The following animation shows the movement of a P-wave through rock. As the P-wave makes contact with the rock, the spacing between rock molecules contracts cause the volume of the material to decrease. The rock expands and returns to it original shape after the P-wave passes. S-waves or secondary waves are a second type of body wave. These waves are slower than P-waves and can only move through solid materials.
Surface waves travel at or near the Earth's surface.
These waves produce a rolling or swaying motion causing the Earth's surface to behave like waves on the ocean. The velocity of these waves is slower than body waves. Despite their slow speed, these waves are particularly destructive to human construction because they cause considerable ground movement. The strength of an earthquake can be measured by a device called a seismograph . When an earthquake occurs this device converts the wave energy into a standard unit of measurement like the Richter scale .