Seismic spectral microzonation of the Yverdon basin (Switzerland)

T. Bailly & C. LacaveRésonance Ingénieurs-Conseils SA, Carouge (GE), Switzerland

ABSTRACTThe Yverdon basin is characterised by very loose deposits down to more than 100 m, as well as by a complex

bedrock geometry with underground faults. In such a situation, spectra proposed by building codes are not

sufficient to account for site effects. A detailed study was conducted by means of 1D/2D simulations, in order to

produce specific spectra for the region, to be used for the seismic design or assessment of structures. Since the

seismicity of the Yverdon region is rather low, an equivalent linear approach was sufficient to deal with non-

linearity. A sensitivity study was carried out to evaluate the influence of parameters such as the Vs profile, the

Vs of the bedrock, the level and incidence angle of the input motion, the damping coefficient, as well as the

account for 1D/2D configurations. The results were used to determine areas of similar response with specific

spectra associated to each area.

Keywords: site effects, microzonation, 2D numerical simulation

1. INTRODUCTION

The Yverdon basin, located in the Vaud canton (Switzerland), is characterised by the presence of very

loose soils, down to a depth of more than 100 m, over the bedrock. They mainly consist of lacustrine

deposits, including a superficial layer of peat. Furthermore, the Yverdon basin is characterised by a

complex geometry of the bedrock, due to the presence of several hidden faults. In such a situation, the

spectra proposed by building codes, like the Eurocode 8 or the Swiss code SIA-261, are not sufficient

to account for the site effects due to this particularly unfavourable geological situation.

2. METHODOLOGY

A detailed site effect study was conducted by means of 1D and 2D simulations, in order to produce

specific spectra for the region, to be used instead of the code spectra, for the seismic design or

assessment of structures. Since the seismicity of the Yverdon region is low (but not negligible), an

equivalent linear approach was sufficient to deal with non-linearity. The 1D computations were

conducted using the CyberQuake code (BRGM, 1998), similar to SHAKE (Schnabel et al., 1972),

whereas the 2D computations were conducted using the Aki-Larner (1970) method combined with an

account for the linear-equivalent behaviour of materials, in the case of plane incident waves, with a

horizontal SH type polarisation. The code that is used in this study for 2D site effects modelling

(especially the equivalent linear module) was developed in the framework of the SISMOVALP

European project. As a first application of the above described methodology, equivalent linear 2D

Aki-Larner computations were used to perform the seismic microzonation study of the upper Rhone

valley, in the Visp area (Switzerland), a typical 2D alpine valley (Lacave and Koller, 2007 ; Lacave et

al., 2008).

3. MICROZONATION OF THE YVERDON BASIN

3.1 Basin and deposit properties

Twelve 1D and two 2D profiles where obtained from geophysical, geological and geotechnical data

available for the Yverdon region (Figure 3.1 top). Below a superficial layer of filling, peat or

lacustrine chalk, the basin is mainly filled with lacustrine deposits, overlaying the bedrock constituted

by the molasse (sandstone and marl). Figure 3.1 bottom shows the shape of the basin along the 2D

profiles P1 and P2.

Figure 3.1. Top: Location of the 1D (blue dots) and the 2D (blue lines) profiles. Bottom: Bedrock 2D profiles P1

and P2 across the Yverdon basin.

0

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0 4 0 0 0 4 5 0 0

D i s t a n c e ( m )

Pro

fon

de

ur

(m)

R 1 - ~ 2 H z R 3 – 0 .7 5 H zR 2 - ~ 1 .3 H z R 4 - 1 .1 H z R 5 - ?

N W S E

0

20

40

60

80

100

0 500 1000 1500 2000

Distance (m)

Profondeur (m)

SSO NNE

R1 - ? R2 - ~ 0.8 Hz R3 - ? R4 - ~ 2.7 Hz

Some H/V measurements of the ambient vibrations were conducted on the whole Yverdon area, at 40

points. This made it possible to draw a resonance frequency map of the area. Furthermore, two

MASW profiles were conducted in order to better estimate the S-wave velocities in the upper 30 m of

deposits (peat over gravel and sand as well as silt over gravel and sand). Using available geotechnical

and geological information, especially SPT values when possible, best estimate S-wave velocity

profiles were defined at each 1D and 2D profile. Then, for each profile, a first computation is

conducted for very weak motion. Then the natural frequency of the obtained transfer function is

compared to the measured value. Finally, the Vs-profile is adjusted so that the computed resonance

frequency matches the measured one. As an example, Table 3.1 shows the S-wave velocity profile

determined for the centre of the 2D profile P1.

