Site effect study in urban area: Experimental results in Grenoble (France)

Three methods are used to determine the site effect in the town of Grenoble, located in the Western Alps. First we use the classical spectral ratio method in 14 sites to calculate the transfer function of the basin. We find an amplification of 10 in the frequency range of 0.25 to 10 Hz. Second, we compare these results with the H over V spectral ratio method, and propose a map of resonance frequency of the basin. We find a lower resonance frequency in the center of the basin than on the edge, that is consistent with the structure deduced from a gravity Bouguer anomaly map. Finally we use the empirical Green's function method to simulate a M-w 5.5 earthquake at a distance of 20 km from the town. The simulated acceleration reaches the level of 2 m/s(2) in the center of the basin compared to 0.2 m/s(2) on the edges. The simulated ground motion we compute is smaller than the French seismic codes on the edge of the valley but significantly larger in the center.