Two main types of waves are generated during a quake. Surface, or L (long), waves travel along the outer layers of the earth, (the crust). Body waves travel through the interior of the earth. Information derived from surface waves is critical to planning and hazard mitigation. Body waves tell us most of what is known about the interior of the earth. 

Body Waves

There are two basic types of body waves. P (primary) waves get their name from the fact that they are the first to arrive at a seismographic station. They are push-pull waves, compressing and stretching materials along the direction in which they travel. This type of wave can travel through solids, liquids and gases. S (secondary) waves are slower than P waves, but faster than L waves. They oscillate materials at right angles to the direction of travel and can be described as shake waves. They travel only through solid materials. 

How we Locate Earthquakes 

Because the ratio of the velocities of P and S waves in different materials is well known, we can use the difference in arrival times to locate the focus and the epicenter of earthquakes. Unfortunately, there is no easy way to establish the direction from which waves originate. For a single station, there is an infinite amount of possible epicenter locations, all lying on the perimeter of a circle whose radius is the distance calculated from the difference in wave arrival times. If we have a record from two stations, the location of the epicenter becomes restricted to two points at the intersection of the two circles generated by the distance from the two stations. In order to eliminate uncertainty, a third station is needed to locate the epicenter.