Earthquakes travel outward from the epicenter in three different types of waves. The fastest of these, called the P-waves, are pressure waves that travel through the ground like sound waves. Then come the S-waves, or shear waves, which move the earth from side to side. Last, but certainly not least are the so-called Raleigh Waves, which result in ripples on the surface of the earth - this is what makes the telephone poles appear to dance. Like counting the second between lightning and thunder to judge the distance to a storm, you can judge the distance to the epicenter of an earthquake by noticing the time between the shaking (S-waves) and the wiggles (Raleigh waves).


Suppose you are in an earthquake. You see the earth start shaking with a period of 2.0 seconds. Approximately 30 seconds later, you see the telephone poles wiggle at a frequency of 5 Hz. Your friend, the seismologist tells you that S-waves have a wavelength of approximately 7 kilometers, while Raleigh waves have a wavelength of 160 meters.