Effects of surface geology on the ground-motion at New Borg El-Arab City, Alexandria, Northern Egypt

The effects of the near-surface geology on the ground-motion at New Borg El-Arab City were evaluated in the current work based on the analysis of the ambient noise records (microtremor). Sixty-nine microtremor measurements have been done in the studied area. The dataset was processed using horizontal-to-vertical-spectral ratio (HVSR) technique to estimate the fundamental frequencies corresponding to the ground-motion amplification due to the soil deposits. By spatial interpolation of the resulted fundamental frequencies (f0) of all the measured sites, the zonation map was produced. This map was correlated with the geological features of the study area and demonstrated that the fundamental frequency ranges between 5.8 Hz and 7 Hz were corresponding to the sites located over Quaternary deposit. However, the fundamental frequencies (f0) increased in the middle of the study area due to presence of parallel Alexandria limestone ridge. Finally, site effect was highlighted by performing a site response analysis. It indicated that, the PGA at surface of the analyzed site is 0.047 g and the maximum spectral acceleration (SA) is 0.157 g. It was also found that, the maximum spectral period from site response analysis is in a good agreement with that one from HVSR technique. This confirmed the robustness of HVSR for determination of fundamental period or frequency.