Site effect evaluation using microtremor measurements in the city of Shiraz

Natural frequency of soil is an important factor in site effect studies. In order to determine this parameter, microtremor measurements were applied as a fast, simple and economic tool. For this purpose, microtremors were recorded in 74 points with time durations of 5 to 10 minutes throughout the city of Shiraz. Natural frequency and amplification factor of soil were then estimated considering the main peak in spectral ratio between horizontal and vertical components (H/V) that is also known as Nakamura’s technique. Fourier and power spectrums were also used to determine this spectral ratio. The results demonstrate that natural frequency varies from 0.5HZ to 6HZ in different parts of the city, so that it increases in the east and north part of the city where alluvial deposits over the bedrock are thin. Also, amplification factor of the alluvial deposits obtained by this method has values between 1.3 and 5 throughout the city. In order to investigate time-dependence of the natural frequency, after 8 years of data recording new data were recorded in five stations. The results indicate acceptable correlation and consistency between old and new data results in these five stations.