HR: 0803h
AN: NS11D-0798    [Abstracts]
TI: 3D Geotechnical Soil Model of Nice, France, Inferred from Seismic Noise Measurements, for Seismic Hazard Assessment.
AU: * Bertrand, E
EM: etienne.bertrand@equipement.gouv.fr
AF: CETE Méditerranée LRPC de Nice, 56 boulevard Stalingrad, Nice cedex 4, 06359, France
AU: Duval, A
EM: anne-marie.duval@equipement.gouv.fr
AF: CETE Méditerranée LRPC de Nice, 56 boulevard Stalingrad, Nice cedex 4, 06359, France
AU: Castan, M
EM: marion.castan@equipement.gouv.fr
AF: CETE Méditerranée LRPC de Nice, 56 boulevard Stalingrad, Nice cedex 4, 06359, France
AU: Vidal, S
EM: sylvain.vidal@equipement.gouv.fr
AF: CETE Méditerranée LRPC de Nice, 56 boulevard Stalingrad, Nice cedex 4, 06359, France
AB: In seismic risk studies, the assessment of lithologic site effect is based on an accurate knowledge of mechanical properties and geometry of superficial geological formations. Therefore, we built a 3D subsurface model in the city of Nice, southeastern France, using not only geological and geotechnical data but also geophysical inputs. We used especially ambient vibration recordings to supply the lack of borehole data over the city. Nice spreads over 72 km² and roughly 20% of the city is built upon recent alluvium deposits. Other parts of the city lie on Jurassic and Cretaceous rocks to the east and thick Pliocene conglomerates to the west. Nearly 450 boreholes located mainly in the alluvial valleys were used. Because they are essentially linked to previous planned constructions (such as road network or important building), their distribution is rather heterogeneous over the studied area. In the valleys moreover, less than 40% of the boreholes are reaching the rock basement. These boreholes have been analyzed and a representative soil column made of 9 sedimentary layers has been recognized. Shear wave velocity of these layers were obtained from Standard Penetration Test values using several empirical correlation law described in the literature. Because of its cost, an extended boring survey was not feasible to complete our data set. Traditional seismic profiling was also not intended, as it is not possible to use intensive explosive sources in town. Recent years have seen many studies using ambient vibration measurements for site effect estimation. Especially, the very simple H/V technique was proven to be suitable for microzoning studies although some limitation were pointed out when dealing with 2D or 3D structures. Nevertheless, this technique alone provides only the fundamental eigenfrequency of the site under investigation. But assuming the shear wave velocity in the sediment it can helps to constrain the depth of the bedrock thanks to the well known f₀=V_S/4H relationship. We measured ambient vibration at almost 500 points homogeneously distributed in the city of Nice. The recordings were processed according to the Nakamura technique (Nakamura, 1989) obtaining thus the fundamental resonance frequency of the sedimentary infill at each recording point. Adding to this data set some earthquake recordings from the French permanent accelerometric network (RAP) in Nice and a microtremor array recording (Bonnefoy-Claudet, 2004), we were able to better constrain the subsurface model of Nice as well as to extend it where no boreholes were existing. This paper shows the good agreement we observed between geotechnical data and ambient vibration recordings. It particularly points out the contribution of seismological surveys in the subsurface modeling.
DE: 0935 Seismic methods (3025, 7294)
DE: 7212 Earthquake ground motions and engineering seismology
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting