In order to seismic microzonation of Urmai city microtremors were measured at 220 stations during a comprehensive study. Measurements were conducted with two different approaches, those are with-reference and non-reference methods. Different aspects of site effect problem were studied by using the recorded data. Two so called Hs/Hr and H/V methods were examined in this study. Results showed that Hs/Hr method failed to give clear resonance peak at predominant frequency of the site when the distance between the site and reference station was more than 2500 meter. Whereas FL’V method successfully indicated the predominant frequency at any distances. The distance of 2500 meter is introduced as the threshold distance for validity of Hs/Hr method.
Suitability of microtremor method for detection of topography effect was also examined. For this purpose the measured data at 5 stations located at Shahrake Electronic hill, south-east of Urmia city were utilized. Two horizontal components of measured microtremors were compared to the vertical one. Results showed that at the stations located at the top portion of the hill there was significant amplification whereas for the stations located at the hill foot the amplification was negligible. This means the microtremor method could successfully distinguish the topography effect. Moreover, the results showed that for selection of a suitable rock station the microtremors recorded on a rock outcropping should be free of any topography effect; otherwise Hs/Hr method could not show reliable results.
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