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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