Earthquakes – Or Wavy Gravy

Philippines after a large earthquake.

Philippines after a large earthquake.

First of all, this is not an article about earthquakes. It is a series about the sound that earthquakes emit as they happen, how that sound is travelling, and how it is detected. In a way it is not about earthquakes at all, or not even really about sound, instead it is about the strange and wonderful world of waves. A field I have spent 20 years studying and researching.

One thing I have noticed is that even though sound and waves are all around us all the time, and even though they rule our daily life in the most fundamental ways, very few really know about them and they are rarely if ever studied in schools. I think this is a pity, because out of the humble little sound wave you can derive everything in the universe.  And I really mean everything.

But let us not venture out into quantum physics or plucky string theory; instead let us keep to our mutual shared interest of earthquakes and volcanoes. But, as with everything else we need to start at the beginning.

What is sound?

Representation of a spherical pressure wave from a point source. This could be a drum, a stone dropped in a lake, or a small earthquake. Image by Thierry Dugnolle, Wikimedia Commons.

Representation of a spherical pressure wave from a point source. This could be a drum, a stone dropped in a lake, or a small earthquake. Image by Thierry Dugnolle, Wikimedia Commons.

According to physics sound is a vibration that propagates as a function of pressure and displacement. The vibration is a mechanical wave moving through a medium such as a gas, fluid or a solid. On the other side of the wave you have relaxation and replacement. So the net value of a sound wave will always be a zero displacement.

The wave is the physical function of it all, but what we hear is the sound from the event, may it be a violin or an earthquake rumble. And it is also good to remember that the sound we hear is interpreted by our brains and may not always be as accurate as we tend to believe, and to complicate things even more, our ears are not that great.

So to be able to say something for certain about sounds and the waves that causes us to hear the sound we most often need specialized equipment, be it the equipment of a recording studio or a seismograph. But, now I am getting ahead of myself again.

Back to the thing with wave propagation and media

There is a difference between waves moving in gases such as air and fluids like water as in comparison to a solid. And that is that mechanical waves in these types of media almost to a flaw moves as longitudinal waves. In the next installment I will get back to longitudinal waves and their estranged cousin the transverse wave.

As the sound source vibrates it creates vibration-movement in the adjoining media and the vibration in turn creates pressure against the individual atoms that moves in a direction from the point of origin. The movement will though only be straight over a longer distance, but for the poor atom the movement will be following the direction of the waveform. This is important since it means that the mechanical energy will be following a longer distance than the straight road. So, one could see it as the sound is travelling along an undulating road from the starting point to our ear.

A side effect of this is separation of frequencies over distance since a low frequency travels along a bigger undulation compared to its brother the high frequency component. This is though almost never a problem since high frequencies do not travel as far as low frequency components. Ahem, once again getting ahead of things.

Waves and zero net point energy

Now, nature seems to abhor leaving things dangling with excess energy, and a displaced atom will want to go back home, and it will do so in a rather mystifying progression. Well, if you have been a child born in the time where there were swings you will know the solution to the mystery by the seat of your pants, literally.

The four base module waveforms that are possible. By combining these 4 building blocks you can through combinatoric synthesis produce any comples waveform. Notice that all 4 base wave forms follow the 0, 1, 0, -1, 0 progression. Image by Omegatron, Wikimedia commons.

The four base module waveforms that are possible. By combining these 4 building blocks you can through combinatoric synthesis produce any complex waveform. Notice that all 4 base wave forms follow the 0, 1, 0, -1, 0 progression. Image by Omegatron, Wikimedia commons.

If we look at a wave as a discrete function it will move from 0 to 1 and one would then assume it would be happy at being back home at 0, but that is not true since the atom would at the end of the movement backward be going at full speed (think about how a swing behaves and what would happen if the swing came to a crash halt at the bottom with you on it), no instead the atom moves happily onwards from 0 to -1 and then back to 0 as resting place. Now if you sum up the energy states 0, 1, 0, -1 and 0 you get a big fat 0 net motion of the poor atom. Natural longitudinal waves always has a net 0 energy value on the local scale, this is due to the energy in the mechanical wave moving in a direction from the source. This might actually be the most fundamental part of our understanding of the Universe.

Antic chinese seismograph. It works in such a way that a sudden jolt will make a ball drop out of the mouth of a dragon into the mouth of the corresponding toad under it. This will then give the rough direction of the earthquake. The only drawback is that a ball will also drop on the other side too.

Antic chinese seismograph. It works in such a way that a sudden jolt will make a ball drop out of the mouth of a dragon into the mouth of the corresponding toad under it. This will then give the rough direction of the earthquake. The only drawback is that a ball will also drop on the other side too.

Now a solid can suffer from another type of wave, the transverse wave, but we will get back to that one in the next installment. What is true for the longitudinal wave is also true for the transverse wave, it just has an extra step that is quite profound, but I will let that dangle until later.

Now a last thing about the media, the wave will form due to the properties of the origin (a drum, violin or an earthquake) and that will give the waveform, but after that it is the media that Rule the Waves. Speed, longevity and a lot of other factors govern how the wave will conduct through the media and how it will be transformed due to diverse diffraction and filtering functions. But, that my friends are the scope for one of the following installments.

I think this is enough for today. This is probably a bit of a hard read for many so I will keep every part as short and simple as possible. Please feel free to tell me if the level is too hard, to easy or good enough. For me this is the hardest I have ever tried to write since it is my field and I tend to think about it in the form of mathematical formulations and technical jibber-jabber.

I do hope that you all in the end will feel that you have gained a new understanding about seismographs and how they detect distant sounds from earthquakes. And, if I really succeed well I might even give a glimpse of my deep rooted love of physics, the Universe and how waves can help you understand everything.

