Estimating shear-waves velocity structure by combining array methods and inversion of ellipticity curves at a site in south of Tehran



Tehran, the capital of Iran, is under the threat of large magnitude earthquakes (above 7) located on known active faults. Previous studies on the effect of local surface geology on earthquake ground motion using 1D calculation of SH transfer function (Jafari et al., 2001; JICA & CEST, 2000) and 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 overlying very rigid bedrock.
Different methods including microtremors array (FK and SPAC) and H/V calculated by TFA techniques, joint inversion of dispersion curves and ellipticty curves and finally SH transfer function were used to constrain the shear-wave velocity and the bedrock depth at a site exhibiting high ground motion amplification at low frequencies in the south of Tehran. Results show that using the array data alone (arrays with a limited aperture of 100 meters) can only provide Vs profile for the superficial layers. Combining the array methods and single station measurement can give deeper and better constrain on shear-wave velocity models. Knowing the range of fundamental resonance frequencies from earthquake data gives us the opportunity to filter the inverted Vs models obtained from joint inversion of dispersion curve and H/V ellipticity curve, by applying SH transfer functions calculated for various inverted Vs profile and comparing the subsequent resonance frequency to the actual one.
In this paper, array processing by using MSPAC (Bettig et al., 2001) and FK techniques were performed using Sesarray software package (Wathelet et al., 2008). Results from MSPAC analysis are not shown here since it did not add more significant results to the dispersion curve than the one derived from FK analysis. Inversion was performed using the Conditional Neighborhood Algorithm (Wathelet, 2008). Although inverted shear wave velocity profiles fit well the borehole Vs measurements (Figure 3a, black line), they do not provide any constrain of Vs for depths deeper than 100 meters. This is explained by the limited array aperture resulting in phase velocity estimates ranging between 6 and 10 Hz
Applying this procedure gives estimate about the shear-wave velocity and the bedrock depth in the south of Tehran which may lie between 700 to 1200 meters. This procedure should be applied to different sites in Tehran in order to retrieve the Vs profile and the spatial variation of sediment-to-bedrock depth throughout Tehran. This will then allow us to better understand observed site amplification.