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

Name that Lava, Herdubreid Scientific Challenge and Volcano Riddle

Photograph courtesy of Dr Carmen Morataya.

Having the brain rebuilt from top down every Friday never get’s old. I do not know how you guys are doing since I am bad at both riddles and recognizing lavas and volcanoes.

But it is fascinating to witness how the blood hounds of the Volcano Café unerringly zeroes in on the pray. This week I want the name of the lava, the volcano and the name of the local fast food. So, 3 points to be had all in all. I will of course ding when someone has scored.

Tomorrow, or the day after I will post you an explanatory article.

Herðubreið

Photograph by Eggert Nordahl under permission.

Herðubreið is a tabletop volcano (tuya) in Iceland that has been active during the last few years with repeated earthquake swarms and magmatic emplecements. Supposedly the volcano is a defunct part of the Askja volcano. Evidently it is not as defunct as previously believed, and there is quite a lot pointing to it not being a part of Askja.

So, last night I proposed that we should do something with the volcano since it is rather poorly investigated. Ontop of the two questions above we can also have a bit of fun with the youngish looking strato volcano on top of the tabletop mountain. Here is the question, have it erupted after the last glacial period?

I thought we could do this as a case of crowd science. We have access to some equipment, we have access to a lot of people with various expertise, so we could actually collectively make a fairly nice piece of scientific work on Herðubreið.

So, in another word, I propose that Volcano Café officially adopt Herðubreið as our project. Now and then we should post status reports on our progress, if anyone feel up to it, being the collector and editor of what has been said please do not hesitate to come forward! Herðubreið and Volcano Café needs your services.

Have a nice weekend everyone!

CARL

Name Those Volcanoes Riddle

The riddle contained the mixed up clues to 2 volcanoes … 1 point awarded for each …

At the intersection of Rts 36 and 119 Peter was totally over come by the glowing clouds; then Bill, Andie and Chris arrived to cover the celebrations!

1 point Sa’ke for Mount Pelee … “Peter was totally over come by the glowing clouds”

Glowing clouds was the English translation of a French term nuée ardente first used to describe the pyroclastic flows of Mount Pelee that engulfed the town of St Pierre in 1902 killing 30,000 people.

http://en.wikipedia.org/wiki/Pyroclastic_flow

http://en.wikipedia.org/wiki/Mount_Pel%C3%A9e

1 point KarenZ for Candlemass Island … “At the intersection of Rts 36 and 119; then Bill, Andie and Chris arrived to cover the celebrations!”

The clue pointed to the movie Groundhog day (1993) set in Punxsutawney, starring Bill (Murray), Andie (MacDowell) and Chris (Elliott). Groundhog Day is the US term for Candlemass Day.

http://www.fisheaters.com/customstimeafterepiphany3.html

http://www.volcanolive.com/candlemas.html

KILGHARRAH

Countdown to Hekla

It has been a long time since I wrote about Hekla. But, I guess nobody is surprised at what I am about to write.

Everyone with a genuine interest in volcanoes have their favorite volcano. As many in here know Hekla is my favorite volcano to bang my head against. Few volcanoes are as intricate as Hekla, and few have such a short run up before an eruption as Hekla. Normal run up time to an eruption is between 30 – 80 minutes from the first sign.

This time around has been different. But let us first recapitulate what has happened since the last eruption. For those who are curious about how Hekla works I would like to recommend my own post about her innards:

http://volcanocafe.wordpress.com/2012/01/10/deconstructing-hekla-hells-gate-revisited/

Background

In 2004 Hekla had received as much new magma as was discharged during the 2000 eruption and sometime during late 2008 to early 2009 that figure had doubled. After that the inflation stagnated and no real uplift was measured at the GPS-stations with the exception of what was most likely magma moving between the different magma chambers.

During the summer of 2011 earthquakes was registered and a public safety alert was issued stating that Hekla was close to erupting. From then on Hekla has had earthquakes ranging from miniscule to 2M+ without erupting. For those who are not familiar with Hekla one should notice that she normally is aseismic, or in other words, that she does not have a lot of earthquakes.

From 2010 and onwards Hekla started to show a new feature that I dubbed “transients”. The transients are sudden rapid drops in the strain measured at the borehole strainmeters. These transients have only been seen before as Hekla erupted. They had before 2010 never been seen without an eruption occurring. A transient is in short happening as the mountain strains to open up.

