Site effects in the city: Seismic Risk Evaluation

Site Effects in the City of Lourdes, France, from H/V Measurements:

Implications for Seismic-Risk Evaluation

by Annie Souriau, Agathe Roullé, and Christian Ponsolles

Abstract The pilgrimage city of Lourdes in the French Pyrenees has been heavilydamaged by earthquakes in the seventeenth and eighteenth centuries. Assessments oflocal 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 todetermine seismic wave amplification by means of spectral ratios of the horizon-tal-to-vertical (H/V) components. A comparison of this ratio obtained from ambientnoise measurements with spectral ratios obtained from earthquake S-wave recording(H/Href) reveals that the fundamental resonance frequency is well estimated but thatthe site amplification is generally underestimated. Resonance frequencies and ampli-tudes exhibit coherent patterns that relate to the geological structure. Bedrock sites aregenerally characterized by flat spectra whatever the geological nature of bedrock, un-less topography or karst are present. Along the river, in particular at the pilgrimagesite, thick sediments induce low resonance frequencies with large amplifications.Along and across valleys, north and east of the city, H/V spectra show systematicvariations according to sediment filling properties. Finally, on the hill flanks eastof the city, H/V spectra rather relate to topography.

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

Introduction

In countries with relatively low seismicity, the instru-mentally based seismic hazard assessment is difficult, be-cause short instrumental databases lack large events. Thisis the case for France, where the national seismic zonationis currently based on the large scale effects of historicalearthquakes. Even with the new, probabilistic zonation (cur-rently debated among experts), the spatial resolution is toocoarse for urban planning at the city scale, in particular whenlocal strong site effects occur. Localized strong amplifica-tions of seismic motion, called site effects, generally occurabove unconsolidated sediments and/or rugged topography.The city of Lourdes, in the central French Pyrenean foreland,straddles both configurations. It is built partly at the conflu-ence of several alluvial and glacial valleys and partly onsteep Pyrenean hill slopes. Over the past century, this cityhas become an important pilgrimage site, with more than five

million visitors each year. Lourdes has been partly destroyedtwice, in the seventeenth and eighteenth centuries, by seismicevents of intensity VIII–IX located only some 10 km fromthe city. A better assessment of the expected ground motionsinside the city is thus a key element for urban and civil pro-tection planning.

A preliminary site effect experiment with 10 seismicstations deployed on various geological units has been per-formed by Dubos et al. (2003) at the city scale. They foundthat the spectral ratio of the S-wave horizontal componentwith respect to a reference station at rock site condition(H/Href) revealed very rapid lateral variations of site effects.In some cases, two sites separated by less than 500 m exhibitvariations of H/Href amplitudes of one order of magnitude atsome particular frequencies. It thus appears necessary to den-sify spatial sampling to investigate site effects inside the city.

2118

Bulletin of the Seismological Society of America, Vol. 97, No. 6, pp. 2118–2136, December 2007, doi: 10.1785/0120060224

However, because the H/Href method depends on the record-ing of natural earthquakes, which may take several months tooccur given the tectonic context, we preferred a methodbased on estimating the ratio between horizontal and verticalcomponent spectra of ambient noise, called H/V in the fol-lowing discussion (Nakamura, 1989; Bard, 1999). This tech-nique requires recording for only a few tens of minutes ateach site. Although this method is less reliable, joint analysisof densely sampled H/V values, H/Href ratios previously de-termined, and geological and geotechnical information pro-vide the major components for a seismic zonation of the city.After a brief recall of the tectonic context and principle andlimitations of the H/V method, we present the results of theH/V experiment, some data interpretation based on 1D mod-eling, and a discussion of the global results in relation to theseismic zonation of the city.

Geological and Tectonic Context

The Pyrenees result from the convergence of the Iberianand Eurasian plates for the last 65 Ma, which took place afteran extension episode opening a shallow sea between the twoplates (Choukroune, 1992). The limit between the two plates,the North Pyrenean fault, corresponds to a 15 kmMoho jumpwith a thicker crust to the south (Hirn et al., 1980), whereasthe paleo-rift remains as a weak zone (the North PyreneanZone) with thick sediments north of the North Pyrenean fault(Fig. 1). South of the North Pyrenean fault, the PaleozoicAxial Zone contains the highest summits (3400 m in the cen-tral part of the range). The convergence continues today at a

low rate, probably less than 1 mm=yr (Nocquet and Calais,2003), and the seismic activity remains generally moderate.

The historical seismicity in the Pyrenees is well docu-mented back to the fourteenth century (Lambert et al., 1996).It reveals a maximum of activity in the central part of therange on the French side (Fig. 1), with about 25 events ofMedvedev–Sponheuer–Karnik (MSK) intensity larger thanVII since the beginning of the seventeenth century (intensi-ties are given in the MSK scale [Medvedev et al.,1967] com-monly used in Europe). Part of the cities of Lourdes andBagnères-de-Bigorre were destroyed in 1660 by an eventof MSK intensity IX that occurred 10 km south of Lourdesand killed 30 persons. Its magnitude is estimated as 6.0–6.1 (Levret et al., 1994). Two other events caused severedamage, one in 1750 with intensity VIII and another in1854 with intensity VII, both located only 5–7 km southof Lourdes.

Instrumental seismicity is recorded since the 1960s, butdense networks for a global survey of the whole Pyreneanrange were deployed only in 1989 (Souriau and Pauchet,1998), and for this specific region only in 1996. In addition,some temporary experiments were performed to specify theexact location of the active faults and the focal mechanism ofearthquakes in this region (Rigo et al., 2005). They reveala general east–west trend of seismicity north of the NorthPyrenean fault with several clusters, one of them being lo-cated immediately south of Lourdes (Fig. 1).

Lourdes is located in the North Pyrenean Zone, in a verycomplex geological setting (Fig. 2) inherited from the Pyr-enean convergence and from the Quaternary glaciations,

Figure 1. Simplified seismotectonic map of the region of Lourdes (see location in the inset) with instrumental seismicity for the period1989–2006 and historical seismicity for the last five centuries. North Pyrenean fault, NPF.

Site Effects in the City of Lourdes, France, from H/V Measurements 2119

Figure 2. (a) Topographic map with the Gave-de-Pau river and Lourdes Lake, and (b) geological map of the city of Lourdes. Alsoreported are the stations used for the 2001 experiment of site effect measurements (Dubos et al., 2003) using spectral ratios with a referencestation at rock (ROC). Sanctuary, SAN; limits of the Lourdes district, dashed line.

2120 A. Souriau, A. Roullé, and C. Ponsolles

whose northern limit was at the latitude of Lourdes. Lourdesis built at the junction of five valley branches (Fig. 2a). Dur-ing successive glaciations, the Lourdes glacier flowed fromsouth to north across Lourdes and through the valleys north-northeast and northwest of the town. Today, these two val-leys are dry. The river, Gave-de-Pau, flows from south tonorth into town and bends west along the axial drainage. ThisQuaternary landscape evolution explains the present fillingwith glacial moraines with large erratic blocks in the south,north-northeast, and northwest valleys and with alluvial se-diments along the axial drainage (Fig. 2b). The sanctuary(SAN in Fig. 2b) is located along this westward running seg-ment of the river. The castle (CHA in Fig. 2b), in the middleof the city, is built on a 50-m high hill corresponding to arecessional moraine. Two summits flank the valley to thesouth of the city: the Béout (altitude 719 m) to the westand the Pic du Jer (altitude 1948 m) to the east.

