1Microzonation: techniques and examples

Corinne Lacave1, Pierre-Yves Bard2 and Martin G. Koller1

Affiliation: 1 Rsonance Ingnieurs-Conseils SA, Geneva, Switzerland2 LGIT, Universit Joseph Fourier, Grenoble, and LCPC, Paris, France

Key-words: Microzonation, site effects, topographic effects, standard spectral ratio, H/Vratio, Nakamura method, numerical simulations, empirical Green's functiontechnique

1. Introduction

It is now well known, and widely accepted amongst the earthquake engineering community,that the effects of surface geology on seismic motion exist and can be large. Two "famous"examples of such effects are San Francisco and Mexico City. In San Francisco, localamplifications over unconsolidated sediments have been shown to be responsible for intensityvariations as large as two degrees (MM scale) during both the 1906 "big" San Franciscoearthquake and the more recent 1989 Loma Prieta event. In Mexico City, there exist very softlacustrine clay deposits underneath the downtown area of the city. These led to very largeamplifications, which caused a high death toll and large economic losses during the distantGuerrero Michoacan earthquake of 1985. Nearly all recent destructive earthquakes (Spitak,Armenia 1988, Iran 1990, Philippines 1990, Northridge 1994, Kobe 1995, Armenia, Columbia1999, Turkey 1999...) have brought additional evidence of the dramatic importance of siteeffects. Accounting for such "site effects" in seismic regulations, land use planning or design ofcritical facilities thus became one goal of earthquake hazard reduction programs (Bard, 1997).

In the first section, the most important site effects are presented, namely amplification effectsrelated with sedimentary sites and strong lateral discontinuities, as well as motionamplifications related with surface topography. Then, experimental and numerical techniquesare presented that are available to estimate these site effects, in the framework of microzonationstudies. An application is finally presented for the Rhne valley in the Visp area, a narrowalpine valley in the canton Wallis, Switzerland.

2. Site effects

2.1 Effects of soft surface layers

It has been recognised for a very long time that earthquake damage is generally larger over softsediments than on firm bedrock outcrops. This is particularly important because most of urbansettlements have occurred along river valleys over such young, soft surface deposits. Manylarge cities located in earthquake prone areas may be given as examples: Los Angeles, SanFrancisco, San Salvador, Caracas, Lima, Bogota, Tokyo, Osaka and Kobe, Katmandu, Manila,Lisbon, Thessaloniki, Izmit, and, of course, Mexico City. But one should not forget many othermid-size, recently developed cities, especially in moderate seismicity areas, since they might bethe place for future, heavily damaging events, due to the combination of site effects and urbandevelopment.

2Soft soils may have, and did have indeed, dramatic consequences for the inhabitants of thosecities. They have however given engineering seismologists the opportunity to performnumerous macroseismic observations during the last century. These demonstrated very clearlythat the damaging effects associated with such soft deposits may lead to local intensityincrements as large as 2, in extreme cases even 3 degrees on the MM or MSK/EMS scale.

The fundamental phenomenon responsible for the amplification of motion over soft sedimentsis the trapping of seismic waves due to the impedance contrast between sediments and theunderlying bedrock. When the structure is horizontally layered (which will be referred to in thefollowing as 1-D structures), this trapping affects only body waves travelling up and down inthe surface layers. When the surface sediments form a 2-D or 3-D structure, i.e., when lateralheterogeneities such as thickness variations are present, this trapping also affects the surfacewaves which develop on these heterogeneities, and thus reverberate back and forth.

The interference between these trapped waves leads to resonance patterns, the shape and thefrequency of which are related with the geometrical and mechanical characteristics of thestructure. While these resonance patterns are very simple in the case of 1-D media (verticalresonance of body waves), they become more complex in the case of 2-D and a fortiori 3-Dstructures. An illustration of 1-D and 2-D effects is given in Figure 1.

2.2 Topographic effects

It has been often reported after destructive earthquakes that buildings located at hill topssuffered much more intensive damage than those located at the base (see example in Figure 2).There is also very strong instrumental evidence that surface topography considerably affects theamplitude and frequency contents of ground motion: reviews of such instrumental studies andresults may be found in Gli et al. (1988), Faccioli (1991) and Finn (1991). However, thenumber of instrumental studies about topographic effects is extremely low compared to studiesdealing with soft soil amplification, so that it remains impossible to derive any statistics fromthe existing data.

