eaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the earth's crust. Stress in the earth's crust creates faults-places where rocks have moved and slip, resulting in earthquakes. The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, the direction of the plane is usually not vertical; rather it dips at an angle into the earth. When the rock hanging over the dipping fault plane slips downward into the ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the bottom wall, the fault is called a reverse fault or a thrust fault. Both normal and reverse faults produce vertical displacements, or the upward movement of one side of the fault above the other side, that appear at the surface as fault scarps. Strike-slip faults produce horizontal displacements, or the side by side sliding movement of the fault, such as seen along the San Andreas Fault in California. Strike-slip faults are usually found along boundaries between two plates that are sliding past each other.
             The sudden movement of rocks along a fault causes vibrations that transmit energy through the earth in the form of waves. Waves that travel in the rocks below the surface of the earth are called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known as shearing waves, cause the most damage during earthquake shaking, as they move the ground back and forth. Earthquakes also contain sur...