Site Effect Determinations: The H/Href andH/V Methods

Three methods, based on spectral ratio computations,are commonly used to determine the soil response to a seis-mic excitation (e.g., Field and Jacob, 1995): The first one,considered the most reliable one (Borcherdt, 1970), consistsof computing the spectral ratio between each component of asite station and of a reference station without site effect, gen-erally located at a bedrock site without topography. It iscalled H/Href for the horizontal components spectral ratioand V/Vref for the vertical components spectral ratio. Thesecond method (the horizontal-to-vertical spectral ratio[HVSR] method) consists of estimating the amplitude spec-tral ratio of the horizontal-to-vertical components of seismicS waves from natural earthquakes. Finally, the third method(H/V) consists of calculating the ratio between the horizon-tal and vertical component spectra from ambient noiserecords (Nakamura, 1989). The first two methods (HVSRand H/Href) are based on records of natural earthquakes:their main drawback is to rely on the occurrence of earth-quakes at some 10–100 km from the station. If the seismicityis moderate, a long-period recording (a few weeks to a fewmonths) will be necessary. The third method (H/V) is basedon ambient noise recordings and may thus be applied easilyin urban environments. Its main advantage is its promptitude:only a few tens of minutes of record are needed, without useof a reference station. It is thus very convenient for practicalengineering purposes. However, it leads to results that aremore difficult to interpret.

The H/Href method assumes that earthquake inducedground motion at the bedrock–sediment interface is similarto that of a nearby station located on rock. Under this as-sumption, the H/Href ratio gives the transfer function ofthe sedimentary layer for a station installed on a sedimentarylayer above bedrock. This assumption hypothesizes that theincident wave is the same beneath both stations, which im-plies that the distance between earthquake sources and sta-

tions must be large compared to the distance between thestation and the reference site. Source and propagation effectscould be safely neglected. A difficulty of this method is find-ing a good reference site, which can effectively be charac-terized as strong rock, without topographic effect. Themethod is generally applied to S waves, which are respon-sible for most of the damage.

The HVSR method is based on the assumption that, fora station at a rock site, the earthquake induced H/V ratiois close to unity (Lermo and Chavez-Garcia, 1993). Thisis generally the case for a seismic ray path with incidenceangles close to 45° (thus for local and regional events, againwith distances from a few tens to a few hundreds of kilo-meters); it is not true for teleseisms, for which rays arrivenearly vertically beneath the station. These generally givean S signal larger on horizontal components than on verticalcomponents (e.g., Kulhánek, 1990). It may be shown that, ifsedimentary layers are present, the amplitude of the verticalcomponent is only weakly modified by these layers, whereasthe horizontal components are strongly amplified at someparticular frequencies. The HVSR provides the resonance fre-quency peaks of the transfer function, as well as an estimateof local amplification for simple structures (Lermo andChavez-Garcia, 1993).

The H/V method (Bard, 1999) relies on the nature of theambient noise and on the relationships between soil structureand amplification of the different seismic waves present inthe noise. Noise includes natural microseisms, mainly com-posed of Rayleigh waves with characteristic periods in therange of 4 to 10 s, and microtremors due to urban activity.Urban noise appears predominantly at periods smaller than1 s and includes body waves (P and S) and surface waves(Rayleigh and Love waves) generated by traffic, industrialand domestic activity, and wind. Because such activitiesgenerate mostly superficial compressional sources, the shearsignal will be dominated by the fundamental mode of Ray-leigh waves. Higher Rayleigh modes, Love modes, and shearbody waves are also present, due in particular to diffractionon scatterers. The relative contribution of these differentwaves (body waves and fundamental and higher modes ofsurface waves) in the noise seems, however, to exhibit sig-nificant variability (see the review by Bonnefoy-Claudetet al. [2006]).

The Rayleigh-wave ellipticity depends on the structure,on the period, and on the considered mode (Harkrider, 1970).For high impedance contrasts at the sediment interfaces,peaks in the H/V ratio are generated by resonance insidethe uppermost layers. For a single plane layer overlayingthe bedrock and without any topography, the resonance peakfrequency fR can be roughly estimated by fR � vS=4h,where vS is the shear velocity in the layer of thickness h.It has been shown that the most prominent low-frequencypeak differs generally by less than 10% from the fundamen-tal S-wave resonance frequency (Bard, 1999). This is nolonger valid if the impedance contrast between superficiallayers and substratum is weak (Malishewski and Scherbaum,


2004) or if several layers are present. The peak may be gen-erated by the vanishing of the horizontal component, forexample, if the Rayleigh-wave ellipticity becomes pro-grade, which occurs if a thick soft sediment layer is present(Tanimoto and Rivera, 2005). It may also occur for somehigher modes of Rayleigh waves (e.g., Harkrider, 1970).Considering now the amplitude of the H/V spectral ratio,the peak amplitude is not clearly related to the S-wave am-plification and is generally smaller. It is sensitive to the Pois-son’s ratio. It also depends on the smoothing functionapplied to the spectra (Lachet and Bard, 1994). A ratio closeto unity is expected for the fundamental mode of Rayleighwaves at conditions close to a half-space (H=V ∼ 0:7, for ahalf-space with a Poisson’s ratio of 0.25).

All three methods described in the preceding discussionhave been applied to the city of Lourdes.

The Site Effect Experiments at Lourdes

The 2001 H/Href Experiment (Dubos et al., 2003)

The H/Href method has been applied to data collectedduring a seven month experiment performed by Dubos et al.(2003) at 10 sites located on various geological units in thecity (Fig. 2b). The reference station (ROC) corresponds to arocky site with flat topography in the center of the city. Itsgood quality as a reference site is confirmed by its flat H/Vratio over the whole range of frequencies. The H/Href ratios,computed for the horizontal components in the frequency

range 0.2–25 Hz, reveal very rapid spatial variations ofthe site responses inside the city (Fig. 3). The V/Vref ratioshave also been computed for the vertical components: theirmaxima are generally at higher frequency than that of theH/Href spectra (see Dubos et al. [2003], their fig. 7). Thetwo horizontal components (north and east) exhibit nearlysimilar spectra and could thus be combined in a single signal,according to H � �N2 � E2�1=2. Some sites exhibit a flatspectral ratio (HOP and EDF). Most of them have one peakbetween 2 and 10 Hz on the horizontal component, whichsometimes broadens (SAN) or splits into two peaks (PMP)at a high frequency (Fig. 3). For most of the 10 sites, theH/V ratios could be computed for seismic signal and for noise(Dubos, 2003). These spectral ratios generally retrieve thefundamental frequency, but they are unable to restitute thecomplexity of the spectra, and they underestimate the peakamplitude (Fig. 3).

