Posted on by Jón Frímann

Harmonic tremor in Katla volcano

At the moment there is an ongoing harmonic tremor going on in Katla volcano. It is just an small one, just above the background level noise floor. This is most likely an small dike intrusion. But nothing in the data from Icelandic Met Office suggests that an eruption has started in Katla volcano at this stage. But that might change without an warning, since this is an volcano after all. This change in harmonic tremor does not seems to be created by an glacier flood of some type. What is interesting is that this harmonic tremor pulse looks like an harmonic tremor pulse detected in the year 1999, just before an small eruption took place in Katla volcano southern caldera rim.

This harmonic tremor is in the high frequency range at the moment. But no change has been seen in the low frequency range at the moment. This is the same thing that did happen in the year 1999. At the moment however it is too early to know what is going to happen in Katla volcano for sure. All that can be done is to watch and wait. This might quiet down, or it might not. Only time is going to tell us what happens next. At the moment the harmonic is falling at fast rate, so this might be over.

Before the harmonic tremor pulse there was an earthquake swarm in Katla volcano caldera. The largest earthquake was ML2.1 and that was an long period event, but that tells me that magma was the source of this earthquakes in Katla volcano caldera. There where two earthquake swarms in the caldera today, but that makes total of three earthquake swarms in the Katla volcano caldera since the beginning of June.


The two earthquake swarms that took place in Katla volcano caldera today. Copyright of this picture belongs to Icelandic Met Office.


The only earthquake that I did record out of this earthquake swarm. This earthquake was ML2.40 with the depth of ~400 meters. This plot is filtered at 1Hz . This picture is released under Creative Common licence. See Licence web page for more detail.


Spectrogram of the ML2.4 earthquake in Katla volcano from this morning. This is filtered from 1Hz. This picture is released under Creative Common licence. See Licence web page for more detail.


Harmonic tremor can clearly be seen at the end of this tremor chart from the Icelandic Met Office. Copyright of this picture belongs to Icelandic Met Office.


Harmonic tremor can be seen at the end of this tremor chart from the Icelandic Met Office, but it is not clear at this SIL station. Copyright of this picture belongs to Icelandic Met Office.


Harmonic tremor can clearly be seen at the end of this tremor chart from the Icelandic Met Office. Copyright of this picture belongs to Icelandic Met Office.


Harmonic tremor can clearly be seen at the end of this tremor chart from the Icelandic Met Office. Copyright of this picture belongs to Icelandic Met Office.


Harmonic tremor can be seen (not clearly) at the end of this tremor chart from the Icelandic Met Office. Copyright of this picture belongs to Icelandic Met Office.


Harmonic tremor can clearly be seen at the end of this tremor chart from the Icelandic Met Office. Copyright of this picture belongs to Icelandic Met Office.

The harmonic tremor is strong enough on many SIL station to go above the background noise level on the SIL stations (wind, ocean waves etc..). But this is not an strong harmonic tremor. They can be stronger.

This does NOT mean an eruption is imminent in Katla volcano. What this does mean that is an magma is on the move inside the Katla volcano. What happens at later time is something that nobody can guess at current time. But it is now time to watch Katla volcano even closer.

It is also important to notice that nobody really knows how Katla volcano behaves before an eruption. We only get those answers after an eruption in Katla volcano. For the moment we can only wait and see what happens in Katla volcano.

I am going to post more updates if anything interesting happens. Also, there seems to have been an Long period earthquake at 22:14 UTC, most likely in Katla volcano. It appeared on my geophone close to Hekla volcano. I am guessing the size of this earthquake is ML2.0+.

Blog post updated at 22:39 UTC. Spectrogram picture added.

26 Replies to “Harmonic tremor in Katla volcano”

  1. Friday 8.7.2011 at 22:13:20 depth 1,1 km 1,5R Quality 90,04 Loc. 3,8 km NNV af Hábungu

  2. Hi. Are you able to post spectrogram (FFT) of seismic signals you deem interesting as well as waveforms? (for harmonic tremors, etc). I always find them very interesting and helpful, but they seldom get shown to the public.

