A fault is a crack in a plate or a place where two or more plates meet. An example of a fault where two plates meet is the San Andrea's fault in California, where the Pacific and North American plates meet. "The San Andreas Fault forms a continuous narrow break in the Earth's crust that extends from northern California southward to Cajon Pass near San Bernardino. Southeastward from Cajon Pass several branching faults, including the San Jacinto and Banning faults, share the movement of the crustal plates. In this stretch of the fault zone, the name "San Andreas" generally is applied to the northeastern most branch." (Watson). The complex network of faults that make up the San Andreas Fault system in central California is shown in the next figure:.
.
The plates are approximately 30 miles thick under land and can be one to five miles thick beneath the ocean. The plates move because of convection currents. Magma has currents like the ocean does, that move in a circular motion beneath the plates. When two plates are pushing against each other, they are constantly building up tension on the fault. Earthquakes occur because "The crust may first bend and then, when the stress exceeds the strength of the rocks, break and "snap" to a new position. In the process of breaking, vibrations called "seismic waves" are generated. These waves travel outward from the source of the earthquake along the surface and through the Earth at varying speeds depending on the material through which they move. Some of the vibrations are of high enough frequency to be audible, while others are of very low frequency. These vibrations cause the entire planet to quiver or ring like a bell or tuning fork." (Shedlock).
Because of the incredible destructive capabilities of earthquakes, scientists are constantly trying to devise ways to ensure their early detection. Earth scientists have begun to forecast damaging earthquakes in California.
