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 stem from body waves that reach the surface.
There are two main types of body waves. 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. 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.