On the 13^th of March and onwards Hekla had a swarm of earthquakes and once again transients were noticed on the strain-meters. There was also harmonic tremor measured indicating rapid movement of magma. This caused IMO to issue a public safety warning, and the London VAAC issued a flight code warning Orange. This followed the exact pattern of how all the previously instrumentally monitored eruptions had started so far.

As we all know nothing happened in the end. We can now safely say that we know even less than we did before about how Hekla acts before an eruption. Because now we have to figure out why Hekla did not erupt when she should have. I guess someone will have a research career out of it in the end.

Present

After this Hekla entered into a new phase never seen before, this time a phase of very rapid and unbroken inflation started. What happened is most likely that the earthquake swarm removed blockages inside the deep feeder tubes of Hekla enabling fresh magma to flow into the volcanic system.

The rate of inflation varies a lot depending on where the GPS station is placed. The big exception is Mjóaskard situated to the west of Hekla. It has only suffered an uplift of 5mm in the last 5 weeks. For the other stations the rate of inflation is between 15mm in Hestáalda and 32mm at ISAK. Average uplift is 16mm, and 21mm if MJSK is not counted. This type of rapid inflation has so far never been measured at Hekla.

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

During the entire inflation phase there have been scattered earthquakes and micro-quakes.

If the current rapid inflation continues there is a very low chance of Hekla not erupting. Yes, we do not know what is happening with Hekla now since we have never seen this type of behavior. But Hekla is constructed in such a way that she can’t take a huge increase in pressure without erupting.

Image by GeoLurking based on data by the University of Iceland and Professor Sigrún Hréinsdóttir. All areas are showing uplift on this image covering the period from 4th of April up untill now. The area with the highest uplift are due north of Hekla.

If the inflation continues at the current rate Hekla will erupt. When? Well I am not going to make any bets, but any time from 1 hour from when you read this to 4 weeks. Remember that 4 weeks into the future the combined uplift in 2 months will have exceeded 50mm at many GPS stations. As seen on the image above the largest uplift is happening on the northern slopes. This is a known site for one of Heklas primary magma chambers. The area to the northeast are not showing correctly, there is uplift there too, but due to lack of a GPS station there the model get scewed.

Image courtesy of the University of Reykjavik and Professor Sigrún Hréinsdóttir.

I personally would not at any cost get closer to Hekla then 10 km from now on. And then I would stay in the car on the road. If you are closer the chance of you surviving is not good and 5 km the chance of you surviving the initial blast is pretty much nill.

What will the eruption be like? Here I will be guessing since Hekla has changed her behavior compared to the last eruptions. I would say that Hekla has remobilized old evolved magma during all that moving of magma, and this latest inflation phase seems to fill up a lot of old magma chambers. This causes me to fear a rather explosive start of the eruption. I would also say that there is quite a high likelihood of there being more lava erupted then was seen during the last 3 eruptions. I will hedge my bet by saying that I would expect it to be anything between a VEI2 and a VEI4 on the volcanic explosivity index, and that Hekla will effuse between 0.1 to 2 cubic kilometers of lava. Based on the GPS plot above my best judgement is that the eruption will start at the top of Hekla proper and then open up the fissure both to the south, but mainly to the north. Most probably the Hekla fissure will open over all of Hekla proper with a fissure extending to the Northeast.

For those who wish to follow the eruption, here is the Hekluvöktun page:

http://hraun.vedur.is/ja/hekla/

CARL

Langjökull quaking and NTV Riddle and the return of Alan’s Evil Riddle

Post by Inge B.

Photo and Copyright IngeB

Photo from Kalidalur highland road, Langjökull (western side) left, Presthnúkur in the middle, Þórisjökull to the right

There was a unusual swarm of about 20 quakes which took place between Langjökull ice cap and the highland road Kjölur on the western side of the ice cap these last days and it made a new start now.

This is a region which has not been very active lately regarding earthquakes or eruptions.

http://www.vedur.is/skjalftar-og-eldgos/jardskjalftar/langjokull/ April 5th.

