Deep alluvial effects on one-dimensional seismic site response in Qom city

Nowadays in many site response analyses of alluvial، environment with shear wave velocity 600 m/sec < νs < 800 m/sec is considered as the seismic bedrock. Results of these analyses did not confirm those from empirical analyses based on the recording of microtremors or weak earthquake motions. Recently، effects of deep alluviums and contrast between geologic bedrock and alluviums are considered as a possible cause for this inconformity. This study examines the site effects of deep alluviums of Qom city. Qom is located at the northern margin of central Iran zone and on the Quaternary young alluviums. Based on the geoelectrical resistivity surveys، thickness of alluviums in some parts of the city is greater than 250m. Earlier empirical studies based on the recording of microtremors had shown that at the frequencies ranging from 0. 6 to 1. 2 Hz، a clear amplification can be seen in the studied alluviums. Amplification at these frequencies did not confirm results from one-dimensional numerical analysis of the soft sediments on the conventional seismic bedrock. At the current study، in order to determine the geometrical properties and thickness of deep alluviums، resistivity surveys have been conducted in Qom plain was used and dynamic properties of the soil layers were determined by geological descriptions. Considering the uncertainty resulting from the lack of dynamic properties of soil layers، a parametric study was conducted and three models of low، medium and high velocity were considered for alluvial layers and bedrock. One-dimensional numerical analysis was carried out using the software Deepsoil. As the results will be compared with those from empirical analysis of small strain displacements of microtremors، a linear elastic behaviour was assumed. Amplification curves were measured using different dynamic properties of alluvial layers and the bedrock and the results were compared with those from amplification of microtremors. In all analyses conducted using different dynamical properties (three models of low، medium and high velocity)، a specific amplification at the frequencies less than 1 Hz was obtained. Furthermore، amplified frequency resulted from previous empirical studies corresponds with the amplified frequency resulted from numerical analysis with high velocity model. Amplification at this frequency range and its correspondence with results of microtremors studies shows the effects of deep alluviums on the site amplification functions. The importance of deep alluviums insists on attention to the shape of the sedimentary basin and consideration of the effects of deep alluviums on the numerical site effects studies. In the case of inadequate information about the deep alluviums، it is necessary to use empirical analysis recorded motion at the site such as microtremors or weak earthquake motions. At present، consideration of site effects in most of building codes for design earthquake resistant structures are limited to effects of shallow alluviums، however، as discussed in this paper، deep alluviums are effective on site amplification specially in low frequencies and it is necessary to take them into account in the design of tall structures.