Theoretical and numerical models also predict a systematic amplification of seismic motion atridge crests, and, more generally, over convex topographies such as cliffs. Correspondingly,they also predict de-amplification over concave topographic features, such as valleys and thebase of hills. The amount of those effects have been shown (Pedersen et al., 1994) to be rathersensitive to the characteristics of the incident wavefield (wave type, incidence and azimuthangles). Theory also predicts complex amplification and de-amplification patterns on hillslopes, resulting in significant differential motions. Very often, measurements show largereffects than what is predicted by simulations. A possible explanation is that in many casesweathered rock is found at the top of hills that reinforces the topographic effect (Pedersen et al.,1994).

3. Methods for estimating site effects

3.1 Experimental methods

3.1.1 Standard spectral ratio (SSR)The most common procedure (the principle of which is illustrated in Figure 3) consists in com-paring recordings at nearby sites (where source and path effects are believed to be identical)through spectral ratios. These spectral ratios constitute a reliable estimate of site response if the

3"reference site" is free of any site effect, which means that it should fulfil the two followingconditions: First, it should be located near enough to the examined station to ensure thatdifferences between each site are only due to site conditions, and not to differences in sourceradiation or travel path, which is generally warranted when the hypocentral distance is largerthan about 10 times the array aperture (in case of necessity a ratio of 5 might still beacceptable). Secondly, it should also be unaffected by any kind of site effect, which is the casewhen the reference site is located on an unweathered, horizontal bedrock. These two conditionsprove to be rather restrictive in practice. The SSR technique, introduced first by Borcherdt(1970), is still widely used. Measurements are usually available only for events weaker muchweaker indeed - than the event engineers are interested in. Therefore, due to non-linearity, thesite effect estimated by SSR techniques may be considered as an overestimation or upper limitof the actual site effects at high frequencies, and, correlatively, a slight underestimation atfrequencies below the "elastic" fundamental frequency.

In a nutshell, the principle of this method may be described as follows: for a network of I siteshaving recorded J events, the amplitude spectrum of the ground motion Rij(f) recorded at site iduring the event j can be written as:

Rij(f) = Ej(f) . Pij(f) . Si (f) (1)with Ej(f): source function; Pij(f): path contribution between the source and the local site; andSi (f): local site contribution.

Written in logarithmic form, it leads to a simple linear equation:

ln (Rij(f) ) = ln (Ej(f)) + ln (Pij(f)) + ln (Si (f)) (2)The traditional spectral ratio technique corresponds to the fact where the path term Pij(f) isassumed to be site independent, i.e., when the distance to the reference site is small compared tothe source-to-site distance.

3.1.2 H/V noise ratio (or "Nogishi-Nakamura" technique)The H/V ratio, i.e. the ratio between the Fourier spectra of the horizontal and vertical com-ponents of ambient vibrations (often called "microtremors" or "ambient noise"), was introducedin the early seventies by several Japanese scientists (Nogoshi and Igarashi, 1971; Shiono et al.,1979; Kobayashi, 1980; Nakamura, 1989).

Although Nakamura's qualitative explanation looked at least questionable (as indicated inLachet and Bard, 1994, and again in Kudo, 1995), various sets of experimental data (Ohmachiet al, 1991; Lermo, 1992 ; Field and Jacob, 1993b; Duval et al., 1994; Duval et al., 1995; Fieldet al., 1995; Seekins et al., 1996; Lachet et al., 1996; Gitterman et al., 1996; Fh et al., 1997;Lebrun, 1997; Riepl et al., 1998) confirmed that these ratios are much more stable than the rawnoise spectra. In addition, on soft soil sites, they usually exhibit a clear peak that is well correl-ated with the fundamental resonant frequency. These observations are supported by severaltheoretical investigations (Field and Jacob, 1993b; Lachet and Bard, 1994; Lermo and Chavez-Garcia, 1994; Cornou, 1998), showing that synthetics obtained with randomly distributed, nearsurface sources lead to horizontal-to-vertical ratios sharply peaked around the fundamental S-wave frequency (Figure 4a), whenever the surface layers exhibit a sharp impedance contrastwith the underlying stiffer formations.