Source: Icelandic Met Office (IMO), 5 April 2013

There are three main factors influencing Langjökull and its surroundings also regarding the earthquakes occurring there:

1. glacier mass balance
2. plate spreading
3. volcanism

Langjökull is the next biggest Icelandic ice cap (after Vatnajökull), its dimensions being acc. to Wikipedia 925 km² at an volume of 195 km³ and up to 580 m (1,900 ft) thickness. This big glacier though has been receding heavily these last years, probably due to climate change. Scientists even prognosticate this glacier being reduced to next to nothing very soon, if the tend in climate change is continued.

This will of course have an influence on the glacier and its surroundings, eg. due to glacier rebound. This last technical term means that when the enormous weight of the glacier is lifted in a rather short time span, the earth’s crust beneath will move upwards. Near Höfn in the east under Vatnajökull, there has been measured a uplift of over 11 cm in the last years.

Earthquakes may be caused by this kind of “resettling” of earth’s crust.

Wikimedia Commons: Present and former rift zones of Iceland. 1v: Western Rift Zone (WRZ); 1n: Northern Rift Zone (NRZ); 2: former Snæfellsnes-Skagi Rift Zone (SRZ); 3: former Westfjords Rift Zone (WRZ); 4: Eastern Volcanic Zone (EVZ) – most likely a future rift zone.

From commons.wikimedia.org <, 1n=NVZ; 1v=WVZ with Reykjanes; 2 and 3 are former rift zones; 4=EVZ

Then there is the spreading which influences the MAR (Middle Atlantic Ridge) and all parts of Iceland on active spreading ridges. These ridges have been going through several rift jumps in some millions of years. The active rift zone was placed over Snaefellsnes and Vatnsnes, till it shifted over to today’s location which is divided in the south into a Western Volcanic Zone (WVZ) – with Langjökull as its center – and an Eastern Volcanic Zone comprising a.o. the more active volcanoes Hekla, Katla and Grímsvötn. This rift zone has been active now for about 2 million years. Northwards this continues in the NVZ (Northern Volcanic Zone) reaching from the Vatnajökull, over Askja and other volcanic systems out to the sea where it combines with the Kolbeinsey Ridge.

The WVZ has been less active than the Eastern Volcanic Zone during the Holocene, so it is proposed that the spreading zone, the rift is again “jumping” in the south of Iceland. In the whole there were only 17 eruptions representing 64% of its whole production and these took place from around 10.000 BP till about 7000 years ago.

Nevertheless, Langjökull is part of one main zone of the Icelandic rift and MAR, so these quakes could also be part of a rifting episode.

And finally, as with all the big ice caps in the centre of Iceland, also Langjökull is concealing some mysteries. The ice of its cap is not flat. There are mountain tops, nunataks and strange indentations. And after some screening done, scientists discovered not only one, but at least two volcanic systems stretching out under the ice with two calderas under the glacier and two high temperature areas near its edge. There could also be a big table volcano under the ice. And a further high temperature area, the one at Geysir in Haukadalur perhaps connected to one of the systems.

The known volcanic systems under Langjökull are

a) Prestahnúkur, in the southwest, there was sometimes talk about it having some quake swarms in the vicinity and someone – was it Newby – intended to drive up there in a high-wheeler.  The volcano’s tentacles in the form of dikes are reaching at least under Geitlandsjökull, an outlet glacier.

b) Hveravellir in the northeast. This is a famous stop-over for people traversing the country on the Kjölur highland road, with an high temperature area forming sinter terraces.

Image Wikimedia Commons: Hagavatn – View from the southeast on the eastern part of the lake. In the background Hagafell and a glacier tongue of Langjökull, in the foreground the outflow of the lake with the remains of a destroyed bridge.

Lake Hagavatn, not far from the hypocenters.

The hypocenters of the ongoing quake swarm are placed beneath an old lava shield.

Where the deeds are done

Langjökull Glacier

Source:http://commons.wikimedia.org/wiki/File:Kjolur.jpg

The last eruptions in the Langjökull area took place around the time of settlement in the 10^th century when 2 craters on the northwestern side of the glacier shield produced an impressive lava field called Hallmundarhraun (length around 50 km).

Will there soon be another?

