عنوان مقاله [English]
The shear wave velocity of soils is one of the most critical and influential factors affecting the nature of the seismic surface motion during a probable earthquake that is known by the term of seismic site effects. In addition to 1D shear wave velocity profile of soils the lateral heterogeneity of shear wave velocity contrast may affect the seismic ground motion at the surface strongly. It seems that such conditions exist in Tehran and especially at the eastern part of the city known as Hakimiyeh valley, in which alluvial deposits have been settled between two chains of Alborz and Sepayeh (AntiAlborz) mountains. In this paper we try to extracts the 2-D shear-wave velocity structure of subsurface layers in the form of two north-south and east-west profiles in this area using array and single-station measurements of ambient noises (microtremors). In this regard, ambient noises are recorded in four array stations and eleven single stations. A circular layout including two concentric circles is considered for array measurements with 11 three-component narrow-band seismometers Lennartz LE-3D/20S in each array. Recorded array seismograms are analyzed using spatial autocorrelation (SPAC) analysis, F-K analysis, and time-frequency analysis (TFA) of ellipticity in Geopsy software. Then, obtained results are used as objective functions in the inversion operation for calculating the shear-wave velocity profile at selected stations. Also, seismograms obtained from the single-station method are only analyzed using the TFA method, and extracted results are used as objective functions in the inversion process. The neighborhood algorithm is used in this study for inversion of objective functions and extraction of shear-wave velocity profiles. In the inversion step, the shear-wave velocity of surface layers at the microtremor measurement site is applied as the boundary condition. This study results in the extraction of two 2-D profiles and recognition of the shear-wave velocity structure in this area. Also, the velocity of 2200 m/s corresponds to the seismic bedrock. Velocity profiles indicated that variations of velocity and dynamic properties of the soil are significant in this area and it could be considered an area with complicated subsurface structure. Lavizan fault can be identified in shear-wave velocity profiles. The dominant frequency of sediments in the Hakimiyeh region, presented in previous analytical studies, have some discrepancies with the dominant frequency extracted from the measurement of ambient noises and 1D shear wave velocity profiles of the present experimental study. The results are in good agreement with the area geology.