In this activity, students learn about the differences between S-waves and P-waves, collectively known as body waves, which move through the inner rock part of the earth and are created during earthquakes.
Seismic waves are the energy that radiates out from the centre of an earthquake (the epicentre). They consist of body waves (P- and S-waves) and surface waves (L-waves).
P-waves (primary or compressional waves) are longitudinal or compression waves, able to move through solids, liquids, and gases at speeds ranging between 300-5,000 metres per second. As they travel through rock, they move particles back and forth in the same direction that the wave is moving. When P-waves strike an object they push and pull the object. It's like a train engine bumping into a railroad car, which then bumps into another. This movement continues through the whole length of the train. In solids, these waves travel twice as fast as S-waves. For example, P-waves move at 330 m/s in air, 1,450 m/s in water, and 5,000 m/s in granite.
P-waves are initiated and transmitted when a sudden release of energy compresses rocks near the earthquake focus. As the waves continue on to compress adjacent rocks the initially compressed rocks expand elastically past their original volume. Then they return to their original shape.
S-waves (secondary waves or shear waves) occur just after P-waves and travel more slowly. They move the ground in an up-and-down motion, similar to the way waves move in the ocean As these waves move, they displace rock particles perpendicular (i.e. up and down) from the wave direction. They can only move through solids.