Combining active and passive seismic methods for the characterization of urban sites in Cairo, Egypt

Abstract

The geology at Kottamiya, Rehab City and Zahraa-Madinat-Nasr to the East of Cairo (Egypt) is composed of low-velocity sediments on top of a rigid rock basement. Such sediments include the loose sands of the Gebel Ahmar formation, marl and shales of Maadi formation, in addition to sparse quaternary soil covers. Due to the contrast of the seismic impedance with the underlying bedrock, these soft sediments have the potential of considerably amplifying the ground motion during an earthquake. For the evaluation of site-specific seismic hazard, we computed the seismic site response in these areas by developing 1-D velocity models and derived average seismic velocities, including Vs 30 . To do that, we applied different active and passive source techniques, including the horizontal to vertical Fourier spectral ratio of ambient vibration recordings and multichannel analysis of artificially generated surface waves. A set of models representing the velocity structure of the site is then obtained by combined inversion of Rayleigh wave dispersion curves and ellipticity functions. While dispersion curves are used to constrain the uppermost low-velocity part of the soil profile, ellipticity helps in resolving the structure at the depth of the sediment-bedrock interface. From the retrieved velocity models, numerical ground-motion amplification is finally derived using 1-D SH-wave transfer function. We account for uncertainty in amplification by using a statistical model that accounts for the misfit of all the inverted velocity profiles. The study reveals that the different sites experience an important frequency-dependent amplification, with largest amplification occurring at the resonance frequencies of the sites. Amplification up to a factor of 5 is found, with some variability depending on the soil type (Vs 30 ranges between 340 and 415 m s −2 ). Moreover, amplification is expected in the frequency range that is important for buildings (0.8-10 Hz), which is additional confirmation for the need of microzonation analysis of the area. The obtained results will be used for the development of a new seismic hazard model.

Hydropower Dam State and Its Foundation Soil Survey Using Industrial Seismic Oscillations

“…Therefore, to determine the sources of damage, it is important to survey both the structure's state and its foundation soil condition. For these purposes, both active and passive seismic control methods and ambient vibration testing along with their complexing can be implemented [3][4][5][6][7][8][9].…”

Probabilistic seismic hazard model for Cairo, Egypt: estimates and uncertainties

“…The PGA is in the range 152-159 cm/s 2 at Kottamiya, 148-162 cm/s 2 at Rehab City and around 153 cm/s 2 at Zahraa. The site response due to dipping sediment-bedrock at Zahraa (Adly et al 2017) is not reflected on estimated homogeneous hazard.…”

“…Compared to the uncertainty, these differences are insignificant. Similarly, at Rehab City, the variability in PGA is related to the variability in the thickness of loose quaternary sediments above Hagul formation (Adly et al 2017).…”

“…Dahshour and Cairo Suez district sources are treated as a single area source in contrast to previous studies (e.g. Adly 2010;Mohamed et al 2012;Ezzelarab et al 2016;Sawires et al 2016). This is done because they both hosted some foreshocks and aftershocks of the 12th Oct. 1992 earthquake.…”

“…The developed seismic hazard model is applied in a site-specific hazard analysis for Kottamiya, Rehab City and Zahraa, to the East of Cairo, where the site response was evaluated (Adly et al 2017). Here we use the Vs30 parameter to introduce the impact of the soil conditions on the hazard levels.…”