A P wave is a seismic wave that alternately compresses and stretches the rock: the rock moves back and forth along the direction in which the wave travels. It crosses solids, liquids and gases, and it is the fastest seismic wave, so it arrives first. An S wave shears the rock: the rock moves at right angles to the direction in which the wave travels. It crosses only solids, and it arrives after the P wave.
উদাহরণ
Example 11.4 (An earthquake away)
Observed travel times are for P and for S at , and at . A station that records an S wave (8 minutes 13 seconds) after the P wave therefore lies from the epicentre: about . At the delay would only be .