The earth’s outer crust is composed of very big, hard plates as large or larger than a continent. These plates float on the molten rock beneath. When these plates encounter each other, appreciable horizontal and vertical ground motion of the surface occurs known as the earthquake. For example, in the western portion of the United States, an earthquake is caused by the two plates comprising the North American continent and the Pacific basin. The ground motion induces a very large inertia force known as the seismic force in a structure that often results in the destruction of the structure. The seismic force acts vertically like dead and live loads and laterally like wind load. But unlike the other forces that are proportional to the exposed area of the structure, the seismic force is proportional to the mass of the structure and is distributed in proportion to the structural mass at various levels.