Seismic Waves

When a sudden break or shift occurs in the earth's crust, the energy radiates out as seismic waves, just as the energy from a disturbance in a body of water radiates out in wave form. In every earthquake, there are several different types of seismic waves.

Photo courtesy USGS Structural damage caused by vibrations from the 1964 Alaska earthquake

Body waves move through the inner part of the earth, while surface waves travel over the surface of the earth. Surface waves -- sometimes called long waves, or simply L waves -- are responsible for most of the damage associated with earthquakes, because they cause the most intense vibrations. Surface waves stem from body waves that reach the surface.

There are two main types of body waves.

• Primary waves, also called P waves or compressional waves, travel about 1 to 5 miles per second (1.6 to 8 kps), depending on the material they're moving through. This speed is greater than the speed of other waves, so P waves arrive first at any surface location. They can travel through solid, liquid and gas, and so will pass completely through the body of the earth. As they travel through rock, the waves move tiny rock particles back and forth -- pushing them apart and then back together -- in line with the direction the wave is traveling. These waves typically arrive at the surface as an abrupt thud.

• Secondary waves, also called S waves or shear waves, lag a little behind the P waves. As these waves move, they displace rock particles outward, pushing them perpendicular to the path of the waves. This results in the first period of rolling associated with earthquakes. Unlike P waves, S waves don't move straight through the earth. They only travel through solid material, and so are stopped at the liquid layer in the earth's core.

Click the play button to start the earthquake.
When P and S waves reach the earth's surface,
they form L waves. The most intense L waves
radiate out from the epicenter.

Both sorts of body waves do travel around the earth, however, and can be detected on the opposite side of the planet from the point where the earthquake began. At any given moment, there are a number of very faint seismic waves moving all around the planet.

Surface waves are something like the waves in a body of water -- they move the surface of the earth up and down. This generally causes the worst damage because the wave motion rocks the foundations of manmade structures. L waves are the slowest moving of all waves, so the most intense shaking usually comes at the end of an earthquake.

In the next section, we'll see how scientists can calculate the origin of an earthquake by detecting these different waves.