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Comment: 12:56 - 14:04 (01:08)
Source: Annenberg/CPB Resources - Earth Revealed - 3. Earth's Interior
Keywords: earthquake, P-wave, S-wave, "seismic wave", "primary wave", "secondary wave"
Our transcription: Geologists divide the seismic waves that travel through the Earth's interior into two basic types, "primary" or "P-waves" and "secondary" or "S-waves."
A P-wave is a compressional wave that makes the rock vibrate parallel to the direction of its movement.
Since it is a very fast wave traveling through rock at between four and seven kilometers per second, the P-wave is the first wave to arrive at a recording station following an earthquake.
An S-wave, on the other hand, has a shearing motion that makes the rock vibrate perpendicular to its path.
This movement slows the S-wave, so that it travels at two to five kilometers per second or about half the speed of the P-wave.
This is why S-waves arrive as secondary waves at the Earth's surface.
There is another important difference between P-waves and S-waves.
Although both can pass through solid rock, only P-waves can also pass through gases and liquids.
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