The Importance of Soil and Alluvial and Quaternary Deposits Conditions in Seismic Risk Analysis

The land of Iran is a seismic active prone that is located in the mountainous and instable crust due to special geological position. Continuous tectonic stress from the Arabic and Indian plates has exposed seismic active faults. The huge and destructive earthquakes that occur every few years in this area reflect the tectonic conditions of the plateau of Iran. Since, the path of faults and fractures, is the proper zone of transmission of groundwater in the semi-arid Iran area, on this basis, our ancestors, have formed ,the first nucleus of population centers, along these springs, or, in other words, along the faults and fractures. Given this fact, in the current situation, many metropolitan cities and towns and population centers of the country are in the vicinity of faults. A look at the distribution map of earthquakes and the map of distribution of population centers, indicates a significant match between these two maps, which has led to higher earthquake damage in Iran because of and based on the statistics of international centers, Iran, is located in very high risk countries group. It is obvious that the occurrence of earthquakes will be continuous and permanent on the Iranian plateau and therefore, considering this fact, seismic risk reduction programs in the country should be followed seriously. The study of various earthquake damage in the urban and rural area of the country shows that the role of soil conditions and its effect on ground motion amplification is very significant, which directly increased the earthquake destruction. The latest experience in Iran is the magnificent Sarplul Zahab earthquake in Kermanshah. In some areas, the soil effect and soil condition on the building damages in the area is clearly visible. In national scale, it seems that geotechnical boreholes are not sufficient to study soil conditions in urban areas and population centers, and in this area, as in most countries of the world we face a shortage of geotechnical data. Studying and zoning the recent alluvium may be a suitable approach for assessing the high seismic risk areas. Therefore, it is suggested that, with the use of geotechnical data, comprehensive studies on the elastic conditions of alluvial and quaternary deposits at the country level should be undertaken and, based on their age and other characteristics, an estimate of their degree of softness or hardness can be obtained. This can be useful in regional and national level seismic risk analysis.