But the forces pushing on the rocks continue to push, and eventually, exceed the strength of the rock. The rock will fracture and snap back, much like a rubber band. This elastic rebound releases the strain on the rock. There are two major groups of seismic waves, or vibrations, that occur as a result of earthquakes. Surface waves , or L waves, travel over the surface of the earth and cause much of the damage in an earthquake.
Body waves travel through the body or interior of the earth.
Thus, we can infer details about the earth's interior by observing these waves. The surface L waves are the last to show up. S waves do not travel through liquid. The body waves change velocities when they pass through different materials inside the earth. Identifying such a change led to the discovery of the crust-mantle boundary.