- © 2009 by the Seismological Society of America
Clear evidence of seismic damage due to local geology was first observed in Japan, during the 1923 Kanto earthquake. Several recent earthquakes (e.g., Michoacan, Mexico, 1985; Dahshore, Egypt, 1992; Northridge, U.S.A., 1994; Kobe, Japan, 1995, and Athens, Greece, 1999) clearly indicate that understanding local site effects is one of the major challenges of seismic risk mitigation. Several techniques are employed to compute the site response for soft deposits (i.e., resonance frequencies and amplification factor). Measuring microtremors based on the analysis of ambient noise has been regarded as one of the preferred approaches in estimating site responses, particularly in highly populated urbanized areas because the data collection is portable and non-invasive and the data analysis is straight-forward and easy to do. This method measures the spectral ratio of the horizontal to vertical (H/V) component of ambient noise. The prominent peak of the spectral amplitude ratio corresponds to the fundamental frequency of the site under investigation (Nogoshi and Igarashi 1971; Nakamura 1989). Many researchers have studied the reliability of this method (e.g., Duval et al. 1995; Field and Jacob 1995; Teves-Costa et al. 1996; Lachet et al. 1996; Bour et al. 1998; Bindi et al. 2000; LeBrun et al. 2001) and concluded that the ambient noise H/V spectral ratios provide reliable estimates of the fundamental frequencies of soil deposits.
Our study is concerned with the Nile delta, which at present covers an onshore area of about 25,000 km2 and an equal offshore area extending to the 200-m isobath (Schlumberger 1984, 1987). The southern apex of the delta is at 30° N, approximately 30 km north of Cairo, where the Nile River splits into the eastern branch (Damietta) and the western branch (Rosetta). The Nile delta is a rapidly growing population …