While our ability to deal with earthquakes as major environmental hazards has been limited, we have been more successful in our attempts to figure out the internal makeup and structure of the earth. This information has been, in large measure, derived from our understanding of how waves behave as they go through materials and layers. 

Wave behavior 

P and S waves behave differently depending on the phase of the material they travel through. S waves travel through solids only, P waves can travel through materials in any state (solids, liquids and gases). Whenever these waves hit a boundary between different materials or different layers, their direction and velocity commonly change. Both types of waves reflect (bounce) off a boundary and refract (bend) as they cross it. In part, the degree of refraction is not only a function of the material itself but also of its density, and of the angle at which waves strike the boundary. Also, as waves pass into different media, their speed changes. 

Wave arrival patterns

In view of the above knowledge of wave behavior, it is especially instructive to analyze the global patterns of body wave arrival at seismic stations. We describe the location of our recording stations by the angle between the point of origin of the quake, the center of the earth and the recording station. 

When we look at the global records of a single earthquake, the arrival pattern is always the same. All stations between 0^o and 103^o record both P and S waves. Stations between 103^o and 143^o record internal echoes but no direct (non-reflected) arrivals of P waves and no S waves at all. This area where there are no direct arrivals is called the shadow zone. Stations beyond 143^o record only P waves. 

The most striking part of this pattern is that there are no S waves arriving beyond 103^o. Because the S waves were propagated outward just as the P waves were, there must be something in the inner part of the earth which suppresses S waves. Because S waves cannot pass through fluids (liquids, gases or plasma), we can conclude that a portion of the interior must be non-solid. Density calculations indicate that this cannot be a gas, therefore this zone must be liquid. We can further deduce that this zone must be spherical, because it always produces the same pattern regardless of the point of wave origin. This internal portion of the earth that cannot be traversed by S waves and is liquid, at least in part, is called the core. 

Finally, we can deduce its size. The P and S waves arriving at 103^o are the last waves to sneak past this zone, so to speak. If it were larger, the last waves to go past it would arrive, not at 103^o, but at some lesser angle. Conversely, if this zone were smaller, S waves would be received beyond 103^o.