However, the first three studies mentioned avove also conclude that the amplitude of this peakis not well correlated with the S wave amplification at the site's resonant frequency (Figure 4b).Instead, it is highly sensitive to some parameters such as Poisson's ratio near the surface.Furthermore, Lachet and Bard (1994) proposed that the good match at the fundamental

4frequency is due to the horizontal-vertical polarisation of the Rayleigh waves, an interpretationthat is in agreement with the early Japanese studies (Kudo, 1995). According to this view, nostraightforward relation exists between the H/V peak amplitude and the site amplification.However, this opinion is not shared unanimously. For instance, a one to one average correlationis claimed by Konno and Ohmachi (1998) on the basis of a comparison between observed H/Vpeaks and numerical estimates of 1-D transfer functions. Furthermore, an empirical relationshipbetween H/V peak amplitude and local intensity increment (MM scale) was argued inToshinawa et al. (1997) from a strictly experimental viewpoint. A thorough comparison (Bardet al., 1997; Bard, 1999) between observed amplifications derived from earthquake records andobserved H/V peak amplitudes at more than 30 sites demonstrates that the latter is almostalways smaller than the former (Figure 5). Such an experimental result, although not yetexplained by theoretical or numerical work, despite the new computations performed by Cornou(1998), could be very useful indeed. It would mean that the H/V ratio technique could provide alower-bound estimate to the actual weak motion amplification. This view needs to beconfirmed, however, by a larger set of experimental data.

Other examples of estimates obtained with this technique are also displayed in Figure 6, forcomparison with other estimates. They show that H/V ratio technique allows obtaining, verysimply, the fundamental resonant frequency, but fails for higher harmonics, and that peakamplitude is somewhat different from amplification measured on spectral ratios.

In practice, the H/V ratios from ambient vibrations are sometimes "non-informative" so that nounequivocal interpretation is possible. This seems to be true particularly in the case ofpronounced 2D or 3D structures, i.e. in the presence of significant lateral heterogeneity.Nevertheless, the method proved to be the most inexpensive and convenient technique toestimate fundamental frequencies of soft deposits. It definitely deserves more research effortsso as to elucidate the factors influencing its reliability.

3.1.3 H/V spectral ratio of weak motionAnother simple technique consists in taking the spectral ratio between the horizontal and thevertical components of the shear wave part of weak earthquake recordings. This technique is infact a combination of Langston's (1979) "receiver-function" method for determining thevelocity structure of the crust from the horizontal to vertical spectral ratio (HVSR) ofteleseismic P waves, and Nakamuras proposal (1989, 1996) to use this ratio with recordings ofambient vibrations (section 3.1.2).

This method is obviously interesting, because of its simplicity and economy. It was first appliedto the S wave portion of the earthquake recordings obtained at three different sites in Mexico byLermo and Chavez-Garcia (1993). These recordings exhibit very encouraging similarities bet-ween the classical spectral ratios and these HVSR, with a good fit in both, the frequencies andamplitudes of the resonant peaks. The same technique has been also checked on various sets ofweak and strong motion data (see Chavez-Garcia et al., 1996; Lachet et al., 1996; Riepl et al.,1998; Theodulidis et al., 1996; Bonilla et al., 1997; Yamazaki and Ansary, 1997; Zar et al.,1999), from which several conclusions appear well established: The HVSR shape exhibits a very good experimental stability. It is also well correlated with surface geology, and much less sensitive to source and path

effects (a warning should be issued however for near field recordings of large events,because of the strong directionality of the near-source "fling" or "killer pulse").

However, comparisons with classical spectral ratios (including surface / down-hole record-ings), as well as with theoretical 1D computations (see also Lachet and Bard, 1994), alsoagree on the fact that the absolute level of HVSR depends on the type of incident waves. It

5follows that the determination of the absolute level of amplification from only HVSR is notstraightforward.

Field and Jacob (1995) also applied this technique in their systematic comparisons, and foundthat the method reproduces very well the shape of the site response, but underestimates theamplification level (see Figure 6e). They also found very different results when applying thistechnique to the P-wave part of the recordings. They therefore conclude that HVSR, whenapplied to the S-wave signals, reveals the overall frequency dependence. Based on our owninvestigations, we agree only partly with this conclusion, since in some cases we did find agood similarity in "spectral shapes", but in a few other HVSR we were only able to identify thefundamental resonance frequency. Furthermore, it should be pointed out that this technique hasbeen applied and checked for soft soil sites only, and might not be valid for other kinds of siteeffects.

3.2 Numerical methods

When the geotechnical characteristics of the site or of the area are known, site effects can be, inprinciple, estimated through numerical analysis. The prerequisite of a sufficient geotechnicalknowledge generally implies that such ground response analyses be made on a site by site basis,but the density of boreholes and geotechnical information in some large cities may be sufficientto allow a numerically-based zoning. Such an approach, however, requires an in-depthunderstanding both of the analytical models and of the numerical schemes that are used. Whenthe required expertise is lacking, it may occur that sophisticated numerical analyses lead to lessreliable results than simpler and cruder, but more robust, approximations. Thus, the presentsection will mainly emphasize the latter ones.

