Keywords: site effect, local geological conditions, Thomson-Haskell method, amplification, wave propagation, El-Asnam earthquake, response spectrum.

When an earthquake occurs, the measured motion on the surface is considered as the result of three effects : the source mechanism, propagation path and the influence of geological and topographical local conditions [1,4]. The different observations and theoretical studies performed previously make possible to distinguish three kinds of site effects : effect of the geological strata and their geometry, topography effects and effects induced by the lateral discontinuities of the local structure of the ground. The strong recent earthquakes, in particular those of Michoacan "Mexico City" (1985), Armenia (1988) and California (1989), highlighted the important influence of the geological conditions on the level and the characteristics of the ground motion, and also on the induced damage.

The purpose of the present study is to perform a model [6] in order to investigate the effect of the local geology. In this frame, an analytical approach based on the Thomson-Haskell's method is adopted [1,2,3,5] and El-Asnam earthquake of October 10th, 1980 is considered. A numerical investigation is performed to analyse the local conditions site effects on the P, SV and SH propagating waves.

The bedrock motion is simulated using a technique based on the empirical Green functions. To simulate the principal event of El-Asnam earthquake at the bedrock, the aftershock of November 8th (M_L=5.6), having a similar focal mechanism with the major event, is considered as Green function.

The obtained transfer functions in the frequency domain for El-Asnam's site show that the height and width of the peaks are controlled by the attenuation. The module of the transfer functions can be in excess of 10 for certain frequencies, and that whereas the amplification of S wave is dominant in the frequency range 1 to 10 Hz, the amplification of P waves is dominant above 10 Hz. Below, 1 Hz, the sedimentary structure has little effect upon either P or S waves.

The surface's seismograms are obtained by the convolution of the calculated transfer function with the simulated seismograms at the bottom of the array. The results highlight important variations of the amplitudes of the motion for points located in free surface compared to the points located in-depth. The PGA at the surface presents an amplification of 1.5 to 5 times the bedrock's one. This is confirmed by the calculation of the response spectra for acceleration and displacement where, on the average, the surface spectra are about 3 times greater than the bottom spectra, although the ratio can be as large as 10. The obtained accelerations are compatible with the results obtained by the performed observations in-situ.

This study shows the importance of the geological and geotechnical structure of the ground and the need to take into account, in an aseismic strategy, the site effects of structural origin for El-Asnam site.

1

J. Coulomb, G. Jobert, "Traité de Géoghysique Interne. Vol.1: Sismologie et Pesanteur", Ed. Masson et Cie, Paris, France, 1973.

2

N.A. Haskell, "Crustal Reflection of Plane SH Waves", Journ. Geophys. Res, Vol. 65, pp. 4147-4150, 1960. doi:10.1029/JZ065i012p04147

3

N.A. Haskell, "Crustal Reflection of Plane P and SV Waves", Journ. Geophys. Res, Vol. 67, pp. 4751-4767, 1962. doi:10.1029/JZ067i012p04751

4

B.L.N. Kennett, "Seismic Wave Propagation in Stratified Media", Cambridge University Press, Cambridge, U.K, 1983.

5

W.T. Thomson, "Transmission of Elastic Waves through a Stratified Solid Medium", Journ. Appl. Phys Vol. 21, no. 1, pp. 89-93, 1950. doi:10.1063/1.1699629

6

K. Tounsi, "Analyse de la propagation des ondes sismiques: Effets des ondes de surface", Thèse de Magister, Ecole Nationale Polytechnique, ENP, Alger, Algeria, 2002.

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