The fast extension and development of cities in areas prone to high earthquake risk increases their vulnerabilities to suffer unacceptable damages. Earthquake microzonation mapping could be a useful approach integrating seismology, geology, built environment, and the socio-economic needs of the community. It is suggested that such an approach, in terms of the earthquake hazard and consequent future mitigation related to land use planning, would be useful to developing projects in the region.
The fact that the site effect is a major control to earthquake damage in the built environment is recently being recognized. Although the site effect is controlled by many factors, but two most important ones are the topographic conditions and the soil conditions. Qazvin City is laid upon a thick deposits of alluvial plain in which the gradation of the soil particles decreases towards the south. Since the plain almost has a flat topography, only the effect of soil condition was considered in this study.
To investigate the effect of the soil condition on seismological response, a number of 22 represented profiles were selected in different parts of the city. The profiles were analyzed on the base of three level of acceleration 0.23g, 0.28, and 0.35g, using a computer program "SHAKE" which is based on the theory of vertical propagation of shear waves. The results were presented in the form of elastic and dynamic periods of alluvial deposits, peak ground acceleration, velocity, displacement, and amplification of ground vibration.
The results showed that the elastic and dynamic periods and the amplification of accelerations of each profile increases as the thickness and the looseness of the alluvial deposits increase. Different seismic microzonation maps were prepared with regard to elastic and dynamic poriods, peak particles acceleration and coefficient of ground surface amplification.