Seismological Measurements for Site Effect Investigation in the Var valley, Nice, France

Abstract eng:
Quantitative assessment of site effects, is a major issue in seismic hazard and engineering seismology studies. The frequency dependent site amplifications, observed since decades during the past earthquakes over the world, are known to be mainly caused by reverberations and resonance effects of S-waves within unconsolidated sediments overlaying stiffer formations. We carried out several seismological investigations of site effects in the Var Valley near the city of Nice, southeastern France. The valley is less than 1 km width but previous geophysical and geotechnical studies show that the bedrock could be reached at a maximum of about 200 m depth. In the studied area, the sediments are mainly composed of sand and gravel layers overlaying Pliocene marl. In order to investigate the effect of this sedimentary filling, two kinds of field experiment were set up through the sedimentary basin during 2005 and 2006. Firstly, several profile of ambient vibration recordings were done through the valley in order to apply the Nakamura's technique and estimate the resonance frequency of the soil column beneath the recording points. We also used array technique in order to estimate the Rayleigh waves dispersion characteristics from ambient vibrations recordings. The inversion of the observed dispersion curves allows us to recover the shear wave velocity profiles of the subsurface. The H/V ratios exhibit amplifications down to 0.5 Hz in the centre of the valley and the shear wave velocity in the sediments is estimated close to 300 m/s. These results are in good agreement with geotechnical observations and 1D linear-equivalent modelling. Secondly, a temporary array of 10 broadband velocimeters was deployed in order to record the surrounding seismicity. These data help us defining transfer functions by the computation of spectral ratio between the recordings at a reference station located on rock site and the ones obtained with the station located in the basin. Resonance frequency in the valley is observed between 1 Hz and 4.1 Hz.

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