عنوان مقاله [English]
Alluvium responses to earthquake motion interference to the damage creation of structure is illustrated in different ways. One of the most important casualty effect that we can refer to is resonance phenomenon. This occurs when the natural period of a Alluvium is equal to the natural period of bedrock. Under these conditions the effect of amplification in ground motion acceleration is maximum.
In this research the microtremor measurements for determination of the natural period of a Alluvium, performed in the NS direction profile in north-western Tehran at 6 stations one of which is located on a bedrock outcrop. Interpretation of data based on the amplitude spectrum, was performed by two methods. In the first method called the bedrock reference station, the average amplitude spectrum of the horizontal components and the vertical component of the station on Alluvium is divided by the average amplitude spectrum of the horizontal and vertical components of the station on the bedrock. In the second method, the Nakamura method, the average amplitude spectrum of the horizontal components are divided by the amplitude spectrum of the vertical component of the same station.
The processing and interpretation of field data show that the bedrock reference station method for calculation of a Alluvium resonance frequency has a better validity than the Nakamura method. Resonance frequency in this area is between 2.4 to 5.5 Hz which reduces from north to south. So the natural period of Alluvium in the profile increases from north to south, which agrees with the increase in thickness of alluvial deposits in the area.
Therefore, microtremor measurements based upon the bedrock reference station method from an economical aspect and to minimize time, are suggested for the determination of the Alluvium natural period.