Seismic base isolation is a technique developed to prevent or minimize the damages to the buildings during an earthquake.It works by separating the structure from ground and therefore the lateral force of earthquake.

A building directly fixed to the ground will move with an earthquake's motion and cause sustainable damage to the structure.When a building is separated from ground,resting on a flexible bearings or pads known as base isolators, it will move a little or not at all during the earthquake.

There are two types of base isolation, elastometric and sliding, both of which are designed to take the weight of structure and let the foundations  move sideways during the earthquake(Fig 1).

                                                       Fig 1. Base Isolated Structure

Flexible elastometric bearing consists of layers of bonded rubber and steel with a central lead core.The lead core softens under pressure absorbing energy that would otherwise be transferred to the building. With a sliding system there is no energy absorption, just deflection through the bearing.

During the earthquake the isolated structure vibrates like a rigid body with large deformation or displacement endured by isolated bearings. The lateral forces of building are not only reduced in magnitude but also fairly redistributed all over the floors, which further mitigates the overturning effect of the structure.

Fig 2. Behavior comparison of isolated building and conventional building during ground movement

The most commonly used base isolators in a building are 
1. Lead rubber(elastmetric) bearing (LRB)
2. High damping rubber bearing ( HDRB)
3.Friction Pendulum System (FPS)
4.Sliding bearing

                                  Fig 3. Typical Base Isolator used in a building