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Theses

Conditions Geotechnique et Alea Sismique Local a Teheran

Abstract : Tehran, capital of Iran, the economical and political centre of the country and one of the biggest and most populated cities in the world, is under the threat of large magnitude (above 7) earthquakes located on very near faults. This high seismic hazard and risk resulted in a French-Iranian cooperation program on the tectonics of central Alborz and the seismic hazard in Tehran. Within this framework, the present thesis is focused on the analysis of geotechnical conditions and their influences on seismic ground motion within the Tehran urban area. In this goal a seismological survey was conducted from February to June 2002,involving the installation of 13 temporary seismological stations on geotechnically representative sites: two on the rock in northern an south-eastern parts, a few on consolidated coarse grained deposits in the northern part, and the most in the southern part with fine grained deposits. The main objective was the experimental estimation of the amplification effect of the soils. In addition the ambient noise was recorded at more than 60 locations within the city. These noise data plus the data of more than 130 other sites recorded during previous microzonation projects by IIEES were processed to help in the interpolation of experimental transfer functions obtained for the 13 seismological stations. The main part of this thesis consists in the analysis of these data in relation with the geotechnical conditions. Various experimental techniques were used to quantify amplification and prolongation of ground motion: standard site/reference spectral ratio, receiver function, H/V spectral ratio on noise, group delay method and at last sonogram. The results reveal a large site effect affecting both the amplitude and duration of ground motion on the majority of the studied sites: in particularly, the south-western part of the city experiences an amplification level reaching 7-8, contrasting with the moderate values (factor 2 to 3) predicted in previous studies with a 1D approach. In addition, this amplification occurs on a very broad frequency band, starting at very low frequency (0.3-0.4 Hz). The thesis also includes a systematic comparison of the information deduced from ambient noise and earthquake recordings, on the basis of a comprehensive data set coming from more than 150 European sites. While the H/V technique does predict the fundamental frequency for approximately 80% of the sites, it sometimes fails, especially for low frequency, continental, stiff sites such as Tehran, where H/V curves remain flat for the majority of the sites. The last part of the present work consists in the simulation of the strong motions with the empirical Green's function technique. Four different scenarios, corresponding to the occurrence of strong earthquakes (Mw from 7.1 to 7.6) on Mosha, North-Alborz and Garmsar faults, were considered. The resulting simulations indicate large peak acceleration values and the predicted response spectra sometimes exceed the design response spectra specified in the present Iranian Building Code.
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Submitted on : Sunday, November 13, 2005 - 11:18:36 AM
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Ebrahim Haghshenas. Conditions Geotechnique et Alea Sismique Local a Teheran. Sciences de l'ingénieur [physics]. Université Joseph-Fourier - Grenoble I, 2005. Français. ⟨tel-00010960⟩

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