Estimating Shear-Waves Velocity Structure by Using Array Methods (FK and SPAC) and Inversion of Ellipticity Curves at a Site in South of Tehran

Abstract eng:
Tehran, capital of Iran, is under the threat of large magnitude earthquakes (above 7) located on very near faults. Previous studies on the effect of local surface geology on earthquake ground motion by using 1D calculation of SH transfer function (Jafari et al, 2001; JICA & CEST, 2000) and by using experimental methods (Haghshenas, 2005) based on earthquake and ambient noise vibration recordings resulted in very different and unexpected results. By assuming a layer with Vs = 700 m/s as seismic bedrock, 1D SH transfer functions show indeed a weak amplification for frequencies above 2 Hz, while the site-to-reference spectral ratios exhibit a significant amplification (up to 8) within a large frequency band from 0.3 to 8 Hz. Such discrepancy might be explained by very thick and stiff sedimentary layers overlaying very rigid bedrock. In order to better constrain shear-wave velocities at depth, we thus derived dispersion curves of Rayleigh waves by using SPAC and FK array methods and extracted Rayleigh waves ellipticity curves by using a recently proposed method based on the wavelet decomposition of the noise wave field. Dispersion and ellipticity curves are then simultaneously inverted to get the shear wave velocities. In this paper, we show results obtained at a site in the southern part of the city that exhibit rather stiff sedimentary Vs structure overlaying a bedrock whose depth is within 700 and 1200 meters.

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