Although t seems that most activities occur on the surface, the inside of the planet is a constant chaos of seismic, tectonic and volcanic havoc. To understand what occurs inside the Earth, scientists study the type of motions that are responsible for earthquakes, volcanoes and other natural disasters with something called diagnostic waves. These waves travel at speeds ranging from 3 to 15 kilometers (1.9 to 9.4 miles) per second and categorize the four different types of seismic movements.
Two of the four waves travel about the surface of the Earth while the other two, Primary (P) or compression waves and Secondary (S) or shear waves travel through the interior of the Earth. Primary waves manipulate the medium (rock or liquid) that they travel through by compressing and dilating it. These waves hold similar characteristics to sound waves.
Unlike Primary waves, Secondary waves move twice as slow as P waves and are only able to travel through rock. However, both Primary and Secondary waves are bent or refracted at points when they encounter the threshold between two different media. For example, when a primary or secondary wave propagates through rock, it refracts when it hits water, or a point when differing physical properties meet. They also reduce speed when moving through higher temperatures. The behavior of these waves around areas can aid scientists to locate seismic discontinuities.
Primary and Secondary Waves 
