Site effects estimation using experimental and numerical methods in Karaj city

In the present study, the Nakamura's H/V spectral ratio method has been used to evaluate the resonance frequency in 37 locations at Karaj site along the north-southwest profile. Calculated dominant frequency changes between 0.4 and 2 Hz. The results of H/V spectral ratio are affected by the local geologic structure. Based on this assumption, we can produce theoretical H/V curve with knowledge of the geologic structure in the area. Therefore, one-dimensional modelling was carried out using Deepsoil software according to downhole, array and geology data. Due to the very small deformations in soils by microtremor and producing a low levels of strain, we applied the linear method to evaluate the ground seismic response during mild earthquake shakes. The results of this modeling shows an effective contrast at depths of 200 to 300 meters. It seems that due to the geology of the region, tuff- andesite of the Karaj Formation as basement plays an important role in the creation of low-frequency peaks. Therefore, to obtain a better model, deep contrast was considered about 2 kilometers due to differences in the type of bedrock with a shear wave velocity of 2500 m/s. For this purpose, according to the properties of the Upper Red Formation, an average constant speed of 1400 (m/s) was considered in modelling and by changing the thickness of this layer, the modelling was continued in a trial and error manner until the numerical model is consistent with microtremor peaks. The modelling results in nine site indicate that there is basement at the depth of 2000 to 2250 meters.