Site Effect Evaluation in Basin Edge Using Microtremor Data and Numerical Modeling

Despite extensive studies on the microtremor processing technics and horizontal to vertical (H/V) spectral ratio analysis, limited studies have been tried to interpret the observations and experimental results and confirming them via two and three-dimensional numerical modeling. The present article deals with this issue by selecting the Urmia alluvial basin. Previous seismic microzonation studies in this city show a broad peak or two neighboring peaks, close to each other at some points near to the edge of sedimentary basin. By examining the transfer functions obtained from one-dimensional and two-dimensional numerical analysis performed on an east-west section of the Urmia sedimentary basin, it was found that this issue could be due to a sudden change in thickness of the soil profiles on either side of measuring point and coupling of shear waves passing through these two different environments. This study shows a very good agreement between the frequency position of the two neighboring peaks between the H/V curve and two-dimensional transfer function. In addition, this study shows that when two-dimensional site effect is predominant, the resonance frequencies are higher compared to one-dimensional conditions. This study also suggests that for recognition of the real peaks in H/V curves, the variation trend of dominant frequencies between the different measuring points in study area should be considered and each curve should not be evaluated individually.