Microtremor HVSR Study of Site Effects in Zanjan City (Iran)

Natural frequency of soils is one of the important factors in the study of vulnerability to earthquakes. In areas characterized by soft sediments, the maximum amplitude of ground motion is common that leads to enhanced seismic hazard and risk. Zanjan city is located in northwest of Iran with high risk of earthquake hazard according to Building and Housing Research Center (BHRC). So, investigation of site effect, beside other parameters, in earthquake vulnerability is considerable. To map natural frequency of soil in Zanjan, microtremor horizontal-to-vertical spectral ratio (HVSR) method has been conducted. Specially, we used Nakamura’s method on ambient noise records. We employed 3-component medium band Guralp seismometer. Ambient noise was recorded in 41 sites in a pre-designed profile. At each site, noise was recorded for at least one hour long. Geopsy software was used for data processing. Evaluation of the gathered data was examined following the recommended guidelines of SESAME (Site EffectS assessment using AMbient Excitations) project. The results of this study represent that there is one amplification peak in most stations. Considering the first peaks, the natural frequency of soil decreases from north to south in the city. The decrease of natural frequency represents an increase in the thickness of soil layer. In some sites, in west and center of the city, parallel to the Zanjanrud river, there are two peaks in spectral ratio. The second peak was always more than 3 Hz. The second peak is related to a shallow thin and low velocity sedimentary layer. To evaluation of results, the data for standard penetration test of boreholes was collected and shear wave velocity was estimated. Using the shear wave velocity obtained from the boreholes, we estimated the thickness of soil from the measured natural frequency of soils employing quarter wavelength law. The estimated thickness of soil shows the presence of a thin sedimentary layer with high velocity in the north of Zanjan. The bedrock slope becomes steeper by moving towards south and flattens within the basin. Inside the basin, the natural frequency is less than 1 Hz and the thickness of the sediments increases to about hundreds of meters. In general, the results of this study investigate one step for seismic hazard assessment and risk qualification of this urban area where great damages can be attained in case of strong earthquakes. Hence, these results should be taken into consideration before establishing the new urban constructions in the area of study.