ABSTRACT
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, which is known as an earthquake. If ground movement could take place slowly, the structure would ride over it smoothly, moving along with it. But the quick movement of ground in an earthquake accelerates the mass of the structure. The basic dynamic property of a structure is its fundamental period of vibration. When a mass of body is given a horizontal displacement, the mass oscillates back and forth. This is termed the free vibration. Two terms are applied to consider the most severe earthquake effects: maximum considered earthquake geometric mean and risk-targeted maximum considered earthquake.
