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Thread: Seismic Waves (P,S,R,L)

  1. #1 Seismic Waves (P,S,R,L) 
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    Hello everyone, I have 4 short question about seismic waves!

    1) "When seismic wave hit the Moho (crust-mantle boundary), the speed of the wave increases. The increase in speed as the waves reach the mantle indicates that the mantle is denser than its crust."
    __The mantle is densier than the crust, so seismic waves speed up as they reach the mantle! How come seismic waves speed up as they travel through denser layers? (Should it be harder to pass through instead, if it is densier?)

    2) In my book, it also says "Seismic waves travel faster through more rigid materials"
    __Is rigidity directly proportional to density?

    3) How does surface waves affect the rocks through which they travel? I know that surface waves are a combination of P and S waves, so would they cause the ground to move BOTH horizontally and vertically and forced the rocks to move in many different directions?

    4) http://www.darylscience.com/Demos/PSWaves.html
    According to this web site, Love Waves causing the ground to move in a horizontal elliptical path ("surface circle", as the web site says), as oppsed to Rayleigh Waves which cause vertical elliptical path! Does it mean that L-wave does exactly the same thing as R-wave but only particles are moved horizontally?
    However, the diagram in the bottom of the web site is not showing a "surface circle" paritcle motion, but straight arrows pointing in opposite horizontal directions...why?

    Can somebody explain please? :wink:


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  3. #2  
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    1. Dense materials are usually faster, there are exceptions, most notably salt - a low density, high velocity material.

    2. No. However denser materials are normally more rigid.

    3&4. Well you'll probably be surprised to hear that there are two types of s-waves, at leat the s-wave must be considered as two components that travel at the same speed. SV waves have osciallations along the line connecting the moving particle to the centre of the earth (vertical), SH waves are perpendicular to SV waves, and particles also oscillate perpendicular to the ray direction (horizontal).

    Love waves are made up of SH waves, the website is wrong! The particles don't move in ellipses, they move in straight lines as shown in the diagram. Rayleigh waves are made up of SV waves and P waves, the combined effect of these is to cause elliptical oscillations in the vertical plane (as shown in the diagram).


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  4. #3  
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    take a load of marbles put them in a line each about a centimetre apart, whack the first marble and it moves, hits the second, it moves then hits the third... etc etc finally if you hit the first one hard enough the last one will move.


    Now make the line more dense, ie keep the line the same length but add more marbles till they are all squashed up and touching. Now whack the first marble and you will see the last one will appear to move at the same time.

    The more dense, the faster the shock-wave travels.

    That's the principle and as for the earth it's a damn good one as the crust is made of pretty much the same stuff as the marbles.

    Oh if you can't find enough marbles, do it on a pool or snooker table.

    And as soon as you reply to this you will become a Forum Sophomore!
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  5. #4  
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    Be careful to distinguish shockwaves from your normal elastic waves.
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  6. #5  
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    Quote Originally Posted by billiards
    Be careful to distinguish shockwaves from your normal elastic waves.
    If that's aimed at me I am generalising on sound waves travelling through a medium, my knowledge of s & P waves through the earth would need revision prior to comment.

    I remember one is direct? the other through the core? or round the crust?
    oh dear - and I only did it about 30 years ago...
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  7. #6  
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    I did it more like 30 weeks ago, from what I remember. surface waves travel round the crust (i.e. love and rayleigh waves). P-waves can pretty much go through everything. S-waves can't go through the outer core, giving an s-wave shadow zone.
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