The material's equivalent linear behaviour was assumed to correspond to published typical curves for

the same type of deposits. A mean curve between the one from Seed and Idriss (1970) and the one

from Ishibashi and Zang (1993) for a plasticity index Ip = 0 was used.

Table 3.1. S-wave velocity profile determined for the centre of the 2D profile P1.

Type of deposits Depth upper interface [m] Thickness [m] S-wave velocity [m/s] Density [t/m3]

Filling, peat or lacustrine chalk 0 3 110 1.6

3 7 150 1.7

10 10 200 1.8

20 10 240 1.9

30 10 270 2.0

40 10 280 2.0

50 10 290 2.1

60 10 310 2.1

70 10 320 2.1

80 10 340 2.1

Lacustrine deposits

(gravel and sand)

90 19 350 2.1

Molasse (sandstone and marl) 109 infinite 1500 2.6

3.2 Input motion

The Swiss Seismological Service re-evaluated the seismic hazard for Switzerland a few years ago

(SED, 2004). For any point in Switzerland, the values of spectral acceleration are available for five

periods between 0.1 and 2 s. These values are valid for a "hard rock" type with Vs around 1500 m/s. In

analogy with what was determined for the Swiss building code SIA 261, a spectrum with the class A

shape (similar to EC8 type 1 shape) and the following design horizontal spectral acceleration value, a0,

is used for the "hard rock" with Vs = 1500 m/s:

( ) ( )

5.2

1

2

10'50 ⋅

+=

HzSHzSa aa (3.1)

where Sa and S'a are the spectral acceleration values for "hard rock" at 5 and 10 Hz respectively. The

corresponding "rock input spectrum" is valid for the motion at an hypothetical outcropping hard rock.

For site effect computations, a set of six acceleration time histories was selected, that all together

cover the input rock spectrum. These accelerograms were either found in the European Strong Motion

Database (Ambraseys et al., 2001) and scaled by a factor if necessary, or they are semi-artifical

accelerograms, created using the Sabetta and Pugliese (1996) program.

3.3 Site effect computations

The response of the Yverdon basin is computed for the six input time histories, with the aid of both 1D

and 2D computations. The following procedure is adopted:

• Computation of the surface response at several 1D points or along the 2D profiles, for each input

motion,

• Computation of the associated response spectra,

• Computation of the amplification functions: ratio between the response spectrum (for 5% damping)

of the resulting motion at the surface receiver and the response spectrum of the outcropping rock

(twice the incoming wave field, as would occur at a free surface of the same rock),

• Computation of the resulting site response spectrum: multiplication of the amplification function by

the regional hard rock hazard spectrum.

Figure 3.2. Examples of the variation of the amplification functions, in the cases of 1D/2D comparison (upper

left), variation of incidence angle (upper right), variation of S-wave velocity profile in the deposits (bottom left)

and variation of the S-wave velocity of the bedrock (bottom right).

Furthermore, using the same procedure, a sensitivity study is conducted in order to account for several

types of uncertainties. For this purpose, the following is done:

• The input incidence angle is varied as to be vertical (0°), as well as oblique with values of -30° and

+30°.

• The S-wave velocity profile is varied by multiplying the values by a factor of 1.4 and dividing them

by a factor of 1.4 (corresponding to a factor of 2 in the shear modulus).

• 1D and 2D computations are compared at some points.

Variation of incidence angle - Profil P1 receiver 5

0

1

2

3

4

5

6

7

0.01 0.1 1 10

Period (s)

Am

plifi

ca

tio

nincidence +30°

incidence 0°

incidence -30°

Comparaison des fonctions d'amplification 1D et 2D au récepteur 5

0

1

2

3

4

5

6

0.01 0.1 1 10Période [s]

Am

pli

ficati

on

moyenne-1D

moyenne-2D

Fonction d'amplification - centre de la vallée - variation de

vitesse

0

1

2

3

4

5

6

7

0.01 0.1 1 10Période (s)

Am

pli

ficati

on

moyenne-Vs/1.4

moyenne-profil moyen

moyenne-Vs*1.4

AF - profil moyen - point P8

0

1

2

3

4

5

6

7

0.01 0.1 1 10Période [s]

Am

plifi

cati

on

acc282x-V1500

acc282x-V1000

acc282x-V2000

acc282x-gradrock

Variation of Vs – Profile P1 receiver 3

Comparison 1D / 2D – Profile P1 receiver 5

Variation of bedrock Vs – 1D Profile P8

• Influence of the bedrock S-wave velocity is shown with values of 1000, 1500 and 2000 m/s, as well

as a gradient from 1000 to 1500 m/s in the 50 first meters of bedrock.