The 2005 Ambient Noise H/V Experiment

In 2005, an ambient noise experiment was conducted bytwo teams, one from the Observatoire Midi-Pyrénées (OMP)and one from the Bureau de Recherches Géologiques etMinières (French Geological Survey, BRGM). It was con-ducted during the winter for two reasons: winter conditionsprovide a stronger natural microseismic noise, and touristicactivity in summer continuously generates noise very close

Figure 3. Results of the site effect determined from three different methods for the sites reported in Fig. 2b. S-wave spectral ratio fromearthquakes, with reference station (H/Href), solid line; H/V spectral ratio measured on earthquakes (H/V signal), dashed line; H/V spectralratio measured on noise (H/V noise), gray line. For clarity, the 1σ confidence level is reported for H/Href only; it is of the same order for thetwo other methods (after Dubos, 2003).


to recording stations. To further diminish human distur-bance, most of the records were performed during the night.

Each OMP recording station includes a velocimeterCDJ-S2A from Chongqing Geological Instruments whosebandpass is broadened at low frequencies up to 0.1 Hz(J. F. Fels, personal comm., 2004). For a few sites, a GuralpCMG-40 velocimeter was also set up. Each seismometer isconnected to a Minititan3 Agecodagis recorder with a sam-pling rate of 125 Hz. The experimental set-up adopted byOMP during the survey campaign used two stations a fewmeters apart and a real time visual display. Duplicate stationswith visual display prevent recording very local perturba-tions (e.g., underground pipes). When possible, the seism-ometers were partly buried in the ground, to remove theresonance frequencies above 25 Hz (Fig. 4a) caused by thecoupling between the legs and the body of the instrument andto decrease the perturbing signal due to the wind. Up to threerecordings of 300 sec were performed at each site.

The BRGM operated at each site a 5-sec Lennartz velo-cimeter connected to a GSR-24 recorder from GeoSig. Theseismometer was up on its legs. The band pass is about 0.2–40 Hz. The sampling rate is 100 Hz, and the record length is660 sec.

At each site, the spectra are computed for each compo-nent using a fast Fourier transform and are smoothed on win-dows with the width set to 20% of the central frequency. Thisremoves of instabilities and preserves the resonance peaks.The two horizontal components are merged according toH � �E2 � N2�1=2, which implicitly assumes that they haveno systematic phase coherence. The mean and standard de-viation are deduced from the processing of tens of indepen-dent signal sampling windows. Each window includes4096 points (32 sec) for OMP. The data processing is glob-ally the same for BRGM, in particular the same smoothing ofthe spectra has been applied, so that the results are compa-tible with each other. As a test, a few sites have been instru-mented simultaneously by both OMP and BRGM. The resultsare similar (Fig. 4b). In some cases H/V ratios increased atvery low frequencies due to strong horizontal shaking causedby windy conditions during the experiment, in particular forBRGM measurements: this increase must not be interpretedas being representative of ground conditions.

Figure 5 maps the 189 points sampled by OMP (dots)and BRGM (squares) superimposed on the geographic–topographic map. In the city center, the distance betweenpoints is on the order of 200–300 m. Dense sampling hasalso been performed in the valley to the north, becauseof the building density. The two mountains to the southand the forested region to the northwest were only sparselysampled, generally in the vicinity of special equipment(e.g., a garbage collection site or a water pumping or sew-age works).

In what follows, sites occupied by OMP will be noted byan (for n from 1 to 111); those occupied by BRGM will benoted by bm (for m from 1 to 68). The 10 sites of the H/Hrefexperiments previously carried out by OMP will be notedwith their three-character code.

Experimental Results

The following section presents the results illustratingspecific characteristics of the method, or specific featuresof the geology in Lourdes, with their implications forseismic-risk evaluation.

Bedrock Sites

Bedrock sites in topographically flat areas are character-ized by flat H/V ratios with an ordinate value close to 1 overthe whole range of frequencies. This property is used to iden-tify a “good” reference station in the H/Href method. In othercases, bedrock sites may present spectra that deviate from aflat line. This occurs when bedrock is altered (Steidl et al.,1996)—an extreme example of which is karstic limestonebedrock with underground cavities. Complex spectra alsooccur when the site is in rugged topography.

Figure 6 gives H/V ratios for some rock sites in Lourdes,corresponding to different geological units (see their location

Figure 4. Tests of the experimental method. (a) Influence of theseismometer set up (on legs or half buried), showing the resonancefrequency due to the coupling between the legs and the body of theinstrument; (b) comparison of the results of OMP and BRGM at thesame site (with 1σ confidence levels).


in Fig. 5). ROC and a17 are hard Upper Cretaceous coral reeflimestone, a97, a30, and b43 are Middle Cretaceous flysch,and a57 is located at an ophitic massif. All these sites haveeither no or very mild topography. The spectral ratios arereasonably flat over the whole frequency domain, with am-plitudes between 1 and 3.

There is no obvious dependence of the spectral ratios onthe type of rock. The station ROC, located on a rock outcropin the center of the city, exhibits the typical flat spectrum of ahigh quality rock site, which justified its choice as a refer-ence site for the H/Href method (Dubos et al., 2003). By con-trast, site a17, which is located on the same type of rock west

Figure 5. Points sampled with the H/V method for site effect determination in the city of Lourdes, operated by OMP, dots, and BRGM,squares. Ten more points are located in the western part of the Lourdes district (not shown). Numbered sites correspond to rock sites shown inFig. 6 (bold numbers) and to the Gave-de-Pau bank sites in Fig. 7 (underlined numbers). The three boxes refer to Figs. 8, 9, and 10. Geo-logical cross section of Fig. 8 (after BRGM, 2006), AB. Topographic background is from the Institut Géographique National.


of the city at the entrance of a cave, exhibits a strong ampli-fication at high frequencies, likely due to the undergroundcavities. The station CHA (Fig. 3) is also located on the sametype of rock but at the flank of a 50-m high hill, mid distancefrom the top. The deamplification observed at high fre-quencies (f > 3 Hz) has been ascribed to a topographic ef-fect (Dubos et al., 2003). It is also interesting to compare a57(Fig. 5) with EDF (Fig. 3), two points close together on anophitic massif at the foot of a cliff. The main differencein the spectra concerns the mean value, close to 3 for57, which is directly at the foot of the cliff, and close to1, as for a flat rock site, for EDF, which is located only100 m away. Similar shifts in the mean value have been ob-served for other points close to cliff walls, either at the top orat the foot.

Sites Along the Gave-de-Pau River

Quaternary soft sediments of glacial (Würm and Riss)and fluvial origin bound the Gave-de-Pau river along mostof its course. Few people live there in permanent housesas this zone is subject to floods. However, some buildingsin the sanctuary, an electric power plant, a sewage works sta-tion, and several car parks, are built on soft soils along theGave-de-Pau. This area thus deserves some consideration forrisk assessment. Figure 7 shows the spectral ratios obtainedfor these points, sorted as natural sites (river bank) and man-modified sites (park places) (see their locations in Fig. 5).

