SUMMARY
The experience of previous earthquakes in the world shows that the structural damages due to the earthquakes are highly influenced by the site condition, which is known as site effects. Since most cities are built on alluviums, studying the seismic response of alluvial basin which are the site of many structures is very important. This study investigates the seismic response of 2D trapezoidal alluvial valleys with slope angles of 31?, 45?, and 71.5?, against P and SV vertically propagating incident waves in middle and lateral areas of the valleys. Then the effect of changes in Poisson’s ratio and specific weight of alluvial materials on the seismic response of the valley in the mentioned areas are investigated. Numerical modeling is done using QUAKE/W finite element software, based on the equivalent linear analysis. According to the results, by increasing the slope angle of the valley, the vertical acceleration and displacement response spectrum in the middle and lateral areas of the alluvial valley decreases and increases respectively. Moreover, by increasing the Poisson’s ratio and specific weight of alluvial materials, the vertical acceleration and displacement response spectrum in the middle area of the valley decreases, but does not undergo tangible changes in the lateral areas of the valley.