Estimation of subsurface structure using microtremor in Karaj city, Iran Nastaran EHSANI (1) , Mohammad Reza GHAEMGHAMIAN (1) , Mohsen FAZLAVI (2) and Ebrahim HAGHSHENAS (1) (1) International Institute of Earthquake Engineering and Seismology, Tehran, Iran E-mail: n.ehsani@iiees.ac.ir (2) Imam Khomeini International University, Qazvin, Iran Abstract In order to estimate the site effects in Karaj city, microtremor H/V spectral ratios were used in 37 locations of the city along the north-south-west profile. The results show that the dominant frequency changes in the range of 0.4 to 2 Hz. 1-D numerical simulation was performed using Deepsoil software based on the linear method at low strain levels and results of theoretical response were compared with fundamental frequencies obtained from microtremor measurements. The obtained modelling results are indicative of an effective contrast at depth of 200 to 300 meters and deep contrast at a depth of about 2 kilometers due to difference in bedrock type. Finally, using the results and according to the geological conditions of the region, subsurface structures in Karaj city were examined in two-dimension along the studied profiles. Keywords: Site effects, Microtremor, spectral ratio H/V, Dominant frequency, 1D site modelling 1. Introduction The earthquake is one of the most devastating natural disasters that always threats human societies in terms of health and financial issues. Iran is one of the most seismic prone countries of the world due to locating on Alpine- Himalayan Orogenic belt. On the other hand, growing population and increased construction of tall buildings, increases the damages caused by large earthquakes, especially in large cities. Karaj is one of the most populous cities in Iran which there has been considerable industrial and economic development in recent years. When an earthquake occurs, seismic waves radiate away from source and travel rapidly through the earth crust. When these waves reach the ground surface, they produce shaking that may last from several seconds to a few minutes. During earthquakes, different alluviums with different structures show various reactions. It is well-accepted that, besides the earthquake magnitude and fault distance, local geologic conditions, known as site effects, can also exert significant influences on characteristics of the seismic waves such as amplitude, frequency content and duration of strong ground motion at a given location (Kramer, 1996). The seismic ground motion at any site is influenced by the type of soil in that region. Younger and softer soils usually amplify ground motion more than older soils or bedrocks (Purnachandra Rao et al., 2011). There are theoretical and experimental methods to evaluate the site response. 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. In addition, a preliminary 1-D site response modelling has been conducted using Deepsoil program according to downhole, array and geology data. Site frequencies obtained from modelling are presented and compared with site frequencies obtained through microtremor measurements. 2. Geology and Seismicity of Karaj Karaj city is situated 20 km west of Tehran, at the foothills of the Alborz Mountains. It is the fourth largest and most populous city in Iran which has been developed on disaster-prone areas and tectonically active region according to the seismic hazard map of Iran. The tectonics of the Alborz Mountain is characterized by boundary conditions, due to convergence between Arabia and Eurasia, which probably began in the Cretaceous simultaneously with establishment of the Alpine-Himalayan orogeny. Andesitic and clastic rocks of the Karaj Formation 1 10th Asian Regional Conference of IAEG (2015)