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

Location of the SIL and GPS stations and borehole strainmeters.
Image courtesy of IMO

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.

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.

Image courtesy of IMO

Búrfell darkblue,
Saurbær blue, Skálholt red, Geldingaá yellow, Stórólfhvoll violet, Hella light blue. Image courtesy of IMO

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).

Image courtesy of IMO

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

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 mage courtesy of IMO

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

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.

Image courtesy of IMO

Upper panel: Volumetric strain rate.
Lower panel: earthquake magnitude (left), horizontal components of tremor amplitudes (right)
Image courtesy of

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 Búfell (blue, 15 km from Hekla) and Skálholt (red, 45 km from Hekla) (Nanostrain per hour) Image courtesy of IMO

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

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!


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
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

263 thoughts on “How to read the Icelandic borehole strain and seismicity plots and NtV Riddle

  1. O/T Recently I started looking to big earthquakes (M>=5) felt in Belgium. I found more earthquakes than I thought, 14 M5+ earthquakes since 1300. That are on average 2 per century. You can divide the epicentra of the earthquakes in two regions:
    1)Lower Rhine Graben (not so surprisingly)
    1504 5 Aachen
    1640 5½ Aachen
    1692 6¼ Verviers
    1755 5¼ Aachen
    1756 5¾ Düren
    1828 5 Hesbaye
    1878 5½ Tolhausen
    1951 5.3 Euskirchen
    1992 5.4 Roermond

    and 2) the area around southern Northsea-the channel
    1382 6 Noordzee
    1449 5½ Noordzee
    1580 6 Pas de Calais
    1896 5 Lens-Arras
    1938 5.0 Nukerke

    Area 2) surprised me, I didn’t know that you could have such strong aerthquakes in that area. Area 1) is related to the Eifel volcanic field which is actually more aseismic ( not unlike the dead zone in Iceland), and has probably the same tectonic stresses as the Eifel that are causing it. The 1592 Verviers earthquake (probably between 6.0-6.5) is the biggest recorded earthquake in West-Europe. It was felt up in scotland. But more important, it had a Intensity of VII-VIII in the epicentral regio which included the cities of Liege, Maastricht and Aachen. An Intensity of VI was recorded for nearly whole Belgium and in West-germany(Köln). London, Amsterdam and Luxembourg city all suffered an intensity of V on the scale of Mercali, Paris IV. Which makes me wondering what would happen if such earthquake would strike today.

    (Source )

          • That’s nice 🙂 passed through there twice last week and will probably be there for obligatory reasons in begin September, as you will understand I think 😉

            But Verviers must be about a 2+ hour drive from there, at least. The second zone quakes must have been much closer.

          • I’m a little more than a one hour drive away from Verviers, little less than an hour for Aachen/Maastricht (good shopping). For the big quakes my area historically got VI-VII.

    • There is a failed rift system under the North Sea. Dunno how it relates to the Area 2) identified.

      According to BGS, the largest EQ in the UK was 6.1 in 1931 at Dogger Bank:

      But does the Lisbon 1755 earthquake count as Western European? If yes, it was in the region of 8.5 to 9.0

      Large intra-plate EQs can happen. What would happen if one struck in or near a large city in western Europe? I don’t know; I don’t know whether building codes include earthquake-proofing. But we have a lot of old buildings in the UK which would pre-date any modern code. We also have a lot of glass-walled buildings ….

    • minor typo in the initial comment by sa’ke (nice start to my morning that btw)
      1592 (in the text at the end) or 1692 (in the list at the top)

    • The Rhine Graben is a rift system like Iceland or the East African Rift, part of a system which extends between the Meditarranean and the North Sea. That would be behind the earthquakes and the volcanic activity in the region (Moho nearer to the surface)..

  2. is it Bob studied by Nemsio Persz? Bob must have a proper name but I dont know what that might be?

