Seismic Ground Acceleration

A series of vibrations are created when seismic waves move through the ground. These movements are transmitted into dynamic loads or inertial forces that cause the ground and any thing attached to it (i.e., the built environment) to vibrate in a complex manner. These inertial forces cause damage to buildings and other structures. Inertial forces are created when an outside force tries to make an object move or change its rate of travel.

Acceleration is the rate of change of motion. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram. The nature of accelerogram may vary depending on
 

- Energy released at source

- Type of slip at fault surface

- Geology along the travel path from fault rupture to the earth surface

- Local soil




They carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content ( amplitude of shaking associated with each frequency) and energy content (i.e., energy carried by ground shaking at each frequency) are often used to distinguish them.

Normally acceleration is not associated with buildings since building is not expected to move. During an earthquake, however, inertial forces may cause the upper part of the building to sway while the foundation remain stationary, or they may cause whole building to “move”. Structures built in seismically active areas must be built to withstand predicted acceleration levels.