For the natural sites (a81, a67, and a109), a low-fre-quency peak is observed at 1–3 Hz, with a spectral ratio am-plitude of 20. The same resonance frequency was obtained atthe sanctuary (Fig. 3), which is also located along the river onsoft sediments. However, the comparison of the H/Href and

the H/V results (Fig. 3) suggests that the resonance frequencybands may include higher frequencies. A seismic refractionexperiment performed in the sanctuary garden along theriver reveals the presence of a 26-m thick layer with S velo-city of 300 m sec�1 (Dubos et al., 2003). The relationshipf � vS=4h predicts a resonance frequency of 2.9 Hz, in goodagreement with the observation (Fig. 3, site SAN). As will beseen in the next section, a modeling with synthetic seismo-grams based on spectral analysis of surface waves (SASW)results also predicts this peak. SASW experiments performedby BRGM along the river (on the north side and the westpart of the river) reveal a generally thinner, 10- to 15-mthick, soft soil layer with mean vS values on the order of250–300 m sec�1 (BRGM, 2006), which would predict reso-nance frequencies higher than those observed near the sanc-tuary and along the south part of the river. It is a clearindicator of a great spatial variability of the structure. Thus,comparison of observations and predictions is only possiblefor sites with a nearby SASW profile available.

The points sampled on river sides transformed as carparks show H/V spectra broadened at low frequencies, com-pared to points on natural sites. Geological data indicate thatit may not be due to an anomalously thick sedimentary layer.It is very likely due to horizontal modes of resonance of theconcrete work set up to reinforce the stability of the parks, asthe largest park place (site a59) also has the broader reso-nance domain at low frequencies. A rough estimate of thefundamental frequency f0 for this l � 550 m long park placemay be made using l � λ=2, where λ is the wavelength(λ � vS=f0, where vS is S-wave velocity). It leads to fre-quencies on the order of 0.3–0.5 Hz, significantly lower thanthe resonance frequency of the natural sites. Horizontal re-sonances are also sometimes observed in narrow sedimen-

Figure 6. H/V spectral ratios determined from noise at some rock sites with no significant topography. ROC and a17 are on Cretaceouslimestone (with a cave at a17); a97, a30, and b43 are on Cretaceous flysch, and a57 is on ophite (see location of points in Fig. 5, except a97,which is located at the very northwest edge of the Lourdes district).


tary basins (e.g., Cornou et al., 2003); they may perhaps ex-plain some low-frequency bumps in the spectra, as observedat 0.3 Hz for a81 (Fig. 7).

The Saux Valley to the North-Northeast

This sediment-filled valley (Fig. 8a) north-northeastof Lourdes is the former bed of the Gave-de-Pau river, beforemoraine deposits deviated its flow to the west. A small creekis now running along this valley from north to south, withformer swamps at some places, which are now drained(Monge swamp, site a36). The valley is the main accessto the city, and many buildings of economical interest arelocated there (e.g., commercial centers, factories, a swim-ming pool, an aquarium). From a geological point of view,the uppermost formation is rather well known thanks to sev-eral boreholes drilled along the valley (Fig. 8a and 8c) andSASW experiments performed by BRGM (see locations onFig. 8a). SASW results are given in Table 1. They reveala structure that has great similarities to the north (near sitea25) and to the very south (near site b13) but that is verydifferent in the Monge swamp. The sediment thickness isgreatest along the axis of the valley and decreases to zeroat the border. We thus expect responses dependent on theposition of the sites within the valley (see, e.g., King andTucker [1984]). Figure 8b shows a series of H/V spectraalong the valley axis. They exhibit a great variability fromnorth to south. Sites to the north (a24, a25, and b44) and to

the very south (b13) generate two peaks, one at 3–5 Hz, theother at 10–20 Hz; these peak positions are well predicted bymodeling from the vS profile (see next section). For the sta-tions in the central part of the profile (a22, a23, a36, anda26), the spectra include lower frequencies in the range of1–2 Hz. Basin-edge-induced surface waves (Kawase andAki, 1989; Narayan, 2005) are very likely at the origin ofthis energy at low frequency, as has also been identifiedin the Grenoble Basin in the Alps (Cornou et al., 2003). Notealso the sharp resonance peak at a low frequency inside theMonge swamp (site a36, fR � 0:9 Hz) and the relatively flatresponse for b15, located on a small promontory.

Two parameters can contribute to the spatial variationsobserved on the H/V spectral ratio within the Saux valley: theeffect of the topography (considering both the shape of thesediment–bedrock interface and the shape of the hills) andthe spatial variations of sediment properties. Several theore-tical studies have aimed to model the topographic effect of avalley on S-wave or Rayleigh-wave propagation (e.g., Kingand Tucker, 1984; Geli et al., 1988; Kawase and Aki, 1989;Sánchez-Sesma and Campillo, 1993; Savage, 2004). In theabsence of filling by sedimentary layers, the horizontalmotion of the Rayleigh wave is strongly deamplified atthe bottom of valleys, compared to a flat half-space, forwavelengths of the same order of magnitude as the valleywidth (Savage, 2004). On the other hand, the horizontal com-ponent of the S wave exhibits a complex amplification–deamplification pattern, which is nonsymmetrical about

Figure 7. H/V spectral ratios determined from noise for sites along the Gave-de-Pau river. (a) Natural sites: the peaks are due to thevertical resonance in the sedimentary layers; (b) Sites on park places: low-frequency ratios are possibly due to horizontal resonance modes(see location of points in Fig. 5).


the valley axis, depending on the direction of wave arrival,on the nature of the incident wave (SV or SH), and on thewave incidence (Sánchez-Sesma and Campillo, 1993). How-

ever, the filling of the valley seems to contribute more toamplitude perturbation than to topography (Geli et al.,1988). The spatial variability of the H/V spectral ratio in

Figure 8. (a) H/V experiment along the Saux valley to the north-northeast of Lourdes (see location in Fig. 5) with sampled sites, dots;soundings, diamonds; SASW experiments, triangles; and the northern segment A′B of the geological cross section AB. (b) Some of therepresentative H/V spectral ratios determined from noise. (c) Geological cross section with indication of the soundings and locations ofsites. Topographic background is from the Institut Géographique National.


the Saux valley may then be primarily related to the spatialvariability of the geological layers.

The Sarsan Hill

The Sarsan hill is a topographic feature located to thenortheast of the city (Fig. 9a). It is made of Campanian

(Upper Cretaceous) flysch. It culminates at 200 m abovethe valley floor and has a half-width of 900 m (Fig. 9b).Houses are built on the southwest flank of the hill, whichis the only one that could be sampled with an easy access.

The H/V spectral ratio (Fig. 9c) exhibits a systematictrend from the base (b11) to the top (a21). At the base, aweak high-frequency peak (∼7 Hz) is observed. In the lowerhalf of the hill, this peak broadens and presents lower fre-quencies with increasing height. In the upper half of the hill,the peak weakens and the spectrum loses its low-frequencycontent. At 50 m from the summit, the spectrum becomesalmost flat. Note, however, that for this last point (a21)the ratio is 2 and not 1, as it would normally be for rockysites in flat areas.

