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USGS CMG InfoBank: Seismic Waves

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Comment: 05:27 - 06:46 (01:19)
Source: Annenberg/CPB Resources - Earth Revealed - 9. Earthquakes
Keywords: earthquake, energy, "plane of slip", fault, "frictional heat", "seismic wave", "P-wave", "S-wave", "primary wave", "secondary wave", "shear wave", "shear stress", "Dee Trent"

Our transcription: Most of the energy in an earthquake is expended when blocks of rock move into new positions.
The energy fractures the rock, forming a plane of slip, called a "fault" along which future earthquakes can also occur.
Some energy generates frictional heat, and a small part of the energy released creates seismic waves.
These waves can be categorized according to the form they take and the speed at which they travel.
Those that travel in such a way that the matter through which the waves are traveling are alternately compressed and dilitated.
These are called "P" waves.
The reason they're called "P" waves is because these are the first ones, the primary waves to arrive at a seismograph.
"S" waves, which are sometimes called "secondary" waves or "shear" waves are waves which travel by a wavelike motion where the matter is vibrating up and down at right angles to the direction that the wave energy is being propagated.
This causes a shear stress to develop in rocks through which it travels is why they're called "shear" waves or "S" waves.
The "S" waves travel somewhat slower than the "P" waves, the difference in speed about 2 kilometers a second.

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