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?
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.
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.
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.
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.
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.
- I would say that the fish bite, but that would be a trap! – Paranha-Etendaka Traps (Inannamoon667, 2pt), home of the Piranha fish
- 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
- I’m going to break the ice: Most of my visitors wear tuxedos! – West Antarctica Rift (KarenZ, 2pt)
- This volcano has an island in a lake on an island in a lake on an island – Taal (Stephanie Alice Halford, 2pt)
- If you’re an admiral, this might be a good place to crash – Loloru, Bougainville (Sissel, 2pt & Evan Chugg, 1pt for the Admiral)