3.2.1 One-dimensional response of soil columnsThere exist a number of simple analytical methods which allow computation of the seismicresponse of a given site with only the help of a small personal computer. Amongst these, themost widely used makes use of the multiple reflection theory of S waves in horizontally layereddeposits, very often referred to as "1-D analysis of soil columns".

Such a soil column is excited by an incoming plane S wave, generally considered as verticallyincident, and corresponding to a surface bedrock motion representative of what is likely tooccur in the area. The specific parameters required for such an analysis are shear-wave velocity,density, damping and thickness of each layer. These parameters may be obtained either throughdirect in situ measurement, or from drillings and subsequent laboratory measurements, or fromknown approximate relationships with other, more usual geotechnical parameters such as theSPT number from standard penetration tests.

These analyses may be performed considering either a linear or a non-linear behaviour for thesoil. The non-linearity is very often approximated by a "linear-equivalent" method that uses aniterative procedure to adapt the soil parameters (i.e., rigidity and damping) to the actual strain itundergoes, according to the curves depicted in Figure 7. The SHAKE program is one of themost widely used for such calculations (Schnabel et al., 1972).

Recently developed packages incorporating true non-linear constitutive models are now avail-able, that also allow to account for liquefaction phenomena (example: CyberQuake program,1998). However , these non-linear analyses require a quantitative knowledge of the actual non-linear material behaviour, which can only be obtained by means of sophisicated laboratory tests.Some generic average curves have been proposed for different types of material, as sand or

6clay, but the actual behaviour of a given soil at a given site may strongly depart from theseaverages. This was precisely the case in Mexico City where the clays proved indeed to behavealmost linearly despite the large strains experienced during the 1985 event, while they werepreviously believed to be highly non-linear because of their very low rigidity.

3.2.2 Advanced methodsAlthough all numerical methods have the same base i.e. the wave equation many differentmodels have been proposed to investigate several of the various aspects of site effects, whichinvolve complex phenomena. For example, one has to consider various types of incident wavefields (near-field, far-field, body waves, surface waves); the structure geometry may be 1-D, 2-D or 3-D; or the mechanical behaviour of earth materials may encompass a very wide range(viscoelasticity, non-linear behaviour, water-saturated media, liquid domains, etc.). Typically,these advanced methods may be classified into four groups: Analytical methods, which can be used only for very simple geometries, are extremely

valuable as benchmarks. Ray methods, which are basically high frequency techniques. It is uneasy to use them when

wavelengths are comparable to the size of heterogeneity, a situation which is generally themost interesting one.

Boundary based techniques (including all kinds of boundary integral techniques, or thosebased on wave function expansions), which are the most efficient when the site of interestconsists of a limited number of homogeneous geological units.

Domain based techniques (such as finite-difference or finite-element methods), which allowto account for very complex structures and material behaviour, but are very heavy from acomputational viewpoint.

As stressed by Aki and Irikura (1991), "() these methods have their advantages anddisadvantages, and, in general, those which can deal with more realistic models are lessaccurate, while those achieving a higher accuracy are more time consuming. Most of thesemethods are still actively developed, because each has its own merit that effectively applies to acertain class of problems ()". It is not our aim in this section to elaborate on the intrinsicreliability of each numerical technique, but simply to discuss the applicability of numericalmethods, considered as a whole, to the estimation of site effects for engineering purposes.

Although these methods need heavy computational processes, their main advantage rests intheir flexibility and versatility (combined with their cheapness on standard computers), whichhave lead to significant breakthroughs in the understanding of site effects during the last twodecades. Not only they allow to carry out phenomenological and parametric studies, they canalso be used to assess the uncertainty in a sites seismic response, given the imperfectknowledge regarding the mechanical and geometrical characteristics of the considered site.Advances could be achieved along two avenues: Proper consideration of diffraction effects in complex surface or subsurface topography.

There now exist many numerical techniques to account for 2-D or even 3-D geometries, asthe Akil-Larner modelling for example (Aki and Larner, 1970), or the combined modesummation and finite-difference technique proposed by Fh and Suhadolc (1994). However,because of computational limitations, as well as lack - at reasonable expenses - ofsufficiently detailed knowledge of the underground structure, analyses with true 3-D modelsare generally restricted to low frequencies (typically below about 1 Hz).