The complex amplification and deamplification patternobserved on the Sarsan hill has been compared to simulationsof topographic effects on hills. Most of the existing modelsconcern the amplitude of the horizontal Smotion, not the H/Vratio. They predict an amplification of the horizontal signal athill tops for both SH waves and Rayleigh waves (e.g., Geli

Table 1Mean Uppermost Structure in the Saux Valley to the North-

Northeast of Lourdes (after BRGM [2006])

Rock Type Thickness (m) vS (m s�1)

North Sand and gravels 10 300Gravels 26 630Substratum — 860

Monge swamp Peat 8.4 220Mud 8.6 170Substratum — —

South Sandy loam 5 300Clayey mud 8 650Substratum — 840

Figure 9. Results for the Sarsan hill, to the northeast of Lourdes (see location in Fig. 5). (a) Sampled sites of the H/V experiment, dots;soundings, diamonds; and SASW experiment, triangle. (b) Topographic profile: origin of distances corresponds to the hill foot. (c) H/Vspectral ratios. Topographic background is from the Institut Géographique National.


et al., 1988; Sánchez-Sesma and Campillo, 1993; Bouchonet al., 1996; Savage, 2004). Amplification at the top is usual-ly observed for wavelengths comparable to the hill width,which corresponds to the fundamental transverse oscillatoryresonance mode of the hill (Geli et al., 1988). The Rayleigh-wave model of Savage for a ridgelike topography predicts anincrease of horizontal amplitude and a reduction of verticalamplitude at the ridge top, thus a high H/V ratio. At the sametime, it predicts a deamplification at the base of the hill. Thisis very different from our observations.

As noted by Bouchon and Barker (1996), the responseof the hill is very sensitive to topographic details, and it isdifficult to model the observed resonance pattern for real si-tuations. For a structure somewhat similar to the Sarsan hill,but with 10 times the reduction in size (height 20 m; half-width 90 m) and also a small plateau at the top, Bouchonand Barker (1996) performed a numerical simulation withSH-waves input. At low frequencies, they predict amplifica-tion at the hill top whereas, for high frequencies, they predicta deamplification at the top and amplification in the upperpart of the flank. We observe a more complex pattern. Thisdiscrepancy between this model and our observations couldbe due to the fact that urban noise includes mostly Rayleighwaves, so that these modeled results cannot be easily com-pared to our field observations.

The Anclades-Pic-du-Jer Profile

This profile crosses the valley to the east and part of themountain to the southeast. For reasons of site access, the pro-file bends and does not follow the steepest slope of themountain (Fig. 10a). The highest point (a84) is therefore lo-cated at a pass rather than on the summit. The mountain ismade of folded Cretaceous marls and reef limestone, withsome glacial deposits to the east. The valley floor at the bot-tom is flat and filled with sediments. Four SASW profileswere performed in the middle and on the edges of the valley.The mean structure includes a 10-m thick sand layer over-laying a 35-m thick compact loam (Table 2). The substratumis reached at a depth of 37 m to the south of the valley and at45 m in its central part.

The H/V spectral ratios exhibit a clear single peak for thesites located in the valley. The central frequency of this peakdecreases from 5–7 Hz for the points located at the border ofthe valley (b22 to the north and a35 to the south) to about1 Hz for the point located close to the valley axis (a33), inagreement with the increasing sediment thickness. At thesame time, the peak width broadens, suggesting reverbera-tions due to a more complex sediment stack, to 3D effects,or to waves bouncing off the basin edges.

For points sited on the mountain flank, the spectra arecomplex and vary from one site to the other, with two to threepeaks in the frequency range of 0.2–20 Hz. Such amplifica-tion variations with oscillations are expected even for homo-geneous mountains, at least for an incident SH wave (Geliet al., 1988). Site a83 may be affected by the presence of

glacial deposits, whereas the other sites are on hard rock.The peak at about 1.5 Hz (sites a84 and a85) correspondsto the resonance of the mountain in its transverse direction.As for the points located along the river, a rough estimate ofthe resonance frequency f0 may be made using the relation-ship l � λ=2, where l is the mountain width and λ � vS=f0.With a mean P velocity of 2:8 km sec�1 in the limestone(Press, 1966), a vP=vS ratio of 1.7, and a mountain widthof 1.5 km (estimated at altitude 550 m), we getf0 � 1:1 Hz, which is close to the observed value. However,we do not have enough knowledge of the 3D-geologicalstructure to model its response with any confidence.

1D Modeling of H/V with Synthetic Seismograms

Modeling may help to better estimate the sensitivity ofthe H/V ratio to small structure perturbations. It allows us tocheck the ability of the H/V ratio to predict the effective re-sonance frequencies and amplitudes. It also gives insights onthe nature of the urban noise.

We have noted previously that the simple relation fR �vS=4h holds true only for a vertical resonance inside a sin-gle layer with a strong impedance contrast with the substra-tum. If this is not the case, or if several sediment layers arepresent, more sophisticated modeling is necessary. We haveperformed two kinds of modeling: a modeling of the H/Vratio based on the computation of noise from synthetic seis-mograms and a computation of the H/Href ratio of S wavesbased on the vertical propagation of a real event though asoil profile.

Methods

Generation of Noise from Synthetic Seismograms

Following Lachet and Bard (1994), urban noise is gen-erated from a summation of synthetic seismograms. It allowsus to generate simultaneously surface modes and bodywaves, which probably have a nonnegligible contributionas noted previously for bedrock sites.

Noise has been generated by summing 1000 syntheticseismograms with a 1024-points length and a sampling rateof 125 Hz (8.2 sec long elementary seismograms). For a gi-ven structure, each elementary seismogram is generatedusing the reflectivity method (Müller, 1985), with parameterschosen randomly inside specified domains. The variableparameters are the distance, the type of source, the far-fieldshape, the dominant frequency, the amplitude, and the timedelay. The sources are randomly distributed around the site atdistances between 100 and 500 m. As in the field experiment,we have avoided sources too close to the recording station.Sources are either explosions or single vertical forces. De-spite the fact that shear sources are supposed to be marginallypresent in the generation of urban noise, we also performsimulations including a proportion of pure shear sourcesin order to simulate conversions from compressional energy


to shear energy at scatterers and at basin edges. Three shapesof far-field sources are possible: a single pulse, a single os-cillation, or a three-arch oscillation. Their dominant periodranges from 0.03 to 0.5 sec with a logarithmic distribution,in order to generate more high frequencies than low frequen-cies, like in real noise. The summation of the 1000 seismo-grams is performed ascribing an amplitude between�1 and 1and a time delay between 0 and 4096 points (0 to 32.8 sec) toeach of them. The length of the synthetic records obtained bysummation is 4096 points, as for the experimental ones. Tendifferent noise series of 4096 points have been generated in

this way for each structure and have been processed exactlyin the same way as the experimental data to obtain a meanand standard deviation of each spectrum.

