Multimethod Characterization of the French-Pyrenean Valley of Bagneres-de-Bigorre for Seismic-Hazard Evaluation: Observations and Models

Abstract

A narrow rectilinear valley in the French Pyrenees, affected in the past by\ndamaging earthquakes, has been chosen as a test site for soil response\ncharacterization. The main purpose of this initiative was to compare\nexperimental and numerical approaches. A temporary network of 10 stations has\nbeen deployed along and across the valley during two years; parallel various\nexperiments have been conducted, in particular ambient noise recording, and\nseismic profiles with active sources for structure determination at the 10\nsites. Classical observables have been measured for site amplification\nevaluation, such as spectral ratios of horizontal or vertical motions between\nsite and reference stations using direct S waves and S coda, and spectral\nratios between horizontal and vertical (H/V) motions at single stations using\nnoise and S-coda records. Vertical shear-velocity profiles at the stations have\nfirst been obtained from a joint inversion of Rayleigh wave dispersion curves\nand ellipticity. They have subsequently been used to model the H/V spectral\nratios of noise data from synthetic seismograms, the H/V ratio of S-coda waves\nbased on equipartition theory, and the 3D seismic response of the basin using\nthe spectral element method. General good agreement is found between\nsimulations and observations. The 3D simulation reveals that topography has a\nmuch lower contribution to site effects than sedimentary filling, except at the\nnarrow ridge crests. We find clear evidence of a basin edge effect, with an\nincrease of the amplitude of ground motion at some distance from the edge\ninside the basin and a decrease immediately at the slope foot.\n