CARL

This series of articles are based on a series of lectures I used to give as an introductory course to wave propagation theory in physics, but rest assured. I will not hand out assignments and there will not be a test at the end.

Riddles

Our new Evil RIddler Matt has produced a new set of confounding and perplexing brainwreckers for the addicted. As usual, 2 points will be awarded up untill the clues are added. After that 1 point will be given out. And do not dispair, during the weekend I will tally up the scores and edit in a scoreboard.

  1. I would say that the fish bite, but that would be a trap! – Paranha-Etendaka Traps (Inannamoon667, 2pt), home of the Piranha fish
  2. My waters are deep, my seals are fresh, and my sponges aren’t very salty – The Baikal Rift (KarenZ, 2pt), home of the Baikal seal and the Baikal fresh water sponge
  3. I’m going to break the ice: Most of my visitors wear tuxedos! – West Antarctica Rift (KarenZ, 2pt)
  4. This volcano has an island in a lake on an island in a lake on an island – Taal (Stephanie Alice Halford, 2pt)
  5. If you’re an admiral, this might be a good place to crash – Loloru, Bougainville (Sissel, 2pt & Evan Chugg, 1pt for the Admiral)

MATT

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124 thoughts on “Earthquakes – Or Wavy Gravy

  1. Speaking of sound waves and earthquakes – I have only experienced 2 in the UK – but it seems that the sound depends on the type of rock they are travelling through. My first earthquake travelled as a double ripple across the fens from a 3.5 under Kings Lynn. I was sunbathing in the garden, lying on the grass, and the only sound was the bump bump of a lorry on the road, going over the ripples. It was a different story when it reached Cambridge where tall buildings swayed and rattled, tiles fell off roofs and stuff fell off shelves. The other one was the 4.0 Birmingham quake at 1.00am which was recorded on a radio programme there, live with Ms Edwina Curry, which rattled the hundred miles or so to Wales, through shale rock, and arriving like a roaring train, shook the house, which swayed like a jelly and then the sound diminished as it went out to sea.

    • One should also recognize that a M4 is 5 times as powerful as an M3.5
      But you are right, the media the wave travells through has a huge impact on things as we will see in later installments. 🙂

    • Experienced a few quakes so far. A few 3.0 in Portugal as minor tremors, but rather lasting several seconds. Then in Iceland I experienced two 3.0 near Katla, one of them was a clear rumbling shake, about 5 seconds. Best of all was the 2014 south Iceland quake, M4.5, a loud wavy noise hitting the house and shaking it back and forward for a few seconds.

    • The only EQ I heard was a bang like someone falling out of bed in the room nextdoor (in fact that is what I thought it was, at first) but then I rembered I was in a concrete building with concrete floors. I then noticed ripples on the surface of my coffee cup, which was on the table. Not sure whether the accompanying dizziness / nausea was from the EQ or my hangover 🙂

      The EQ was a 6.9 in Tokyo.

  2. Hello all and happy Whitsuntide/Pentecost holidays! I am too lazy with the over 30°C outside (and almost as hot in my loft apartment) to write all too much, but I did appreciate all the latest very educating posts. Especially this one on waves had me glued! Thank you!

    The Peony (German: Rose of Whitsuntide) or Paeonia derives its name from the Greek word “Paeonia”, which stands for the gods’ medicus Paian. According to Greek legend he healed with the help of this plant Pluto, the god of the underworld, after he was wounded by Heracles in the war of Pylos.

    • Whitsuntide Carol: The Young Tradition (1967)
      “From their album ‘So Cheerfully Round’. Was collected from Thomas Coningsby of Whaddon in Cambridgeshire. He tells how the men of the village used to go into the woods on Whit Sunday, cut oak branches and lay them on the doorsteps of the houses. Then they would go round in a group singing this carol. It seems strange that the high moral tone of the early verses should somehow be linked with what is obviously a pagan custom.”

      • Well, Oak has figured prominently in several ancient religions. Probably due to it’s strength and relatively high heat output during winter. It’s the wood of choice for a hearth. A rather odd thing that I learned from my granddad. Pine is for the morning stove when you need to get a fire started quickly so you can cook breakfast, hardwoods like oak are for the fireplace at night.

        I have had Oak limbs placed at the front of my house… but it was done by a hurricane and it was the whole freaken tree… and it ate my truck. The good part (if there can be a good part), is that the truck kept it from rolling onto my garage.

  3. I think it hasn’t been mentioned here that an eruption has started at Zhupanovsky volcano, Kamchatka and its aviation code has been set to Yellow.

    Tokyo VAAC
    http://ds.data.jma.go.jp/svd/vaac/data/TextData/20140606_ZHUP_0001_Text.html

    KVERT
    http://www.kscnet.ru/ivs/kvert/
    “Explosive eruption of Zhupanovsky volcano began on June 06, 2014. Ash plumes rise up to 20,400 ft (6 km) asl (data from UHPP and Tokyo VAAC) Moderate explosive eruption of the volcano continues. […] Historic eruptions are associated with the third cone. A fumarolic activity of the volcano is observed at present. […] Data about beginning of eruptions by staff of Nalychevo Natural Park.”

    Photo of Zhupanovsky’s crater by Markus van Almsick on Panoramio:

  4. I just came across this pool of information on Stromboli monitoring data, presenting all the seismographic, infrasonic, latest MODIS, infrared cam and more data in real-time or processed as statistics, and more. If it hasn’t been posted before, here is the link:

    http://193.206.127.20/realtime/roc.php?stz=ROC&ch=1&h6=0
    website of the Laboratorio di Geofisica Sperimentale, Uni Fiorenze

  5. Pingback: Leave your sineful ways behind | VolcanoCafé

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