Propagation of an Earthquake through a Soil Profile

Another approach to modeling site effect is to propagatevertically a real or synthetic earthquake signal (used as inputon bedrock at the bottom of the soil column) through the1D known structure and to analyze the output signal atthe surface. A nonlinear 1D dynamic model was developed

Figure 10. Results for a profile perpendicular to the Anclades valley to the east (see location in Fig. 5) with an uphill segment along thePic-du-Jer. (a) Topographic map with H/V sites, dots; soundings, diamonds; and SASW experiments, triangles. (b) Topography along theprofile. (c) H/V ratio for some of the sites, showing the decrease of the peak frequency in the middle of the valley and the complex response onthe hill flank. Topographic background is from the Institut Géographique National.


at BRGM to simulate the response of soil profiles undervarious hydraulic conditions (e.g., Bernardie et al., 2006);it is implemented in a commercial piece of software, Cyber-Quake (Modaressi et al., 1995). The main advantage of thisapproach is to take into account the mechanical soil proper-ties at the origin of nonlinear responses of the soils, givinga realistic simulation of soil response during both weakand strong earthquakes. The results allow us to estimatethe H/Href ratio (i.e., the transfer function of the sedimentlayer) using the S-wave input signal, at the bottom of thecolumn, as reference. In our case, output signals were usedto compare the experimental H/V response with the predictedH/Href resonance frequencies for earthquakes, thus, to eval-uate the credibility of our analyses for seismic-risk assess-ment. The chosen input is an event of M 5.0 (17 Novem-ber 2006) recorded at a distance of 20 km from Lourdesat a rock site station (PYLS) without site effect (Drouetet al., 2005).

Modeling Results

All the results we present in the following discussionconcern geological structures without topography. Forwardmodeling of synthetic spectral ratios based on SASW resultswill be compared to the experimental spectral ratios obtainedat nearby sites.

Bedrock Sites

Synthetic noise has been computed for the rock modelshown in Figure 11a (right). The H/V ratio computed for thismodel is close to 1 at any frequency (Fig. 11a, left), in agree-ment with the experimental values. The weak frequency de-pendence of the modeled H/V ratio is due to arbitrarymultilayering in the substratum of our model, which is intro-duced for computational reasons.

As previously mentioned, the H/V ratio expected for apure fundamental Rayleigh mode is close to 0.7 (Rayleigh-wave ellipticity) at typical bedrock sites (considered as ahalf-space) with Poisson’s ratios in the range of 0.23–0.27. For the fundamental Rayleigh mode, an ellipticity closeto unity (H=V � 1) may be obtained only in the unrealisticcase of a negative Poisson’s ratio (Malischewski and Scher-baum, 2004). By contrast, S waves from local sources mayinduce large H/V values as well higher modes of Rayleighwaves (e.g., Harkrider, 1970; remember, however, that high-er modes are not excited in a real half-space). Therefore, one

has to invoke the presence of body waves in the noise toexplain the H/V ratio of 1. The independence of Rayleigh-wave ellipticity and body-wave incidence to frequency inthe case of a half-space explains why the H/V ratio is flatover the whole frequency domain. The introduction of ashear sources has no effect (dashed line in Fig. 11a), becauseLove waves are not generated in a half-space.

The Gave Bank at the Sanctuary

A 1D model is given down to 30 m by a SASW profile(Fig. 11b, right) performed close to site SAN (Fig. 2). Fig-ure 11b (left, gray line) shows the H/V ratio predicted by theSASW model when sources do not include shear (here, arbi-trary, 50% explosion, and 50% single vertical force; this bal-ance has no critical influence). The observed peak positionand the amplification close to 5 of the H/V ratio are ratherwell reproduced by the model. By contrast, the slow decreaseat low frequency is poorly reproduced if no shear is presentin the noise, due to a deficit of shear energy in the signal atlow frequencies. If sources include pure shear energy (here,one-third pure shear, one-third explosion, and one-third sin-gle force), the low-frequency trend is much better reproduced(dashed line), and at the same time the peak is slightly shiftedtoward lower frequencies. This fits very well with the obser-vations. The fit of the low-frequency trend is, however,rather sensitive to the proportion of shear energy.

The simulation of the H/Href ratio with CyberQuake(Fig. 12a, gray line) shows several peaks of resonance with,for the fundamental peak, an amplification that exceeds 10.The frequency of this peak is well expressed on the H/V ratioof SAN, but its amplitude is strongly underestimated. Thiswas also the conclusion drawn from the comparison of H/Vand H/Href on natural earthquakes (Fig. 3) at the same siteand for eight other sites in Lourdes. Note that the H/Hrefratio shape predicted from CyberQuake is significantly dif-ferent from that measured from earthquakes (dashed line),where there is a plateau between 1.5 and 20 Hz, as observedin the case of dipping interfaces (a case which cannot be si-mulated by 1D modeling). However, the maximum ampli-tudes of the two spectra are similar.

The Saux Valley to the North

The numerous SASW profiles performed in the valley(Fig. 8) have revealed a great variability in the thicknessand nature of the top 50 m (Table 1); thus, a 1D modelingmay not be appropriate. We present here the results for onlythe northernmost point (a24). The corresponding SASW pro-file (Fig. 11c, right) is defined down to 37 m, but it presents avelocity inversion near 10 m, so that its reliability at a greaterdepth may be poorer than for the sanctuary. The agreement ofthe model with the H/V observations (Fig. 11c, left)is, however, globally satisfactory. The experimental low-frequency peak position is well reproduced, but its ampli-tude is slightly overestimated. Again, the inclusion of shear

Table 2Mean Uppermost Structure in the Anclades Valley to the East (after

BRGM [2006])

Rock Type Thickness (m) vS (m sec�1)

Sand 10 240Loam 35 445Substratum (Upper Cretaceous flysch) — 800


Figure 11. Left: Modeling of the H/V ratio from urban noise generated by a summation of a large number of synthetic seismograms withrandomly defined characteristics. Only compressional sources, gray line; compressional and shear sources (see text), dashed line; the 1σconfidence level is reported. Observed H/V ratio, black line. Right: vS structure deduced from SASW profiles. (a) Rock site; (b) sanctuary;(c) Saux valley; (d) Anclades basin.


source signals as for the sanctuary (dashed line) leads to abetter fit of the observations at low frequencies; moreover,it generates the peak at 15 Hz, but with an amplitude muchlower than the observed one.

The simulation of the H/Href ratio from CyberQuakeagain reveals that the H/V ratio is able to predict the positionof only the lowest frequency peak and that it underestimatesthe expected amplification.

The Anclades Basin to the East

Three SASW profiles are available (Fig. 10), but LOU02does not exceed 15 meters, and LOU25 exhibits poorly con-trolled velocity inversions. We thus considered LOU01,although it is located at the basin’s edge. This profile is de-fined down to 37 m (Fig. 11d, right). As for the sanctuary,the agreement between the observed and the predicted H/Vratio is quite good when shear sources are included in thenoise (Fig. 11d). The H/Href simulation (Fig. 12) againshows very nicely that the H/V ratio detects correctly the re-sonance peak at the lowest frequency but that it underesti-mates the resonance amplitude.

