Introduction: The earthquake is one of the most devastating natural disasters that always threats human societies in terms of health and financial issues. Iran is one of the most seismic prone countries of the world due to locating on Alpine-Himalayan Orogenic belt. On the other hand, growing population and increased construction of tall buildings, increases the damages caused by large earthquakes, especially in large cities. Karaj is one of the most populous cities in Iran which there has been considerable industrial and economic development in recent years. When an earthquake occurs, seismic waves radiate away from source and travel rapidly through the earth crust. When these waves reach the ground surface, they produce shaking that may last from several seconds to a few minutes. During earthquakes, different alluviums with different structures show various reactions. It is wellaccepted that, besides the earthquake magnitude and fault distance, local geologic conditions, known as site effects, can also exert significant influences on characteristics of the seismic waves such as amplitude, frequency content and duration of strong ground motion at a given location. The seismic ground motion at any site is influenced by the type of soil in that region. Younger and softer soils usually amplify ground motion more than older soils or bedrocks....