More realistic modelling of strong non-linear behaviour in soft soils, and especially saturatedsands, as done by the program CyberQuake for example (CyberQuake, 1998).

7Nevertheless, the routine application of numerical models for the evaluation of site effectsraises some concerns: Until recently, numerical models had been only very rarely objectively tested for their ability

to predict actual effects. Indeed, comparisons between observations and theoreticalcomputations had almost always been carried out a posteriori : the predictors knew what tofind.

Numerical methods are not panacea, but can be applied only to some limited cases.Unfortunately, individuals who lack an understanding of their limits may use the softwareimplementing these methods outside of their validity domain, which may consequently leadto wrong estimations.

Even when the methods are properly used, their actual cost may be high, perhaps well in excessof the cost using instrumental means. The reason is that they require detailed geotechnical orgeophysical investigations for the site to provide the constitutive properties needed as inputparameters. This issue may sometimes be overcome through parametric studies, but this isuseful only when the results do not exhibit too much sensitivity (which is rarely the case).

3.2.3 Empirical and semi-empirical methodsEmpirical attenuation laws

Many empirical attenuation laws have been derived on the basis of available strong motionrecordings. They all relate a given ground motion parameter (pga, pgv, Sa, duration, Ariasintensity, etc.) to the magnitude and distance of the seismic event, and they also very often takeinto account a site parameter. Very often this site parameter is simply a binary descriptor, suchas "rock" and "non-rock". Only rarely is the site geology characterized in a more refinedmanner, for instance with distinction between thin and thick deposits, or with S-wave velocityvalues : the reason is that detailed information on strong motion recording sites is generallymissing. Significant efforts are made throughout the world, however, to fill this gap : the moststriking example is the K-NET network installed in Japan after the Kobe event, for which a20 m deep borehole has been drilled at each of the 1000 sites, and the S and P wave velocityprofile has been obtained.

It is thus possible to modify the ground motion parameters according to the site geology.However, as these modifications are based on a very crude classification of soils, and onstatistical studies which, in essence, smooth out the extreme values, such an approach may leadto a dangerous underestimation of amplifications at sensitive sites. Conversely, there is asignificant probability of overestimating the motion at common sites.

Empirical Green's functions technique (EGF)The empirical Green's functions (EGF) technique is known essentially in the seismologicalcommunity, as a tool for studying the source process of past large earthquakes using recordsfrom both mainshock and aftershocks (e.g. Mueller, 1985 ; Courboulex et al., 1998). It is farless known in the engineering community, where its potential to predict the expected strongground motion during future large events has not yet been sufficiently exploited. A key featureof the EGF technique is its capability to synthesise physically realistic, site specific accelerationtime histories. This is of particular interest for the seismic analysis of critical facilities.

Since critical facilities should withstand strong earthquakes with long return periods, typicallyof the order of 10 000 years or more, it is common that no recording of such an event isavailable for the facility's site. This lack of appropriate time histories can elegantly be overcomewith the aid of the EGF technique. Its basic idea is to interpret recordings of small seismic

8events at the site of interest as reasonable approximations of Green's functions and to convolutethem suitably, using earthquake scaling laws, in order to simulate time histories that correspondto larger earthquakes. The EGF technique was first put forward by Hartzell (1978) and hassince been further developed by numerous scientists. Its main interest is that the truepropagation and site effects are automatically accounted for ; its main disadvantage is that itcannot, on its own, account for non-linear soil behaviour. If non-linear soil behaviour cannot beneglected, the EGF technique should be combined with geotechnical methods, as outlined byHeuze et al. (1995).

There are, roughly speaking, two different "families" of EGF techniques. In both cases, the EGFis taken at several times and added up so that a larger earthquake, referred to as the "target"event, of the same focal mechanism is synthesized (see Figure 8). The difference lies in the wayhow the summing up of the EGF is performed : with or without kinematic modelling of thetarget event's rupture process. Irikura (1983, 1986), Hutchings (1994) and Irikura and Kamae(1994) are all representatives of the family of kinematic modelling techniques. The other familyuses essentially statistical tools that allow to sum up the EGFs in a way that the relevantearthquake scaling laws will be respected. An overview on this family is given by Tumarkinand Archuleta (1994).

4. Example: Microzonation of the Visp area (Wallis, Switzerland)

Alpine valleys are often characterised by significant site effects, due to the presence of deepyoung alluvial deposits in narrow glacial valleys, overlaying a bedrock formation. The town ofVisp is located in the Rhone valley, Wallis, Switzerland. The aim of the following work was toestablish a microzonation directly applicable in engineering practice, by means of proposeddesign response spectra for different zones in the considered area (Rsonance et al., 1999).

