Ultrasonics and Seismology

Ultrasonics and Seismology - Ultrasonics and Seismology We...

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Ultrasonics and Seismology We know that the Earth has a core which is molten, at least in its outer parts, because the shear (S) waves do not propagate through this region. The text notes that seismic S- waves cannot pass through liquid. You will remember this better if you think about why this might be so. The easiest way is to think first of a gas, like the air in the lecture room. If I push a molecule in the forward direction, towards you, it will bump into an adjacent one, and bounce back. The one which it hit moves forward in turn and bumps into the next one, and so on, molecule after molecule. In this way, a longitudinal (forward and backward) motion can pass through the gas, like the rattle which propagates down the length of a freight train when the engine bumps into it. But if I push an air molecule sideways, rather than towards you, the disturbance will obviously not be transmitted in your direction: the second molecule along the line between me and you is not much affected by the motion of the first because the molecules are not connected together -- instead, they are free to move independently. (This is a gas, after all.) In other words, even if a lot of molecules are made to move at
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Ultrasonics and Seismology - Ultrasonics and Seismology We...

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