Evaluation of local site effect in the western side of the Suez Canal area by applying H/V and MASW techniques
Introduction
The Suez Canal is located in the northeastern part of Egypt. It is the most commercially used, and the longest excavated waterway in the world (Fig. 1) rivaled only by the Panama Canal in Panama. The Suez Canal is an inland coastal-ocean saltwater body that links the Red Sea in the south with the Mediterranean Sea in the north. It mostly transmits a desert environment and is bound by sand banks, except the northern 60 km where extensive wetlands bordered it. The canal conditions such as poor construction quality and the presence of the soft soil conditions may increase the damage ratio in a scenario of major earthquake occurrence in the study area. Therefore, the construction of important buildings in the Suez Canal corridor needs suitable construction methods for these geotechnical conditions. In regions that have low seismicity, it would be necessary to wait for a long time to get a complete data set. Therefore, due to this reason, the use of ambient noise is becoming popular as an excellent alternative technique for determining the fundamental frequencies of the soil and corresponding H/V amplitude (Bard, 1998).
The study area is affected by moderate seismic activities due to its closeness to the junction of the Red Sea and the movement along the Gulf of Suez and the Gulf of Aqaba-Dead Sea transform fault system. The area is also affected by the Suez-Cairo Shear Zone (wadi Hagul Seismic Source); the seismicity emanates from northeast Cairo (Abu-Zaabal area) due to the relative motion between three plates (Eurasia, Africa, and Arabian plates). This area has experienced damage from local earthquakes between 1955 and 2007 (e.g., the Mediterranean offshore 1955 Alexandria earthquake, Ms = 6.8, the 1969 Shadwan earthquake, Mw = 6.9; the 1995 Gulf of Aqaba earthquake, Mw = 7.2; the 1996 Cyprus earthquake, Mw = 6.8, and the 2007 Wadi Hagul earthquake with ML = 3.8.
In general, the softness of the surface layer not only amplifies ground motion at certain frequencies but also extends the duration, which may cause further damage during earthquakes. The identification of the fundamental frequencies of soil deposits is gaining increasing importance for seismic site effect assessment because it carries implicit information about the bedrock depth and shear wave velocity of the soils, which is related to soil shear stiffness. Site amplification is considerably an effect on the building response, especially in the near field and large scale earthquakes (Polat et al., 2009). According to the frequency range in nature; the natural frequencies of high storey buildings (3–10 floors) range from 1 to 3 Hz, while the natural frequencies for low storey buildings (1–3 floors) range from 3 to 10 Hz, and the natural frequency of very high storey buildings (10–50 floors) range from 0.1 to 1 Hz (Mandal et al., 2005).
The MASW is a newly method. It was developed by Miller et al. (1999) and Xia et al. (2000) to estimate the shear wave velocity profile (i.e., Vs versus depth) from the surface wave energy by analyzing Rayleigh-type surface waves on multichannel recordings. The surface wave measurements were carried out around the area and processed using the 1D multichannel analysis of surface waves to get the shear wave velocity profiles through two main steps; the first is the calculation of the dispersion curves and the second is the estimation of the shear wave velocity profiles from the inversion of these dispersion curves. The shear wave velocity VS30 in the upper 30 m of sediments is a very significant parameter to evaluate near-surface stiffness and for characterizing the given site (Stokoe et al., 1994). This parameter is important to derive the resistant structural design for the earthquakes (Kanai, 1983).
The National Earthquake Hazard Reduction Program (NEHRP, 2003) provides a classification of the given sites based on VS30. This code was developed after strong earthquakes occurred in the USA, Japan, Europe and in other countries. In this study, the effects of local site conditions were determined by conducting HVSR measurements along the Suez Canal. HVSR peak frequencies of the soil and HVSR peak amplitude values were calculated, and the site Classification is determined based on VS30.
Section snippets
Geologic and tectonic setting
The study area and its surroundings occupies a semi-flat terrain from the north to the south, except the area to the south where topography rises defining the southern mountains as in Gabal Geniefa, which rises on average to 234 m above sea level and Gabal Shabraweet, which rises on average to 226 m above sea level. Many wadis drainage channels in the study area are filled by alluvial deposits. The most important wadis are; Wadi El Ashara, Wadi Yasara, Wadi Al Agrama, and Wadi El Tumilat (
Seismicity
The study area is characterized by low seismic activity (Fig. 3) except the surrounding regions that have moderate seismic activity due to the interactions between African, Arabian, and Eurasian plates and the Sinai subplate (Fig. 4). These plates are provide a tectonic framework and show the current plate movements and crustal deformation. So, the study area can be affected by different seismic sources; e.g. Gulf of Suez, Gulf of Aqaba, North Red Sea, Cairo-Suez district Shear Zone and Nile
Data acquisition and instrumentation
The ambient noise measurements were carried out at 61 sites along Suez Canal area in three cities: Suez, Ismailia, and Port-Said. The duration of recording ranging from 45 to 60 min except some sites has a recording of 100 min due to the strongly disturbed environment (dense and close to car traffic, pedestrians, etc.). These measurements carried out using a single station consists of high sensitivity seismometer (Trillium 120 compact), equipped with three-component digital seismograph data
The horizontal-to-vertical spectral ratio method
Ambient vibration recorded signals were processed using (H/V) method or Nakamura's technique (Nakamura, 1989). The software used for data processing is the GEOPSY Software (version 2.7.1, geopsypack-2.2.1) (http://www.geopsy.org). The data was converted from the Nanometrics format to (SESAME ASCII data format). The SESAME consortium prepares the Geopsy software package to calculate the H/V amplitude ratios, after required waveform operation such as offset removal and bandpass filter were done,
Results and discussion
HVSR curves were computed as the average of both horizontal component spectra divided by the vertical spectrum for each window. The total measurements of H/V were conducted in different sites indicate that frequency peaks of an industrial origin not identified. The MASW method emphasized the minimization of near field and far-offset effects, acquisition speed, sampling redundancy, and overall data accuracy. The present study concludes several important data listed in (Table 5) such as follows:
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Conclusion
The Suez Canal is an inland coastal-ocean connected saltwater body that links the Mediterranean Sea in the north with the Red Sea in the south. It is surrounding by different seismic source zones. In this study, the fundamental frequency (fo) values are ranging from 0.57 to 1.08, 0.64–0.91, and 0.6–0.97 Hz, while the corresponding amplitude (Ao) values up to 4.05, 4.4 and 6.46 at Suez, Ismailia, and Port Said respectively. These values of frequency fall in the natural frequency range of
Acknowledgements
The authors are very grateful thanks to the National Research Institute of Astronomy and Geophysics, Egypt, for the financial support. We thank also the staff of the Egyptian National Seismic Network for facilitating our works in the field. The authors are grateful thanks to the anonymous reviewers for their constructive comments.
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