Abstract

The pilgrimage city of Lourdes in the French Pyrenees has been heavily damaged by earthquakes in the seventeenth and eighteenth centuries. Assessments of local seismic hazard have been performed through a seismic microzonation of the city. A campaign of urban noise recording has been conducted at about 180 points to determine seismic wave amplification by means of spectral ratios of the horizontal-to-vertical (H/V) components. A comparison of this ratio obtained from ambient noise measurements with spectral ratios obtained from earthquake S-wave recording (H/Href) reveals that the fundamental resonance frequency is well estimated but that the site amplification is generally underestimated. Resonance frequencies and amplitudes exhibit coherent patterns that relate to the geological structure. Bedrock sites are generally characterized by flat spectra whatever the geological nature of bedrock, unless topography or karst are present. Along the river, in particular at the pilgrimage site, thick sediments induce low resonance frequencies with large amplifications. Along and across valleys, north and east of the city, H/V spectra show systematic variations according to sediment filling properties. Finally, on the hill flanks east of the city, H/V spectra rather relate to topography.

An additional issue with H/V spectral ratios based on ambient noise is whether this indicator reflects meaningful geological variations. This question is tested on sites where the S-wave ground velocity structure is known. A first test consists of estimating H/V spectral ratios by means of simulation of urban noise through the summation of synthetic seismograms with random properties. A second test consists of estimating H/Href spectral ratios by propagating an earthquake record through a 1D soil profile. These two modelings successfully predict most of the observed spectrum characteristics. In addition, they give some information on the nature of the urban noise, which may include a significant amount of shear energy.

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