Multimethod characterization of the French Pyrenean valley of Bagn`eres-de-Bigorre for seismic hazard evaluation: observations and models

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. General good agreement is found between simulations and observations. The 3D simulation reveals that topography has a much lower contribution to site effects than sedimentary filling, except at the narrow ridge crests. We find clear evidence of a basin edge effect, with an increase of the amplitude of ground motion at some distance from the edge inside the basin and a decrease immediately at the slope foot.

💡 Research Summary

The paper presents a comprehensive multimethod investigation of the Bagnères‑de‑Bigorre valley in the French Pyrenees, a narrow rectilinear basin that has experienced damaging earthquakes in the past. The authors deployed a temporary seismic network of ten stations along and across the valley for two years, recording direct S‑waves, S‑wave coda, and continuous ambient noise at each site. Classical site‑response observables were extracted: spectral ratios (SR) of horizontal to vertical motion between each site and a reference station using direct S‑waves and S‑coda, and horizontal‑to‑vertical (H/V) spectral ratios derived from ambient noise and from S‑coda records.

Vertical shear‑wave velocity (Vs) profiles at each station were obtained by a joint inversion of Rayleigh‑wave dispersion curves and ellipticity measurements. These 1‑D Vs models served as the basis for three distinct modeling efforts: (1) synthetic noise‑based H/V ratios, (2) H/V ratios of S‑coda waves calculated from equipartition theory, and (3) full 3‑D seismic response simulations of the basin using the spectral element method (SEM).

The synthetic H/V and S‑coda H/V results match the observed ratios within the expected variability, confirming that the inverted Vs structures capture the essential frequency‑dependent amplification characteristics. The 3‑D SEM simulations incorporate both the actual topography and the sedimentary fill of the valley. The comparison between simulations and observations shows generally good agreement across the frequency band of interest (0.5–10 Hz).

A key finding is that sedimentary infill, rather than the rugged topography, dominates the site‑effect amplification, except at the narrow ridge crests where topographic focusing contributes modestly. The simulations also reveal a clear basin‑edge effect: ground‑motion amplitudes increase at a certain distance inside the basin from the edge (approximately 200 m) and then drop sharply at the foot of the slope. This “edge amplification” is consistent with theoretical predictions for 3‑D basins but is often underestimated in 2‑D models.

The study demonstrates that a multimethod approach—combining field measurements (direct S‑waves, S‑coda, ambient noise), joint inversion for Vs, and high‑resolution 3‑D numerical modeling—provides a robust framework for evaluating seismic hazard in complex valley settings. The authors suggest that incorporating basin‑edge effects and the dominant role of sedimentary fill into regional hazard maps will improve the reliability of ground‑motion predictions and inform better engineering design and land‑use planning in similar mountainous regions. Future work is proposed to extend the frequency range, include nonlinear soil behavior, and explore the impact of different source mechanisms on basin response.