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.

The shadow zone.
Why then no direct P wave arrivals between 103^o and 143^o? This can be explained by refraction. Because the earth is density zoned, internal layers will refract waves more than the external layers. The last direct P waves to skim off the core will exit at 103^o. A portion of these P waves, however, will cross the mantle/core boundary and be refracted inward. After these waves cross the core, they recross the core/mantle boundary on their way out, refract outward this time, and resurface at 143^o. Because of this double refraction, there is no possible way for P waves to cross this part of the earth, and therefore, there are no direct arrivals in this shadow zone. 

Wave arrival patterns also give us information as to the internal density of the earth. We know from exploration and direct measurements that the specific gravity of the surface materials of the earth is near 3.0. We also know that on the average the whole earth's specific gravity is closer to 5.5. We therefore know that the interior must be denser than the surface. 

Suppose then that we compare the arrival times of the two types of waves (surface and body) at two points that are the same distance from the epicenter. If the earth were homogeneous, both sets of waves should arrive at expected times and, if we are correct for all factors other than density, they should arrive at the same time. But, in fact, body waves arrive earlier than expected, indicating not only that internal materials are more dense (which we knew already, although it is always reassuring to have one's calculations confirmed), but also that density increases with depth. 

When we look at body wave behavior, there is an abrupt increase in velocity for all waves anywhere from 3 to 35 miles down. This tells us that there is an abrupt change in composition, or in materials, or both at that depth. Such an abrupt change in wave behavior is called a discontinuity. The upper layer, above this seismic discontinuity, is called the crust, and the layer below it, in which there are no major discontinuities till we reach the next major discontinuity at the core, is called the mantle. From this information we can therefore conclude that the earth is density zoned into three layers, the crust, mantle and core, separated by two major discontinuities. This is not too surprising given that our earth has been an active planet for the past nearly five billion years and that these materials have had ample time to separate according to density, under the influence of gravity. Moreover, this internal density zonation is just an extension of the pattern we can observe at the surface, where the less dense materials of the atmosphere overlie the oceans, and these in turn cover the denser upper part of the solid earth.