feel the need to relocate yourself quickly, go to the eastern part of the Island. It should be fairly safe since there has been no activity there. In the light of things, if you are really worried, or things heat up even more, do not wait for an official call for evacuation. Just go if you can or feel the need. Remember that in the end it is you who are responsible for your decisions regarding you and your loved ones.

CARL

El Hierro – Day 7

Photograph by Cestomano. The lighthouse of the Orchillas was once known as the end of the world. Under this lighthouse is where the new magma chamber is forming.

This will mainly be a short update. Not much new has happened during the last few days, I will just try to explain why.

What we are seeing now is the formation of a new part of the magma chamber(s) under the island of El Hierro. The progression of earthquakes that we have seen moving roughly from Tanganasoga volcano towards the WSW is magma following a weak seam between two rock layers in the crust. This is creating a rather low, but wide, layer filled with magma. You can think of it as a cake, where the magma is a layer of custard cream between two pieces of bread.

Initially I thought that this layer would not be able to take a lot of magma before the magma would break through into the two older layered magma chambers that were created before the last eruption. My line of thinking was that as soon as that happened magma would move up into the old feeder channel leading towards the volcanic vent known as Bob south of La Restinga.

Image by IGN. The darkblue and read area is where a new magma chamber has started to form like a stacked layer in a cake.

Instead a large quantity of magma has gushed into this new formative magma chamber, which is continuing to grow. There is a rule in fluid dynamics that state that as a bladder (balloon) grows the rate of growth will decrease as the volume increases. This is why it takes more and more magma to keep the pressure at a constant level during the expansion. This is why we see fewer earthquakes as the size grows. It takes a larger amount of magma going into the system to create the pressure for the earthquakes to happen. We will also soon see that the rate of rapid uplift will be decreasing. Do not take this as a sign of the risk for an eruption decreasing. It is quite the opposite. Why? The rate of magma arriving is still constant, so, in due time it will break through.

Magma seems to have entered into the old feeder channel to Bob, how far up though is anyone’s guess. But, it seems to have stopped flowing upwards now, probably due to the opening being plugged up.

Instead we are now seeing a small amount of earthquakes entering into the zone between 16 and 8 kilometers from the surface. What is most likely is that we will see more of them during the next 48 hours; with a bit of luck we will be able to pinpoint the formation of an earthquake stack. That would be good since that could point towards a general area where there could be an eruption. Currently that is pointing towards the western area of the island, but that might change rapidly.

Image by IGN. A few earthquakes have started in the 16 to 8km depth range.

When the magma enters the region 8 kilometers and above the earthquakes will most likely stop, or be few and far apart. The reason for this is that there is a layer of old sediment there. Then there will be a brief flurry of earthquakes as the magma breaks through to the surface, unless it finds an old lava-tube, then it would be a quiet onset of eruption. The eruption would be of basalt or basanite, so it would most likely be a quiet effusive eruption. In the beginning it could be more vigorous due to gas pressure release.

I would still not rule out that the eruption will happen somewhere along the old feeder channel that lead out to Bob. If that happens I believe the eruption to happen somewhere closer to land than during the last eruption. It could also happen on land.

During this second phase of the eruptive cycle the level of information given out has been much improved. IGN is now giving out real GPS data instead of cumulative data. That is a huge improvement. Also Pevolca has started to write reports that are filled with technical details that helps a lot, and removes any chance of people accusing them of hiding data. If IGN and Pevolca keep up with this new openness they will find that people will be much happier. So, from us a big thanks for this new approach.

I would also like to point out to the political establishment of El Hierro that the last eruption will in the end be a big boon for the Island. Before nobody pretty much knew that El Hierro existed. Now many do. And that should in the end raise the number of visitors, especially from the rather large cadre of volcano aficionados. So, instead of trying to hide your volcano, flaunt it a bit. Be proud of it, and people will come to watch your beautiful island.

CARL

El Hierro – Day 2

The face of El Hierran politics, Alpidio Armas.

This will just be a short update with the information that has come at hand.

There seems to be a full on war between Pevolca, Involcan and José Luis Barrera VP of the ICOG (Spannish Association of Geologists). Pevolca has stated that there are some reneval of tectonic earthquakes, with no risk for the population or any need for any measurments to be taken. They also point out that there is no increase in gases. They do though mention that they will be watching things. They have also stated that there is no inflation at El Hierro (GPS).

Involcan has stated there is inflation after studying the GPS system of Professor Sagiya from Nagoya. They also point out that the reason for Pevolca not being able to see any heightened gas levels is that they have not measured the gas since April 5.

http://www.facebook.com/pages/INSTITUTO-VOLCANOLÓGICO-DE-CANARIAS/134042953295772

José Luis Barrera and the ICOG have issued a statement that the more than 350 earthquakes are tectonic, but that they might be a run-up phase for renewed volcanic eruption. They also note that the activity is unusual.

http://www.europapress.es/comunicados/noticia-comunicado-colegio-geologos-muestra-incertidumbre-repunte-actividad-simica-hierro-20120626152109.html

Meanwhile in the real World

While the Spannish authorities and organisations are involved in their usuall pissing contest there are some things worthy of comment.

A little tidbit on the earthquakes during the 48 last hours…
Yesterday had the fifth highest recorded number of earthquakes (241), the highest recorded number is close to 454, and that was in August as the former wad of magma had it’s peak of arrival.
But what is really interesting is that if one take a look at how those quakes break down into size…
Yesterday first, then August number-record.
0-2M 55 (448)
2-3M 180 (6)
3<M 8 (0)

And that would have made yesterday into releasing 5 times as much energy and destroying about 32 times as much rock. It was the record of all time energywhise. This leads me to believe that the amound and speed of arrival is higher this time around. So, the last two days have had the largest accumulated seismic energy release since onset of activity at El Hierro. Energy record in short. And that is note noteworthy according to Pevolca.

LP Earthquakes

The earthquakes today has long amplitude component to them normally associated with magmatic movement into the cracks. These LP earthquakes is considered to be magmatic. There has during the last 8 hours been at least 3 of them. The 17.16 is one of them.
I believe this is onset of movement upwards of magma into the actual system of Tanganasoga.

First LP

Image by IGN. The First LP Earthquake.

Second LP

Image by IGN. The Secong LP Earthquake.

Third LP

Image by IGN. Third LP Earthquake, and what is most likely onset of a heavy magmatic intrusion upwards.

The third LP seems to have opened a conduit somewhere, most likely from the crustal boundary (MOHO) up to the chamber under Tanganasoga, or directly towards Bob. The Long Periodicity Earthquake is caused by an initial earthquake that opens up a fissure, sill or dyke, after that magma moves in to fill the opening, and that creates and unusual type of earthquakes.

Conclusion

There is no reason that I should recant on what I wrote yesterday. I still believe that there is a rather high risk of a new eruption at El Hierro. I still see no reason to not believe it will be in the southern part of the island, or out in the ocean south of La Restinga. I still feel that La Restinga is not entirely safe for it’s population.

Update

While I was writing this post the signal changed sufficiently for me to believe that there is risk that the eruption is either about to start, or has already started. We are all waiting for news, and think about the unprotected civilians in El Hierro.

CARL