Journal Paper
Abstract
Tehran basin is located in the central Alborz seismic zone, a region with very high seismicity due to the existence of numerous large active faults. According to the model proposed by Engalenc (1968), for the Tehran sedimentary-geology, the Plio-Quaternary alluviums of Tehran consist in homogenous cemented conglomerates estimated up to 1000 m thick. In the city of Tehran, analysis of earthquakes recorded by a temporary seismological experiment (Haghshenas, 2005) has outlined a significant amplification of ground motion (up to 7-8) over a wide frequency range from about 0.4 Hz to 8 Hz. Haghshenas (2005) suggested that such amplification is due to the existence of thick alluvial deposits associated to a strong impedance contrast at large depth and also the presence of strong lateral discontinuities leading to multidimensional site effects. In order to better understand and predict the effects of the geometry and mechanical properties of the Tehran basin on surface ground motions, we developed a 3D shear-wave velocity model of Teheran’s basin by integrating all available geophysical, seismological and geological data. Geological data include 197 available geotechnical or geological logs within Tehran city. For geophysical data, it has been used 33 seismic ambient vibration arrays with aperture ranging from 100 m to 200 m and 13 active surface waves profiles in order to derive shear-wave velocity profiles. Finally, 884 single-station ambient vibration recordings (H/V data) have been also integrated. Interestingly, H/V method applied on seismic ambient noise was in most cases failing to provide fundamental resonance frequency of the site, most probably as a consequence of the low level of seismic ambient noise at low frequency (below 1 Hz). Shear-wave velocity down to the deep seismic bedrock were thus derived by joint inversion of dispersion curves, ellipticity of Rayleigh waves and fundamental resonance frequency. The final three-dimensional structure of the basin is then achieved by integrating the geological and geophysical information. This model outlines a heavy change in the bedrock depth ranges (from 89 m to 910 m) with strong lateral variation from north to south of the basin.
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