4.1 Geological context and soil parameters

The dimensions of the valley are approximately 220 m in depth (down to the substratum) for awidth of 1500 m.

Geological information were gathered from a systematic bibliographic study. Soil parameterswere deduced from SPT (Standard Penetration Test) values with the aid of empirical relations.The Rhone valley in the region of Visp can schematically be described as follows : In the centre of the valley, recent alluvial deposits are found down to a depth of 170 m, with

S-wave velocities estimated to range from 250 m/s at the surface to 900 m/s at depth. Thesedeposits are characterised by silts and fine sands in alternation with gravel and sand layers.

Then a morainic layer of 50 m is found, characterized by velocities estimated around 1000 to1200 m/s.

Finally, the rock substratum is characterized, in this study, by an assumed S-wave velocityof 2500 m/s.

4.2 Methodology

4.2.1 Regional seismic hazardThe regional seismic hazard to be taken into account in the area of Visp, for ordinary buildings,is characterised by a peak ground acceleration ag = 0.16 g, following the seismic zonation ofSwitzerland (SIA 160, 1989). This zonation is based on damage estimation from historical

9catalogues and empirical correlation with intensity. The corresponding acceleration values arethus representative of a soil type which is neither a "good" rock site, neither an alluvial site (asvillages were rarely built in alluvial valleys, due to the flow hazard). Following the work doneby Rttener (1995), and Smit and Rttener (1998), we chose to consider a reduced value of ag =0.12 g, for an ideal rock site in the Visp area.

4.2.2 H/V measurementsMeasurements of the ground natural frequency were conducted at 80 points in the consideredarea, using the H/V ratio experimental technique. The a priori estimated S-wave velocities ofthe numerical model were then adjusted, keeping the layer thickness fixed, so as to find themeasured natural frequencies for weak motion. In the present case, velocity adjustmentsbetween 10 and 25 % were necessary. An example is given in Table 1.

4.2.3 Numerical simulationsThe geological profile determined as described above was used to perform numerical simul-ations of the local amplification due to site effects. A pragmatic methodology was chosen forthese calculations, using SHAKE (1-D, pseudo non-linear approach) and Aki-Larner (2-D,accounting for geometrical effects) programs with the following procedure: 2-D calculation by means of the Aki-Larner technique for weak motion, adjustment of the

S-wave velocities according to the measurements of the H/V ratio, in order to obtain thecorrect fundamental natural frequency in the transfer function;

1-D calculation for strong motion, accounting for the approximate influence of non-linearbehaviour of the soil, followed by an adjustment of the S-wave velocity and the damping (Qvalues), as a function of the mean soil deformation, for each layer (Table 1 gives anexample of the pseudo non-linear velocity adjustments and Q values in the centre of theRhne valley);

2-D calculation for the strong motion with the new values for S-wave velocities anddamping.

4.3 Results

The numerical simulations made it possible to characterize site effects in the Visp area. Theresults obtained are of the same order as site effects already observed or calculated in othersimilar areas. This study proposed to distinguish three zones : zone "Rhne valley", zone "alluvial cone", zone "rock".

The whole Rhne valley has been defined as a single "zone" in this microzonation, accountingfor a "mean" 2-D effect of the valley. This approach is in agreement with the recent conclusionof the work performed by Chavez-Garcia and Faccioli (2000), indicating that "the 2-D basinamplification factor can in first approximation be assumed constant throughout the surface ofthe valley".

Following the results of the microzonation study of Visp, an elastic response spectrum has beendefined, for each zone, for the design of standard buildings (see Figure 9).

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5. Conclusions

One of the main points of this overview is that the instrumental approach based on analyses ofearthquake recordings is principally a reliable technique. However, the existence of non-linearsoil effects a firmly established reality compromises the validity of amplification factorsobtained from weak motion measurements to a certain extent. Whenever non-linear effects mustbe taken into account in numerical approaches, however, the computations are significantlyimpaired by the uncertainties in the measurement or estimation of the non-linear constitutivecharacteristics of the soil. These uncertainties are at least as large as those that affect themeasurement or estimation of the soil characteristics at small deformations.

