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Seismic waves

  1. May 30, 2010 #1
    HI,
    I want to know how seismic waves are produced?. I cannot find any information on it on google. Any help will be helpful.
    Thanks.
     
  2. jcsd
  3. May 30, 2010 #2
    Earthquakes, meteorite impact, explosions, drilling activity, hitting the ground with a hammer, kangaroos jumping on the ground, wind blowing trees and shaking the trees. Seismic waves are elastic waves (to a very good approximation), so anything that squishes, squeezes, or twists the rocks will propagate as seismic waves provided that the elastic limit of the rocks is not surpassed.
     
  4. May 30, 2010 #3
    So what would happen if the elastic limit was surpassed?
    It would simply break, right?
     
  5. May 31, 2010 #4
    Yes makes very sense, thank you so much.
     
  6. Jun 1, 2010 #5
    That depends somewhat on the properties of the rock. There are some materials, like for example a spring, that when you exceed the elastic limit the spring is permanently deformed -- it doesn't break or snap, it is just stretched beyond its elastic limit.

    In rocks, most of the time the rock will break when it has been squeezed or stretched too much (bearing in mind we are sticking to short time scales here -- over very long time scales the rock can behave very differently and change shape a bit like the spring). In that case the rock is squished until it can stand it no longer and it is broken, seismic energy is still propagated but the elastic limit of the rock puts a cap on the maximum amount of seismic energy that can be propagated. Interestingly, the strength of rocks puts a limit to the maximum magnitude of an earthquake (measured in ground shaking), you cannot get earthquakes larger than about a 10 on the magnitude scale, if you try to put in more energy the energy is spent in tearing up the rocks locally and is not propagated as seismic waves.
     
  7. Jun 3, 2010 #6
  8. Jun 3, 2010 #7
    Well as I said it depends on the rocks.

    There are though at least two things here that need mentioning:

    1) The Chicxulub event was not an earthquake, it was a huge meteorite impact that probably wiped out the dinosaurs 65 million years ago. Actually, a lot of the energy from the Chicxulub impact would've been transmitted as shock waves (which travel faster than seismic waves) which are definitely NOT elastic waves. So meteorites can make things happen which go off the chart, meteorites can also vapourize rocks and excavate huge craters -- you just can't get that with earthquakes.

    2) The Richter scale is obsolete. Seismologists now use the moment magnitude scale http://en.wikipedia.org/wiki/Moment_magnitude_scale.
     
  9. Jun 4, 2010 #8
    Arrrr.... moment magnitude scale. Thanks for that information. Quite impressive. Obsolete indeed. Thank you.
     
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