Multimethod Characterization of the French-Pyrenean Valley of Bagnères-de-Bigorre for Seismic-Hazard Evaluation: Observations and Models by Annie Souriau1, Emmanuel Chaljub2, Cécile Cornou2, Ludovic Margerin1, Marie Calvet1, Julie Maury1, Marc Wathelet2, Franck Grimaud3, Christian Ponsolles3, Catherine Pequegnat2, Mickaël Langlais2, and Philippe Guéguen2 1. CNRS and Université Paul Sabatier Observatoire Midi-Pyrénées 14 Avenue Edouard Belin 31400— Toulouse, France
[email protected]!mip.fr 2. ISTerre, CNRS-Université Joseph Fourier Grenoble I, IFSTTAR, IRD, Maison des géosciences BP 53X, 38041 Grenoble Cedex, France 3. Observatoire Midi-Pyrénées 57 Avenue d"Azereix 65007-Tarbes, France Abstract! A narrow rectilinear valley in the French Pyrenees, affected in the past by damaging earthquakes, has been chosen as a test site for soil response characterization. The main purpose of this initiative was to compare experimental and numerical approaches. A temporary network of 10 stations has been deployed along and across the valley during two years; parallel various experiments have been conducted, in particular ambient noise recording, and seismic profiles with active sources for structure determination at the 10 sites. Classical observables have been measured for site amplification evaluation, such as spectral ratios of horizontal or vertical motions between site and reference stations using direct S waves and S coda, and spectral ratios between horizontal and vertical (H/V) motions at single stations using noise and S-coda records. Vertical shear-velocity profiles at the stations have first been obtained from a joint inversion of Rayleigh wave dispersion curves and ellipticity. They have subsequently been used to model the H/V spectral ratios of noise data from synthetic seismograms, the H/V ratio of S-coda waves based on equipartition theory, and the 3D seismic response of the basin using the spectral element method.