ESTIMATION OF THE NEAR-SURFACE ATTENUATION PARAMETER OF THE SEISMIC MEDIUM AT THE NORTHERN PART OF EGYPT

Abstract

The northern part of Egypt is an area of moderate to high seismic activity. The seismicity regime corresponds to a complex tectonic framework. A wide variety of sedimentary rocks are present along the main fault systems in the northern part of Egypt. The complex geological and seismo-tectonic settings are basic elements to consider, regarding the attenuation of propagation of seismic waves. The high-frequency ground motions are controlled by the earthquake source spectrum, its attenuation with distance and their local site effects. Fractal dimensions (D) and Near-Surface Attenuation Parameter (kappa) are estimated at different distances and through selected stations in the study area. In the hypothesis of fractal dimensions less than 3, the model predicts a dependence of the quality factor (Q) of the direct seismic waves with distance. Kappa factor for each individual record is obtained from the slope of the smoothed log acceleration Fourier spectral amplitude at high frequencies, generally greater than 10 Hz, where the frequency is in linear scale. A least-squares fit to the spectrum of each record is done. Kappa factors are calculated for the vertical and horizontal components of all the acquired records versus distance. The best-fit line to the distribution of kappa versus distance is fitted to the form k = k R+ ko. The best-fit coefficients for the vertical componenst are k = 0.00011s and ko =0.025 s, while k =0.00015s and ko=0.02s for the horizontal components.