In fact a real earthquake ground motion at a particular site is much more complicated that than the simple waveform depicted. It would be beneficial if we make comparison between the surface of the ground under the impact of an earthquake and the surface of the water in a pond. We can bring the surface of a pond in motion by just throwing few stones into it. The first few stones generate a series of circular waves which begin to collide with each other and  after some time these collisions,  which of circular waves which begin to collide with each other and after some time these collisions, which are called interference patterns, begin to dominate over the circular pattern of waves. After a short while, the whole surface of the water is full of ripples, and we cannot recognize the original waves. In the event of an earthquake also, the ground vibrates in a similarly complex manner due to interaction among the waves of different of different amplitudes and frequencies.

The complex nature of earthquake ground motion can be attributed to the following three factors:

1.      The seismic waves created during the fault movement are not all of a uniform character.

2.      During their movement through the earth from the fault to the site of any building, the waves get modified by the rock and soil media through which they pass.

3.      Moreover, when these waves reach the building site they are further modified de pending upon the characteristics of the ground and the soil beneath the building.

These three factors are known as source effects, path effects and site effects.