Sources:

re. the glacier

• Comparison of energy balance and degree-day models of
• summer ablation on the Langjökull ice cap, SW-Iceland by Sverrir Guðmundsson, Helgi Björnsson, etal. (2006?)
• Geometry, Mass balance and climate change response of Langjökull, Iceland, by Helgi Björnsson, etal. (Conference poster, Obergurgl) [https://docs.google.com/viewer?a=v&q=cache:ltfiXqdsA7AJ:www.raunvis.hi.is/~sg/langj1.pdf+Langj%C3%B6kull&hl=de&pid=bl&srcid=ADGEEShVpiITP8jVmBZbPlTCNnegXWpIVioUEPomrjsCqi3ciYV7cOFS_HjK085M5c69YZ4jahI9cD3GMs9YBa3Ja9pwQ90sFghnGPuqWHzWTa5ZUOFeI_bZque_XSyOxHcJnAuON5Ne&sig=AHIEtbTzFsBz4pRIsoQG4u4V-TfJmCYxBw]
• Holocene climate conditions and glacier variation in central Iceland from physical modelling and empirical evidence by Gwenn E. Flowers, etal. (2008; paywalled)

re. the volcanism beneath:

• GVP zu Hveravellir: http://www.volcano.si.edu/world/volcano.cfm?vnum=1701-08=
• GVP zu Prestahnúkur: http://www.volcano.si.edu/world/volcano.cfm?vnum=1701-07=
• Th. Thordarsson, Armann Hoskudsson: Iceland. Geology of Europe 3. Harpenden 2002 (book)
• Jon Sinton, etal. Postglacial eruptive history of the Western Volcanic Zone, 2005.

http://www.soest.hawaii.edu/GG/FACULTY/SINTON/publications/wvz_g3.pdf

Inge B.

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NTV Riddle

*** Beware the red herrings ….
1 point for the volcano 1 point for the red herring

No 1 - Pixar; Sakhalin Oblast; 2001; Citroen; SOLVED Nemo Peak
No 2 - Yellow alert; Dog breed; D.O.C.; Licking County; SOLVED Mt Etna
No 3 - Shield volcano; Scenic railroad; Lahar alert system; Ice caves; SOLVED Mt Rainier
No 4 - Mosquitoes; Inaccessible; Desert; Salt lakes; SOLVED Waw an Namous
No 5 - Spiders; Tiger flowers; Smoke rings; Local Capital; SOLVED Colima

Points …

Diana Barnes 2, Grimmster 3, Inge B 2, El Nathan 2

Kilgharrah

————————————————————————————————

Alan’s Evil Riddle ….

All this stone writing could really give printers the bird!

For a change, 3 alternative answers, so what are they?

Alan C

———————————————————————————————— 

Hekla Eruption Sweepstake …. Who’s gonna wear the ‘bragging rights’ T?

NOT ….
Chryphia March 26th at 19.17
Islander March 26th/27th up to 4pm
Jamie March 26th 23pm to midnight
LAKAT March 27th anytime
Diana Barnes March 27th at 7.30am
Lamiah87 March 27th 12.37
Talla March 28th at 2pm
Georgiade March 29th at 2.30pm
Sam March 29th 4pm onwards
Karenz. March 30th 4pm tp 5pm
Stoneyard April 1st midnight to 4am
Spica April 1st 00am to 06am
Floodwarn April 1st/2nd midday to midday
Inannamoon667 April 1st midday to midnight
CJWINWIN April 2nd anytime
Ursh April 2nd anytime

BUT MAYBE ….
Irpsit April 6th at 8.30am
Newby April 7th 3am to 7am
Cowboy Andre April 9th 1pm to 5pm
El Nathan April 9th late afternoon
Barbara Germany April 12th 6am to 8am
harrie April 12th/13th 8pm to 4am
Bobbi April 14th 2am to 6am
Stephanie Alice Halford April 14th 5.05pm
Denise April 17th midday to midnight
Jim Ludwell April 17th 4am to 11am
Lughduniense April 19th, in the (late) morning
Kilgharrah April 19th midday
Kobba April 20th 9pm and 10pm
Schteve42 April 23rd 6am to 8am
Christian Thordin April 27th 3.30am
Tyler Mannison April 30th midday and 5pm
Bo Minik May 2nd 1pm to 3pm
microcollector May 18th at 8.32am Granyia May18th daylight hours
Grimmster May 21st anytime
Alison May 25th midnight to midday
Summer May 30th anytime
GeoLurking June 25th anytime
Bruce Stout July 1st 1pm
Stefan August 16th 00am to 6am

Kilgharrah

TFZ – Tjörnes Fracture Zone and Etna is erupting right now!