      2. This the best that the software that I use can offer. It is not much, as it is not as good as the professional level software out there.

      3. If you had the raw waveform, you could pass it through something like CoolEdit and generate that plot.

        I’ve done it with a wav filed extracted from the Iris BUD seed data, but had to go through SeisGram2k. Generally, it wants to interpolate anything going to wav format (so you can hear it) and that tends to muddy the data.

  4. Magma moving both under Katla and Hekla volcanoes… hmm… this stuff sounds a bit like “the attack of the killer worms”. 🙂
    Let us wait and see.
    We don’t want another disappointment here as what has happened with Etna in Sicily: a failed eruption

  5. Heheh! I am just contemplating the press-induced panic across Europe if Katla and Hekla were to erupt at the same time. Dangerous thoughts!

    Anyway, about this harmonic tremor thing… I see the blue and green bands going up together, but not the red. Is the red one not so important?

    Thanks, as ever, in advance.

    1. We should send the press elaborate and detailed descriptions of the events of 1783 and associate them with Katla and Hekla and there will be great knashing of teeth fueled by the media.

      Personally, I agree with Renato that this is all earth worm activity. I have long thought that giant green earthworms planted and controlled by aliens would descend upon the earth and devour all human life.

      1. Well, indeed! In fact, it would make perfect sense that if alien-controlled earthworms were plotting to devour us, that they should want to cook us first. We would be much tastier medium rare!

  6. I am not yet sure. But the last earthquake in Katla volcano caldera appears to have increased the harmonic tremor again.

    This does not look good. But so far am not yet ready to call it an eruption.

  7. Hi jon,

    Quick question. When were seismographs first used to monitor Icelandic volcanoes. I’m going to imagine it is much more recent than in other countries because of how poor the country was.

    1. I think it was around the year 1960 or something. There where seismometers in use when eruption took place in Heimey in the year 1973. But the network was not as large as it is today.

  8. Godabunga looks much more altered now than when you posted. I see also changes in Lagu-Hvolar tremor graphs.

    1. Yes, after the last earthquake the tremor stopped falling and started to rise again.

      This is what I call not good. I am already at my own alert level 2 at the moment. There are changes taking place in Katla volcano and I do not like them one bit.

      1. The source is closest to that station. I am guessing at the location where the other earthquake swarm is located. Where the ML1.5 earthquake took place earlier tonight.

  9. If I see that the frequency of 0.5 Hz tremor at some stations is somewhat higher, I think it’s still too early to see something clear

    1. Its in the middle of the night – especially in europe. The next, who could spot some activity are american news outlets.

  11. The harmonic tremor is still rising. But at uneven rate so far. I do not think that this an glacier flood that is creating this tremor that we are seeing.

    I am going to post an update around 00:30 UTC on what is going on at Katla volcano.

  12. http://hraun.vedur.is/ja/Katla/god_trem.gif

    the tremors have gone through the roof. or if we would like to call it somewhat less strong: the tremor level has risen significantly over the last few hours.

    also: there seems to be inflation which can be seen on two of the GPS stations.
    http://hraun.vedur.is/ja/gps/predorb/soho_enu_p.png
    http://hraun.vedur.is/ja/gps/predorb/gola_enu_p.png

    my guess is, that there is a dike intrusion taking place, which causes the deeper earthquakes. and my instinct tells me, that the shallower earthquakes are caused by increased geothermal activity, which might be caused by the intrusion.

    but still, lets wait a few hours more and we will be a bit wiser. good night to everybody and good morning to those who are allready getting up : )

    ps. i like the earthworm-theory a lot : ) we should make a movie out of it^^

    1. At this point, we need some holy virgins. Its a perfect window of opportunity to throw some in before it erupts.

      We have to appease the gods….they appear to be very angry. The hell with the earth worms.

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