IJST, Transactions of Civil Engineering, Vol. 39, No. C2, pp 319-332 Printed in The Islamic Republic of Iran, 2015 © Shiraz University EXPERIMENTAL EVALUATION OF SEISMIC SITE RESPONSE OVER AND NEARBY UNDERGROUND CAVITIES (STUDY OF SUBWAY TUNNEL IN CITY OF KARAJ, IRAN) * M. J. KAZEMEINI 1 , E. HAGHSHENAS 2** AND M. KAMALIAN 3 1 Dept. of Civil Engineering, Science and Research Branch, Islamic Azad University, Tehran, I. R. of Iran 2, 3 International Institute of Earthquake Engineeing and Seismology, I. R. of Iran Email: haghshen@iiees.ac.ir Abstract– The effect of underground cavities in site seismic response was studied over the subway tunnels that are under construction in the city of Karaj, using ambient noise measurements as well as numerical modeling. The idea for this research comes from the observation of differences in experimental site transfer function, calculated on more than 100 locations of the city, for the areas near the under construction subway tunnel, compared with other parts of the city. In the present study a series of multi stations ambient noise measurements at 11 test sites across the tunnel were performed to evaluate the effect of the tunnel on seismic site response. This paper shows the results of these measurements as well as the result of a numerical modeling for one of the locations. The results show that the site effect in areas near the tunnel are affected by tunnel and vary based on dimension of excavation and distance from tunnel axis. Keywords– Seismic site response, subway tunnels, Karaj, Iran 1. INTRODUCTION The effect of surface and subsurface irregularities is one of the well known local site effects that may affect the ground surface motion and numerous studies have been developed on the effect induced by valleys and topographies. Despite these large numbers of studies, understanding and quantifying the effects of underground irregularities, such as buried cavities or tunnels, is still at a research stage. Similarly, the identification and characterization of either natural or artificial subsurface obstacles, such as cavities or petroleum reservoirs, constitute a challenging issue for geophysical subsurface investigations [1]. On the other hand, almost all of the researches that have been previously carried out in this topic are of analytical calculation or numerical modelling type [1-3]. There have been only a few experimental efforts in characterizing the effect of the subsurface cavities on seismic ground motion by Lombardo and Rigano [4, 5] and Sgarlato et al [6]. The present paper deals with evaluation of local seismic response related to the presence of subway tunnels in the city of Karaj (Iran) in the framework of a comprehensive research project of the local seismic hazard studies in this town. This city, with a population near to 3 million is situated 25 km west of Tehran and adjacent to some active faults, capable of creating an earthquake with magnitude greater than 7. A subway network, consisting of 5 lines with a total length of 72 Km (Fig. 1) is under construction by the municipality of the city that covers considerable area of the city and can affect the seismic site  Received by the editors March 2, 2014; Accepted August 23, 2014.  Corresponding author