Site response estimates for the city of Lourdes, Pyrenees, France, using the spectral-ratio method

The city of Lourdes, in the central part of the French Pyrenees, has been damaged several times by earthquakes, in particular in 1660 and 1750, two events which partly destroyed the city [Lambert et Levret-Albaret, 1996]. The Pyrenees is the most seismically active area in France. Historical seismicity indicates the possible occurrence of a magnitude 6 event in the Lourdes area. As Lourdes is an important place of pilgrimage with more than five millions of visitors each year, the evaluation of the seismic risk is of major importance. To this purpose, it is necessary first to correctly identify the active faults near the city, second to determine the ground motion response to local events, also called site effects. The active faults are rather well known thanks to the permanent network of the Observatoire Midi-Pyrenees [Souriau et Pauchet, 1998], and complementary studies [Dubos, 2000]. In the present paper, we present the results of an experiment devoted to the determination of the site effects in the city of Lourdes, using a classical spectral ratio method based on the record of small local events. Although the extrapolation from weak motion to strong motion is often non linear [Darragh et Shakal, 1991; Seekins et Boatwright, 1994], this approach gives crucial information for the evaluation of the ground response in case of a magnitude 6 event. The Pyrenees are an intraplate collision range [Choukroune, 1992]. A major tectonic feature is the North Pyrenean Fault (FNP), which separates the Paleozoic Axial Zone to the south from the North Pyrenean Zone, made of Mesozoic sediments, to the north. Lourdes is located inside this latter unit. Quaternary glaciations have added various superficial structures [Alimen, 1964]. The city, at an altitude of about 400 m, is at the junction of five glacial beds, two of them corresponding to the Gave de Pau river. Two heights, the Beout (719 m) and the Pic du Jer (1948 m), are located south of the city. The experiment has been conducted during seven months using ten digital three-component instruments with broadband sensors. They have been set up with interstation spacing of 200 m to 1200 m. The station locations were chosen by taking into account the importance of some sites for the population, but also the geological nature of the ground. It is well known that site effects are strongly influenced by the lithological features of the superficial structures, which may induce resonance phenomena, and by the topography of the surface and layer interfaces, which may induce focusing effects [Gao et al., 1996]. In our experiment, all the stations are at about the same altitude (400 m + or -40 m), except CIT located on the slope of the Beout at an altitude of 535 m. ROC, CIT, HOP and CHA are on the bedrock, EDF is at the foot of an ophite cliff. CHA is located half-way up the 40 meter-high hill of the castle. The other stations are on the sediments. GEN and PMP are at the base of buildings of 5 levels and 7 levels respectively, these structures may possibly modify the site effects [Wirgin and Bard, 1996]. 19 local events with magnitude 1.4 to 3.2 at epicentral distance 5 to 62 km have been analysed, as well as two regional events. The local events are well representative of the local seismicity, they have been recorded at many stations (table II) with a very good signal to noise ratio. As shown by Riepl et al. [1998], eight to ten events per station are sufficient to define correctly the site effects. Only GEN does not fulfil this criterion because of its later installation. The records show that the amplitudes and frequency contents of the signals are very different from one station to another. For instance, PMP and ROC are distant of 300 m, but the signal at PMP is about four times larger than at ROC. Also, the signal at CHA appears depleted in high frequencies compared to the others stations. The method used is a classical spectral ratio method with a reference station [Borcherdt, 1970]. It consists in determining the site amplification at each site with respect to a reference site located on the bedrock, thus poorly affected by site effects. This method is applied to the signal of local events, whose distance to the stations must be large compared to the interstation distance. The site response is computed by dividing the amplitude spectrum of a record at a station by the amplitude spectrum of the similar record at the reference station. The spectra are computed for the S-wave, which is more sensitive than the P-wave to sediment amplification. An important step is the smoothing of the spectra: it removes instabilities, but it may also remove resonance peaks which could have some physical significance. Spectral ratios are computed for both the vertical (V) and horizontal (H) components. H is a combination of the N-S and E-W components. We present logarithmic average ratios for each station, plus or minus one standard deviation.