Synthesis and Discussion of H/V Results

In the previous sections, we have presented the resultsfor a few particular geological and topographic structures ofinterest in Lourdes. We have also shown that some of theobserved features of the H/V spectra may be correctly repli-cated with simple 1D models but that in many cases model-ing is limited by complex 3D structures that are poorlyknown. Thus, site effects for seismic-risk evaluation will stillrely on experimental results.

A global analysis of the results is made on the basis ofthe frequencies and amplitudes of the highest peaks observedin the spectra. Figure 13 gives a map of the H/V sites withthese spectrum characteristics. Stations without site ef-fects are those for which amplitude remains below 2 overthe whole spectrum (0.5–20 Hz). For the other sites, ampli-tudes (amp) are represented with two classes: amp < 5

and amp ≥ 5. Five classes are considered for the dominantfrequency: the highest frequency domain being for f >10 Hz, and the lowest one being for f < 2 Hz. Roughly,the highest frequency range corresponds to a frequency do-main that has almost no impact on buildings; the lowest fre-quency range concerns five-storey and taller buildings; andthe other classes concern individual houses and small build-ings. This statement is based on the relationship fR � 10=N,where fR is the resonance frequency (in Hz) of the struc-ture and N is the number of storeys, a relationship whichholds for only one standard type of reinforced concrete build-ing. For seismic-risk assessment, we have to keep in mindthat the H/V method gives the position of only the lowestfrequency peak and that the H/V peak amplitude is lower thanthe real soil amplification (Figs. 3 and 12). Additional infor-mation, concerning in particular geology and seismic groundvelocities, are thus necessary to establish maps of seismicrisk (BRGM, 2006). Here, we will limit our analysis to theH/V spectra.

Figure 13 reveals large zones where coherent featuresare observed. The points without site effect (squares) or witha very small amplification (open circles) are mostly locatedon rock areas to the north and southwest of the city. In thecity center, large amplifications in the frequency range of

Figure 12. H/Href ratios of S waves modeled from the propa-gation of an earthquake through the soil profiles given in Figure 11,gray line, compared to the experimental H/V ratios, black line.For the sanctuary, the experimental H/Href ratio is also reported,dashed line.


1–4 Hz are generally observed (see also Fig. 3), but the am-plifications tend to decrease to the edges of the basin. Lowresonance frequencies with moderate amplifications (smallblack dots) are also observed to the west-northwest of thecity along the Lourdes lake, which is contained in a basinof thick recent deposits. Results in the valley to the north-northeast are very scattered, with a predominance of reso-nance frequencies in the range of 4–6 Hz, but again weobserve generally smaller amplifications at the borders ofthe valley. It may be related to a thinning of the sedimentarylayers. Points immediately along the river correspond tolarge amplifications with various resonance frequencies.Sites south of the river along the east–west oriented segmentof the river bed exhibit rather homogeneous characteristics,with moderate amplifications (amp < 5) in the frequencyrange of 2–6 Hz. From a general point of view, the regionswith similar spectra are also those with similar geologicalcharacteristics; the topography seems to be of secondaryimportance.

Conclusion

In the city of Lourdes, H/V measurements performed onurban noise have been used to get a dense spatial sampling ofsite effects. They come in complement to H/Href measure-ments, applied to Swaves of natural earthquakes, which werepreviously obtained at a few places in the city. If the H/Vmethod is easy and rapid to implement, it has, however,some drawbacks. Its limitations for predicting the site re-sponse during earthquakes is clearly shown by the compar-ison of H/V ratios with experimental and synthetic H/Hrefvalues: H/V spectral ratios predict correctly the position ofthe lowest resonance frequency peak but not the peak widthnor the amplification value, which is generally underesti-mated. It is, however, a valuable tool for seismic-risk assess-ment so long as it is used in combination with other methods.Because of the difficulty to perform reliable numerical mod-eling, due to the natural complexity of surficial geology, theH/V measurements on noise will long be the most efficientmethod to quantify soil responses inside cities. In the case of

Figure 13. Summary of the results of the H/V ratios with indication of the dominant frequency, depicted by the level of gray, and the H/Vamplitude at this frequency, depicted by the size of the dot. Sites for which a resonance frequency could not be defined, squares. Topographicbackground is from the Institut Géographique National.


1D simple geological settings, two numerical methods basedon (1) the modeling of urban noise from synthetic seismo-grams, and (2) on the propagation of earthquake data througha 1D soil column show good coherency between geologicaldata, SASW results, and H/V measurements.

In the case of Lourdes, we have been able to identifyregions with similar characteristics of the H/V ratios corre-sponding generally to regions with similar geological char-acteristics. The comparison between the simulation resultsand the different kind of available observations or experi-mental results (e.g., H/Href, SASW profiles, and soundings)shows that, despite their limitations, the measurements of theH/V-noise ratios are very informative for seismic-risk evalua-tion because they provide a confident estimate of the reso-nance frequency. They are thus a key input in urban seismicmicrozonation, in the elaboration of a large earthquake sce-nario, and in seismic hazard mitigation.

The H/V method remains mostly empirical: first becausethe input signal is still poorly understood and second becausethere have been few theoretical studies of standard situationssuch as hills, valleys, cliffs, karsts, etc. We have shown from1D modeling with synthetic seismograms at a few sitesthat the urban noise cannot be reduced to the fundamentalRayleigh mode. Noise includes body waves, higher modesof surface waves, and an important contribution of shear en-ergy. A complete modeling of the observations will requirenot only an accurate knowledge of the 3D structures and to-pography, but also a complete model of all the waves thatpropagate through these structures.

Acknowledgments

Financial support has been provided by the French Ministry of Ecol-ogy, by the Bureau de Recherches Géologiques et Minières, and by the cityof Lourdes. We thank Sébastien Benahmed and Stéphane Drouet for theirparticipation in the field experiments and the city of Lourdes for its logisticalhelp. A special thank you to Georges Delpont for his interpretation of thegeological data. Many thanks also to two anonymous reviewers and to theAssociate Editor Arben Pitarka for their helpful comments on the manuscriptand to Thomas Dewez for his help in correcting it. Topographic maps arefrom the French Institut Géographique National (IGN), Saint-Mandé, France(Convention 8869/IGN-BRGM); Geological maps are from the Bureau deRecherches Géologiques et Minières (BRGM), Orléans, France.

References

Bard, P.-Y. (1999). Microtremor measurements: a tool for site effect estima-tion? in The Effects of Surface Geology on Seismic Motion, K. Irikura,K. Kudo, H. Okada and T. Sasatani (Editors), Balkema, Rotterdam.

Bernadie, S., E. Foerster, and H. Modaressi (2006). Non-linear site responsesimulations in the Chang-Hwa region during the 1999 Chi-chi earth-quake, Taiwan, Soil Dyn. Earthq. Eng. 26, 1038–1048.