Post by Inge B.

Tjörnes Fracture Zone is the northern one of the two big fracture zones in Iceland. They connect the northern resp. the southern parts of the rift and there is also a theory about microplates underlying these zones (s. Foulger, etal.).

Bouguer anomaly map showing gravity anomalies of Iceland and its surroundings showing very clearly how it is “embedded” in the MAR, region we are talking about is in the north of the country at the connection of the Arctic part of the MAR; image from Commons Wikimedia

The main geological features in the north of Iceland are this TFZ and the Northern Volcanic Zone which is connected by the TFZ to the Kolbeinsey Ridge, the part of the MAR north of Iceland out in the Arctic Ocean. The Northern Volcanic Zone comprises 5 volcanic systems (from S to N): Kverkfjöll, Askja, Fremrinámur, Krafla and Þeistareykir.

Two, probably even three constituent parts make out this zone from N to S: The Grímsey Oblique Rift (GOR), sometimes also called Grímsey lineament, the Húsavík-Flatey-Fault (HFF) and sometimes there is named a third fault reaching from south of Húsavík over to the Eyjafjarðar Trough, where the last earthquake swarm event took place during the winter. (for a map, see Metzger, p. 422)
http://www.n.ethz.ch/~smetzger/download/GJI_2012.pdf )

The now ongoing quake swarm didn’t come as a surprise to the scientists involved, because there has been continuously ongoing research on the region and the newest GPS measurements indicated very strongly a locking at rather shallow depth. This means that plate movements are arrested in a way, and therefore stress builds up – explaining the non-continous rifting events as described eg. by Irpsit. Plates seemingly often move in jumps and bolts and seldom smoothly (silent slip as an exception from the rule).

The locking was found out by analysing eg. seismology, also seismologic history of the region, which has often had heavy quakes in the past (2 magn. 6,5 in 1872), but also last not least GPS. And this is where the newest research comes in. The scientists discovered strong uplift in parts of the TFZ, i.e. in the southeast esp. And that even, after they had taken out by calculation the 2007-8 uplift at the volcanic system of Þeistareykir . While at the same time there was some subsidence in the west, which describes a plate movement to the southeast, meeting some hindrance around the valley of Jökulsá á Fjöllum, the biggest river valley in the region. Such blocking normally is released in considerable quake activity. And that is what is going on at the moment.

Copyright: Icelandic Meteorologic Institute, via RÚV, 03. April 2013

Up to now, there have been 4 big earthquakes and over 700 earthquakes within 3 days at a whole, the map showing the location of the hypocenters of the big ones and the rupture direction. Interesting that three of them have the same rupture direction and that the development is such that there is another center of activity now to the southwest of the first action.

Copyright: IMO by Inge B. (disclaimer: I am just an interested layman, no geologist). Literature:

• Th. Thordarson, A. Hoskuldsson: Iceland. Classic Geology in Europe 3. Harpenden 2002, esp. pp.136 …
• S. Metzger, et all.: Present Kinematics of the Tjörnes Fracture Zone, North Iceland, from compaign and continues GPS measurements. (2012) http://www.n.ethz.ch/~smetzger/download/GJI_2012.pdf

Images by IngeB and me. Please also check Irpsits comment from yesterday explaining how rifting works in Iceland. And keep in mind, we are all no volcanologists or geologists, just layman.

image: http://www.norvol.hi.is/~thora/ondvegi_frett.html

Inge B.

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Etna’s southeast crater is erupting right now:

The action is taking place at the new southeast crater again. Best cams are http://www.guide-etna.com/webcam/ and this one http://www.radiostudio7.it/webcam.asp?web=2&id=2 And White Island showed some incandescence last night:

White Island Crater Floor

Check http://www.geonet.org.nz/volcano/info/whiteisland El Hierro experienced some earthquakes again. For info on this check http://www.01.ign.es/ign/head/volcaSenalesDiasAnterioresCuasiReal.do?nombreFichero=CJUL_2013-04-03&estacion=CJUL&tipo=1&Anio=2013&Mes=04&Dia=03 http://earthquake-report.com/2011/09/25/el-hierro-canary-islands-spain-volcanic-risk-alert-increased-to-yellow/ and http://elhierro1.blogspot.com.es/

Spica