Site characterization in Ismailia, Egypt using seismic ambient vibration array

Abstract Seismic ambient vibration array measurements were carried out at seven sites in Ismailia, Egypt in order to obtain one-dimensional (1D) shear wave velocity (Vs) profiles and horizontal-to-vertical spectral ratios (HVSR). We applied the spatial autocorrelation (SPAC) method to the ambient vibration data for the estimating Rayleigh-wave dispersion curves and obtained 1D Vs profiles. The deeper parts (90–350 m) of the estimated models have Vs values ranging between 500 and 700 m/s, while the subsurface parts have Vs varying between 140 and 200 m/s. The averaged Vs in the upper 30 m based on the results indicate that site classes of all the sites are classified as C or D according to NEHRP soil classification. The computed site amplification factors derived from the estimated Vs profiles show that the resonant frequencies at sites on the east side of the Suez Canal are below 1 Hz, while those at sites on the west side vary between 1 and 2 Hz, indicating the difference of subsurface geology across the Suez Canal. The HVSR curves at each site are generally similar for all sensors, indicating nearly flat subsurface layers beneath each site. We also compared the dominant frequencies of HVSRs and those derived from 1D amplification factors. The comparisons show that both the frequencies are similar to each other, but there are slight differences possibly due to the lack of deeper sedimentary layer structures and lower resolution of HVSRs in the lower frequency range, indicating the importance of further ambient vibration survey with broadband sensors. The results show that the combination of SPAC and HVSR methods is advantageous to estimate proper Vs profiles to evaluate site effects for earthquake disaster mitigation.