MAPPING THE FUNDAMENTAL FREQUENCIES IN THE CITY OF BEIRUT USING AMBIENT NOISE MEASUREMENTS

Marleine Brax (1, 2) , Cécile Cornou (1) , Christophe Voisin (3) , Pierre-Yves Bard (1)

1 ISTerre - Institut des Sciences de la Terre
2 National Center for Geophysical Research, CNRSL, Bhannes
3 Cycle sismique et déformations transitoires

Historically, Beirut the capital of Lebanon has been destroyed several times by earthquakes, the most devastating of which was the 551 A.D. offshore earthquake. Assessing the local seismic hazard and risk is therefore of primary importance for the whole country. The present study is part of a comprehensive set of experimental surveys providing physical insights and quantitative constraints on the site response variability within the city (Brax, 2013). The geology of Beirut constitutes an exception to the geological landscape in the area, in relation with its sub-horizontal platform (Fig.1). Stiff to hard Miocene and Cenomanian units are outcropping on two hills whereas the rest of the platform is covered by alluviums or sandy soils, resulting in highly variable near surface geological conditions throughout the city, which may in turn result in highly variable ground shaking characteristics In order to evaluate the soil effects, 30' long ambient noise measurements were conducted in 615 sites distributed over a region that covers the city of Beirut and parts of its suburbs. The dataset has been processed with the horizontal to vertical spectral ratio technique HVSR according to the SESAME guidelines, resulting in the identification of fundamental frequencies over the study area. Different assessments of the predominant frequencies were made individually by the authors in order to test the robustness of the results. A good correlation between H/V frequencies and the geology of the soil was found. Different zones with large amplitude peaks have also been identified. A detailed map of the fundamental frequencies of the soil was derived for the whole region of study. Despite its limitations, such a map provides a useful information on sub-surface conditions of the city, and highlights several zones where possible amplifications can be expected in future earthquakes, and result in higher risk. It also allows identifying zones, which should be the focus of further investigations. This map could be an important step on the way towards a seismic microzonation of the Greater Beirut area, which is essential for city planning and the optimal design of earthquake resistant structures.

Domaines: Géophysique [physics.geo-ph]

Marleine Brax, Cécile Cornou, Christophe Voisin, Pierre-Yves Bard. MAPPING THE FUNDAMENTAL FREQUENCIES IN THE CITY OF BEIRUT USING AMBIENT NOISE MEASUREMENTS. Second European Conference on Earthquake Engineering and Seismology (2ECEES), Aug 2014, Istanbul, Turkey. ⟨insu-01742677⟩