Numerical approaches remain of primary importance to help understand the physics of siteeffects. Their advantages for practical estimations of amplification factors at specific sites mustnot be overlooked, since the instrumental approach may not always be applicable, for instancein urban areas with weak seismicity (gap areas, or intraplate seismicity zones with large returnperiods). However, sensitivity studies (see Field and Jacob, 1993a) draw attention to the needfor multiple, redundant geotechnical measurements (which increases the actual cost ofnumerical estimations).

Ambient vibration methods are clearly important because of their low expense, as the H/V ratiotechnique based on horizontal to vertical spectral ratios, for example. Additional research mustbe carried out to establish the reliability and actual limits of these techniques. For instance, canthe H/V ratio method be used to provide quantitative estimations of site amplification factors ?

The main lessons learned on the physics of site effects are i) the growing reconciliation ofseismologists' and engineers' viewpoints on non-linear soil effects and ii) the accumulatingexperimental and numerical evidence on the engineering importance of 2-D or 3-D effects(wave diffraction by surface or subsurface topography).

Much work, both in research and of regulatory character, remains to be done in order to transferthe accumulated knowledge about site effects to the engineering practice. For instance,regulations are still needed on how to anchor peak ground accelerations for soil and rock sitesin design-response spectra so as to account for amplification effects caused by surface and sub-surface topography.

In summary, although significant advances have been achieved in recent years, some issuesregarding the physics of site effects as well as the manner in which to consider them inengineering practice remain unresolved (Bard, 1997) : Basic research is needed, with both theoretical and experimental approaches, in order to

better understand some particular aspects of site effects: surface topography effects, effectsof strong lateral discontinuities, actual importance of non-linearity in soil response, actuallevel and effects of differential motion on structures, site-city interaction effects in denselyurbanized areas.

Methodological work is required to better assess and compare the reliability, cost andusefulness of the various methods available for the prediction of site effects.

Last, but not least, some regulatory work is needed to better account for site effects inseismic codes.

The extreme importance of site effects in recent damaging earthquakes calls for special effortsto apply right now what we already know regarding site effects without waiting for results offurther research. The state of the art is such that it is now possible to perform "present day"

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microzonation studies. The task at hand is certainly not simple, and it will not only encounterjust technical problems. It must be remembered that the results of such microzonation studiesare intended for use by local authorities, city planners, land-use specialists and civil engineers,whose technical background and preparation differ greatly. Thus, the recommendations must bevery clear and based on sound judgement. Programs recently launched in several countries willeventually clarify the methodologies required for this important goal.

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6. References

Aki K. and K.L. Larner (1970), Surface motion of a layered medium having irregular interfacedue to incident plane SH waves, J. Geophys. Res., 75, pp. 933-954.

Aki, K. and K. Irikura, 1991. Characterization and mapping of earthquake shaking for seismiczonation, Proceedings of the Fourth International Conference on Seismic Zonation,August 25-29, Stanford, California, E.E.R.I. (editor), Oakland CA, 1, 61-110.

Bard, P.-Y. (1997), Local effects on strong ground motion: Basic physical phenomena andestimation methods for microzoning studies, Advanced study course on seismic risk(SERINA), Thessaloniki, Greece, pp. 229-299.

Bard, P.-Y., A.-M. Duval, B. Lebrun, C. Lachet, J. Riepl and D. Hatzfeld, 1997. Reliability ofthe H/V technique for site effects measurement: an experimental assessment. In: SeventhInternational Conference on Soil Dynamics and Eathquake Engineering, July 19-24,Istanbul, Turkey.

Bard, P.-Y., 1999. Microtremor measurements: a tool for site effect estimation ?, State-of-the-art paper, Second International Symposium on the Effects of Surface Geology on seismicmotion, Yokohama, December 1-3, 1998, Irikura, Kudo, Okada & Sasatani, (eds),Balkema 1999, 3, 1251-1279.

Bonilla, L.F., J.H. Steidl, G.T. Lindley, A.G. Tumarkin and R.J. Archuleta, 1997. Siteamplification in the San Fernando valley, California: variability of site-effect estimationusing the S-wave, coda, and H/V methods, Bull. Seism. Soc. Am. 87-3, 710-730.

Borcherdt, R.D. 1970. Effects of local geology on ground motion near San Francisco Bay. Bull.Seism. Soc. Am. 60, 29-61.

Bour, M., 1993. Simulation de forts mouvements du sol partir de petits sismes utilisscomme fonctions de Green empiriques, PhD thesis, Universit Louis Pastuer, Strasbourg

Chavez-Garcia, F.J., L.R. Sanchez and D. Hatzfeld, 1996. Topographic site effects and HVSR.A comparison between observations and theory. Bull. Seism. Soc. Am., 86-5, 1559-1573.

