Seismic Waves

When crustal rock breaks, stored energy is released in a single, massive event called an earthquake. The vibrations that travel through the Earth are called seismic waves.

A seismograph detects and measures three types of seismic waves. P waves and S waves travel from the focus of an earthquake through the Earth's interior. When the energy reaches the surface of the crust, surface waves are created.

P waves, or primary seismic waves, are the fastest moving waves. They travel through solids, liquids and gases. The pulsing of P waves causes rock to move back and forth, first compressing and then stretching the rock.

S waves, or secondary waves, are slower than P waves and can travel through solids, but not liquids. S waves move in an S pattern, creating shear stresses that cause the crust to move from side to side and up and down.

The slowest and most destructive seismic waves are called surface waves. One type of surface wave travels in a circular motion, moving the surface up and down. Another type causes back and forth motion.