  3. Well I never…
    NeMO studies the dynamic interactions between submarine volcanic activity and seafloor hotsprings at an observatory, Axial seamount. A volcanic eruption occured at Axial in January 1998, destroying some hydrothermal vent sites and creating new ones. Since then NeMO scientists have been assessing the impact of the eruption and documenting the on-going changes in Axial’s summit caldera.

  4. My impression, the focus of activity is moving to the west.

    And the focus of the area is right down the rift (MAR)

    With no focal mechanisms, all I can say is that it is probably a series of normal faulting either side of the ridge, and that grabens either side of the ridge midpoint are opening up and fresh magma may be seeping up to fill the gaps.

    • And here be the point to ponder.

      Like a peice of paper, the strain releif (was gonna say “crack” but its not really a contiguous opening… I dont think) is moving into regions that havent let go yet (recently). The further south along the MAR you go, the greater the spreading rate. In the area of the swarm, its about 18 mm/yr total. Further North, its slower. At the SISZ, its transform motion (strike slips).

      Does this translate as more for Hekla to deal with later down the line? How long does it take that stress field to get there? Sure the swarm is headed west, but what of the other side of it? The strain is there, you just dont see it manifest as quakes right now… or its in free slip aseismic mode. (only gps can really say one way or the other… though probably too small for us to see it on public data)

      • I was thinking along similar lines . Why doesn’t IMO South Iceland area get huge swarms like we have seen on the Reykjanes Ridge and on the TFZ or is it that they do have big swarms and we just haven’t seen them yet?

        • That is perhaps because the South Iceland Transform Zone between Hengill and Hekla is probably a microplate, and the borders of TFZ as well as the Reykjanes Ridge are rift zones. Big swarms of (smaller) quakes characterising opening rifts, heavy earthquakes with aftershocks (which would also be quake swarms but of a different kind) characterising convergent plate boundaries and locking, ie. the plate can’t move, subduction plates eg. may be stopped by seamounts. But then TFZ is probably also a microplate (in the middle between two rifts) and has both kinds of swarms because of that. You have eg. quake swarms also to the west and east of South Iceland Transform Zone, i.e. in Hengill or Katla. (no expert)

          • The thing with the microplate is that it is getting broken up by the pressure as it progresses towards Hekla. It is actually easier thinking of books getting neatly stacked at the seismic faultlines accross the SIFZ.
            And at Hekla it is getting pushed down (most likely). So, the pressure stress mostly get assimilated as it traverses the SIFZ. And the rest just slides down quite asseismically at Hekla.
            The interesting thing is what happens on the other side of Hekla. There we have new crust comming moving from the rifting fissure line fault running from Katla up through Vatnajökull. That pressure also seems to magically dissapear at Hekla.

            Back to Lurkings walking strain. It will probably cause a bit of mischief sooner or later in the area in Reykjaness and then we get another big southern Icelandic earthquake and a new sprungur forms. When? Hm, yeah, who knows.

            • Sprungur are fissures, and in quakes there is not only one of them which forms , but lots of them. Also Almannagjá or other big ones at the Thingvellir graben formed in many earthquakes. Even during rather big quakes (magn. 6 or so), most of the fissures formed are rather small, but in big numbers. Saw a lot of the fissures from the 2000 and 2008 quakes in Iceland myself.

              What is a “walking strain”? Strain is normally accumulating in certain areas, and at a certain break point, the pressure releases in an earthquake or a series of quakes. The bookshelf faulting in southern Iceland may be caused by magma pushing up in the Reykjanes region, but I doubt that the connections between this and the SISZ are yet very well understood. There was no eruption in Reykjanes since the Middle Ages (i.e. since around 700 years), and we don’t know how much magma is welling up beneath the rift there.One could imagine that it would initiate some bookshelf faulting, but if so, should the bigger earthquakes there not arrive more often? There was nothing like that for 30 years before the 2000 quake.