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Table 1 : Estimated and adjusted S-wave velocities and Q factors for the soil layers in thecentre of the Rhne valley at Visp.

Depth

[m]

A priori estimatedS-wave velocityfor weak motion

[m/s]

S-wave velocityadjusted according to"noise" measurements

[m/s]

Pseudo non-linearS-wave velocity

for strong motion

[m/s]

Pseudo non-linear Q factor

for strong motion

[]

0-4 243 290 262 10

4-9 286 340 282 6.0

9-18 412 490 420 7.4

18-22 444 530 444 6.3

22-45 539 650 555 7.2

45-70 641 770 741 33

70-100 800 960 930 38

100-130 900 1080 1050 38

130-170 1000 1200 1170 42

170-210 1100 1320 1290 42

210-250 1200 1440 1410 45

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Figure 1 : Differences between 1-D and 2-D behaviors for a perfectly elastic (no damping)sediment-filled valley. These diagrams represent the spatial (x) and temporal (t)evolution of the surface motion of a sediment-filled valley impinged by a SH signalwith a characteristic frequency f

p=_

1/4h (_

1 is the S wave velocity in the sediments,

h is the valley thickness).a) Results obtained with a 1D approximation ( considering only, for each site x,the local sediment thickness)b) Results obtained a 2D modeling, in the case of a shallow valley: h/w = 0.06c) Results obtained a 2D modeling, in the case of a thick valley: h/w = 0.70

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Figure 2 : An example of a large topographic effect at Pine, France. The 5 top diagramsdisplay average spectral ratios obtained at 5 surface sites ( one strandarddeviation), for the EW (in-plane) component. The reference station is not the basestation ("Roya"), but the westernmost, mid-slope station ("Bramafan"). A cross-section of the investigated topography is shown on bottom. (Reproduced fromNechtschein et al., 1995).

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Figure 3 : An example of use of microtremors through H/V ratio technique. Horizontal tovertical spectral ratios of ambient noise are shown for various locations around analluvial coastal plain in the city of Nice. The H/V ratios (thick continuous line) arecompared with classical spectral ratios with respect to a rock reference site (thincontinuous line), and with 1D numerical estimates of the transfer function (thickdashed line). After Duval, 1994.

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Figure 4 : Theoretical checks of the H/V ratio technique. Numerical horizontal to verticalspectral ratios were computed from noise models in various (about 15) soilprofiles.a) Comparison between the S-wave resonance frequency (fs, computed forvertically incident S-waves) and the peak frequency "observed" in theoretical H/Vratios (fn): the agreement is very satisfactory.b) Same thing for the spectral amplitude at the resonant frequency (As is theamplification for vertically incident S waves, An is the amplitude of the H/V peakobtained from noise modeling): the agreement is very poor... (Reproduced fromLachet and Bard, 1994).

Figure 5 : Experimental checks on the H/V ratio technique, based on instrumental recordingsat 33 different soft sites in different areas from France and Greece. Relationshipbetween the amplitude of the H/V peak and the peak amplification measured onearthquake spectral ratios with respect to a reference site. There is no correlationbut an apparent inequality : the former is almost systematically lower that thelatter.

a)

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Figure 6 : Comparison between various techniques for the estimation of site responsetransfer function, for two sites in Oakland (California). Adapted from Field andJacob, 1995.a) Traditional spectral ratios;b) Generalized Inversion spectral ratios whatever the noise ratio);c) Generalized Inversion spectral ratios obtained when only data with signal to

noise ratio larger than 3 are kept (and then given an equal weight).d) Parameterized inversion estimates;e) Average Horizontal-to-vertical component spectral ratios for S wave part of

earthquake records.f) Nakamura's estimates (average horizontal-to-vertical spectral ratio of ambient

vibrations)Curves a) to c) correspond to site-reference techniques, while curves d) to f)correspond to non-reference site techniques. The dashed-lines represent 95 %confidence limits of the mean.

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Figure 7 : An example of strain dependency of normalized shear modulus (top) and damping(bottom), for soft soils with varying plasticity index PI (after Dobry and Vucetic,1987).

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Figure 8 : Schematic representation of the different sources of time delays in the summationof the EGF : path length, finite rupture velocity and dislocation rise time (afterBour, 1993).

Figure 9 : Chosen elastic response spectra for the three zones "Fels (Rock)", "Schuttkegel(alluvial cone)" and "Rotten (Rhone valley)".