Earthquakes, the trembling or shaking movement of the earth's surface, are related to compressional or tensional stresses that have built up at the margins of the large moving lithospheric plates that comprise the earth's surface (Earthquake). The majority of earthquakes are minor tremors. Most often, large earthquakes begin with small tremors that evolve into one or more violent shocks, and end in vibrations of diminishing force, referred to as aftershocks (Earthquakes). The subterranean point of origin is called the 'focus,' while the surface point directly above the focus is called the epicenter (Earthquakes). The magnitude an intensity of an earthquake is determined by the use of scales, such as the Richter Scale and the Mercalli Scale (Earthquake).
             Most shallow earthquakes are caused by the sudden release of stress along a fault or fracture in the earth's crust, which results in the movement of the opposing blocks of rock past each other (Earthquakes). The movements create vibrations that pass through and around the earth in the form of a wave, much like the ripples that result when a pebble is dropped into water (Earthquakes). Although volcanic eruptions, rockfalls, landslides, and explosions can cause an earthquake, the majority of these are of only local extent (Earthquakes). Yet, shock waves from a large earthquake can cause smaller earthquakes in areas that are hundreds of miles away if the geologic conditions are favorable (Earthquakes).
             The different types of earthquake waves include: P, or primary waves, which are compressional and travel with the greatest speed; S, or secondary waves, are transverse and cause the earth to vibrate perpendicularly to the direction of their motion; L, or long waves are surface waves (Earthquakes).
             Since the velocities of the P and S waves are affected
             by changes in the density and rigidity of the material
             through which they pass, the boundaries be...