Bonnefoy-Claudet, S., F. Cotton, and P.-Y. Bard (2006). The nature of noisewavefield and its applications for site effects studies. A literature re-view, Earth Sci. Rev., 79, 205–227, doi 10.1016/j.earscirev.2006.07.004.

Borcherdt, R. D. (1970). Effects of local geology on ground motion nearSan Francisco Bay, Bull. Seismol. Soc. Am. 60, 29–81.

Bouchon, M., and J. S. Barker (1996). Seismic response of a hill: the ex-ample of Tarzana, California, Bull. Seismol. Soc. Am. 86, 66–72.

Bouchon, M., C. A. Schultz, and M. N. Toksöz (1996). Effect of three-dimensional topography on seismic motion, J. Geophys. Res. 101,5835–5846.

Bureau de Recherches Géologiques et Minières (BRGM) (2006). BRGM/RP-53846-FR Microzonage sismique de Lourdes, Rapport Final.222 pp.

Choukroune, P. (1992). Tectonic evolution of the Pyrenees, Annu. Rev. EarthPlanet. Lett. 20, 143–158.

Cornou, C., P. Y. Bard, and M. Dietrich (2003). Contribution of dense arrayanalysis to the identification and quantification of basin-edge-inducedwaves, part II: Application to Grenoble basin (French Alps), Bull. Seis-mol. Soc. Am. 93, 2624–2648.

Drouet, S., A. Souriau, and F. Cotton (2005). Attenuation, seismic moments,and site effects for weak-motion events: application to the Pyrenees,Bull. Seismol. Soc. Am. 95, 1731–1748.

Dubos, N. (2003). Contribution à l’évaluation du risque sismique dans lesPyrénées centrales, Ph.D. Thesis, University Paul Sabatier, ToulouseIII, 211 pp.

Dubos, N., A. Souriau, C. Ponsolles, and J. F. Fels (2003). Etude des effetsde site dans la ville de Lourdes (Pyrénées, France) par la méthode desrapports spectraux, Bull. Soc. Géol. Fr. 174, 33–44.

Field, E. H., and K. H. Jacob (1995). A comparison and test of various site-response estimation techniques, including three that are not reference-site dependent, Bull. Seismol. Soc. Am. 85, 1127–1143.

Geli, L., P.-Y. Bard, and B. Jullien (1988). The effect of topography onearthquake ground motion: a review and new results, Bull. Seismol.Soc. Am. 78, 42–63.

Harkrider, D. G. (1970). Surface waves in multilayered elastic media, part II:Higher mode spectra and spectral ratios from point sources in planelayered Earth models, Bull. Seismol. Soc. Am. 60, 1937–1987.

Hirn, A., M. Daignières, J. Gallart, and M. Vadell (1980). Explosion seismicsounding of throws and dips in the continental Moho, Geophys. Res.Lett. 7, 263–266.

Kawase, H., and K. Aki (1989). A study on the response of a soft basin forincident P, S and Rayleigh waves with special reference to the longduration observed in Mexico City, Bull. Seismol. Soc. Am. 79,1361–1382.

King, J. L., and B. E. Tucker (1984). Observed variations of earthquake mo-tion across a sediment-filled valley, Bull. Seismol. Soc. Am. 74,136–151.

Kulhánek, O. (1990). Anatomy of seismograms, in Developments in SolidEarth Geophysics, Vol. 18, Elsevier, Amsterdam, 178 pp.

Lachet, C., and P.-Y. Bard (1994). Numerical and theoretical investigationson the possibilities and limitations of Nakamura’s technique, J. Phys.Earth 42, 377–397.

Lambert, J., A. Levret-Albaret, M. Cushing, and C. Durouchoux (1996).Mille Ans de Séismes en France, Ouest Editions, Nantes, France,70 pp.

Lermo, J., and F. J. Chavez-Garcia (1993). Site effect evaluation using spec-tral ratios with only one station, Bull. Seismol. Soc. Am. 83, 1574–1594.

Levret, A., J. C. Backe, and M. Cushing (1994). Atlas of macroseismic mapsfor French earthquakes with their principal characteristics, Nat. Ha-zard 10, 19–46.

Malischewsky, P. G., and F. Scherbaum (2004). Love’s formula and H/V-ratio (ellipticity) of Rayleigh waves, Wave Motion 40, 57–67.

Medvedev, S., W. Sponheuer, and V. Karnik (1967). Seismic intensity scaleversion 1964, in Inst. Geody. Publ., Vol. 48, Inst. Geody., Jena,Germany.

Modaressi, H., E. Foerster, D. Aubry, and A. Modaressi (1995). Researchand professional computer-aided dynamic analysis of soils, FirstInternational Conference on Earthquake Geotechnical Engineering,Tokyo, Japan, 1171–1176.

Müller, G. (1985). The reflectivity method: a tutorial, J. Geophys. 58, 153–174.


Nakamura, Y. (1989). A method for dynamic characteristics estimation ofsubsurface using microtremor on the ground surface, Q. Rep. Railw.Tech. Res. Inst. 30, no. 1, 25–33.

Narayan, J. P. (2005). Study of basin-edge effects on the ground motioncharacteristics using 2.5-D modelling, Pure Appl. Geophys. 162,273–289.

Nocquet, J. M., and E. Calais (2003). Crustal velocity field for Western Eur-ope from permanent GPS array solutions, 1996–2001, Geophys. J. Int.154, 72–88.

Press, F. (1966). Seismic velocities, in Handbook of Physical Constants,S. P. Clark (Editor), Publications of the Geological Society of Amer-ica, Vol. 97, Yale Univ. Press, New Haven, Connecticut, 195–218.

Rigo, A., A. Souriau, N. Dubos, M. Sylvander, and C. Ponsolles (2005).Seismotectonic interpretation of a microseismic analysis in the centralpart of the Pyrenees (France), J. Seism. 9, 211–222.

Sánchez-Sesma, F. J., and M. Campillo (1993). Topographic effects for in-cident P, SV and Rayleigh vaves, Tectonophys. 218, 113–125.

Savage, W. Z. (2004). An exact solution of effects of topography on freesurface waves, Bull. Seismol. Soc. Am. 94, 1706–1727.

Souriau, A., and H. Pauchet (1998). A new synthesis of the Pyrenean seis-micity and its tectonic implications, Tectonophys. 290, 221–244.

Steidl, J. H., A. G. Tumarkin, and R. J. Archuleta (1996). What is a referencesite, Bull. Seismol. Soc. Am. 86, 1733–1748.

Tanimoto, T., and L. Rivera (2005). Prograde Rayleigh wave particule mo-tion. Geophys. J. Int. 162, 399–405.

CNRS, Laboratoire de Dynamique Terrestre et PlanétaireObservatoire Midi-Pyrénées14, Avenue Edouard Belin31400 Toulouse, France

(A.S., C.P.)

BRGM/ARN/RIS3 Avenue Claude Guillemin45060 Orléans, Cedex 2, France

(A.R.)

Manuscript received 25 October 2006


Education

Site effects in the city: Seismic Risk Evaluation