            • Reykjanes is a spreading ridge, but at the same time composed of oblique arrangements of volcanic sytems. So the one does not forcibly exclude the other. At TFZ you find two volcanic mountain ridges opposite one to the other, with a graben in between. And at each ridge, sort of two conveyor belts placed also opposite one to the other where the magma comes to the surface and enforces the spreading by pushing the old material to the side.

        • No, they do not get huge swarms, they get huge singular earthquakes instead. They are not ripped apart, they get compressed untill it cracks in 6M+ earthquake as a Sprungur forms.

          • But yes, they do get huge swarms, too. Remember the afterquake swarms of 2008 in SISZ? They were continuing for months.

            • Those are afterquakes, something rather different then the normal quakeswarms we are talking about. A regular quake swarm does not have a large initial quake. Instead it is a series of smaller earthquakes with medium sized earthquakes interspersed in the ongoing sequence. There is not pattern of going from large to smaller. Two very different puppies really.

      • Should be more than 18 mm a year on Reykjanes Ridge, because for Central Iceland, it is 18.2 mm, more south of it, less north of it, acc. to P. Einarsson: Plate boundaries, rift and transforms in Iceland.

  5. New post is up with comments on A Tale of Three Cities by Rudiger Escobar Wolf, a scientist that does research on Fuego in Guatemala.

  6. While looking up about the 1692 earthquake, I found a letter from Huygens (the guy from the Huygens principle) about this earthquake. Here is the google translation :

    “1692, Sept. 18. in Hofwijck in Voorburg at half past three in the afternoon, while I was reading a book, I suddenly felt an earthquake and not without fear. The house shook clear, and moved back and forth, so were beaten in the dining room hanging paintings at the gold leather that covered the walls. The stone floor on which I stood was slightly lifted and dropped again, and several times for about 10 or 12 seconds. The moat around the house, 60 feet wide, moved with some broad waves to the sides. Employees in the kitchen, under the dining room, had felt the same movement, and were anxiously rushed to me.
    There was no wind. I had some time on the assumption that the arsenal of Dunkirk was completely destroyed by gunpowder, as was expected every day that that city would be and would be. With rockets and artillery fired besieged by our army But such a large distance could be barely as large print data to the air, and there was heard no sound or blow. Two days later we understood that there was nothing done to Dunkirk, and it was so really been an earthquake. And this also has to Amsterdam and Antwerp created all afraid. At an Amsterdam observer it seemed that, as it were waves from North to South (‘t NNW at’ t SSE) went forth, and all staggered. It is said that especially towers swung on an amazing way, and that naturally went ringing. Bells in some
    My caretaker, seized by work in the garden, had not felt anything, I think because he was moving. Shortly after the movement had ceased, I went upstairs to the barometer *) to inspect. He was 12 degrees, while he had the previous day stood at 14 ° and 16 °. But the next day he fell further to 10 °, and it has abundant rain.
    Throughout Zealand, in Flanders in the forts of King William, Liège, Cologne, Paris, London and Scotland is the same movement occurred. In Hamburg appears to be. He is not observed In Liege he was violent, without any damage. The time was there a quarter past two, thus earlier than here °).

    If the Earth through a kind of wave rises and collapses, she should be underground cave, or rest on water that then so moves. But from where is its movement? More likely is a cavity in which gather vapors, although they do not ignite as in the air, when it thunders.
    Could be something made out of the distance over which this sequence extends, about the depth of the holes and the vapors?”

    The last paragraph I found actually very interesting. I don’t know when seismology came into existence but here he is already giving the principle behind calculating the hypocentre. And he also gives very clear the time and place where the earthquake was felt. I think this letter must have been a huge help for modern seismologists to study this earthquake.

    • Some people just get it, regardless of the time they are in and the general knowledge that society posesses. And unfortunately, also some people will never get it, no matter how much knowledge society will ever gather.

      Bow to scientists of all ages, and not to Kings and Presidents.

      Fantastic find btw!

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