• Influence of the damping coefficient is shown by increasing its value by a factor of 1.5.

Figure 3.2 shows examples of the resulting variation of the amplification functions, in the cases of

1D/2D comparison (upper left), variation of incidence angle (upper right), variation of S-wave

velocity profile in the deposits (bottom left) and variation of the S-wave velocity of the bedrock

(bottom right).

As described earlier, the final response spectra were computed for each point, each input motion, and

with the different varied parameters. Then, the resulting spectra are gathered in groups of similar

shape – i.e. similar response –, which will lead to the definition of corresponding zones.

In the case of the area of Yverdon, four zones are distinguished, based on the results, within the basin:

• Zone S1: located at the northern shore of the lake, where a compact moraine or the molasse

bedrock is found at depths smaller than 10 m.

• Zone S2: located at the western side of the basin, and in the thermal area, where the lacustrine and

fluvial sedimentation is not thicker than 45 m,

• Zone S3: located at the eastern side of the basin, characterised by a deep (> 60 m) quaternary

sedimentation, including a moraine deposit,

• Zone S4: located at the centre of the basin, characterised by lacustrine alluvium over a thickness

greater that 60 m, without any moraine deposits at its base, contrary to the case of zone S3.

Figure 3.3. Response spectra resulting from the computations in northern shore of the lake (zone S1, top left), at

the western side of the basin and the thermal area (zone S2, top right), at the eastern side of the basin (zone S3,

bottom left) and in the centre of the basin (zone S4, bottom right). In each case, the red curve is the proposed

elastic design spectra for the corresponding zone, the black curve is the code spectrum that should be used

without any specific site effect study.

Spectres de réponse - Zone S1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0.01 0.1 1 10Période [s]

Ac

cé

léra

tio

n s

pe

ctr

ale

[m

/s2

]

Spectres de réponse - Zone S2

0

1

2

3

4

5

6

0.01 0.1 1 10Période [s]

Ac

célé

rati

on

sp

ec

tra

le [

m/s

2]

Spectres de réponse - Zone S3

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.01 0.1 1 10

Période [s]

Acc

élé

ratio

n s

pectr

ale

[m

/s2]

Spectres de réponse - Zone S4

0

0.5

1

1.5

2

2.5

3

3.5

0.01 0.1 1 10

Période [s]

Ac

célé

rati

on

sp

ec

tra

le [

m/s

2]

Figure 3.3 shows these spectra for zones S1 (top left), S2 (top right), S3 (bottom left) and S4 (bottom,

right). On this figure, all spectra are presented, including the account for the uncertainty on the

velocity profile, input motion, incidence angle, and for several positions within each zone. The aim of

a microzonation study is to define site specific spectra that better account for the local site effects than

the average spectra proposed in the building codes. The next step is then to use the results obtained in

order to draw simple "code type shaped" response spectra, for each zone. Out of the computed spectra,

a unique design spectrum is chosen for each zone (red curves on Figure 3.3), rather in a conservative

manner, to cover all the above mentioned uncertainties. The proposed spectra are:

• rather conservative in the long period range where the input hazard is not very reliable and

conservative,

• are characterised by a plateau which "cuts" some insignificant peaks in the short period range,

which is not so important for structures, and where the input spectrum is more conservative.

3.4 Seismic microzonation of Yverdon

Figure 3.4. Left: Extract of the seismic microzonation map of Yverdon. Right: Proposed elastic design spectra in

each zone, compared to the code spectra, that would be used without accounting for the specific site effects (thin

curves).

The zones to which the above spectra are attributed have to be delimited on a map. To do this, several

aspects are taken into account, such as:

Site specific spectra - Yverdon

0

0.5

1

1.5

2

2.5

3

3.5

4

0.01 0.1 1 10Period [s]

Sp

ec

tra

l a

cc

ele

rati

on

[m

/s2

]

Zone S1

Zone S2

Zone S3

Zone S4

SIA 261 - classe D

SIA 261 - classe E

• the shape of the bedrock,

• the geology of the deposits,

• the resonance frequency distribution,

• the surface topography,

• the parcels or roads for the fine drawing of the limits.

Figure 3.4 shows an extract of the seismic microzonation map of Yverdon. The shape of the spectra

associated to each zone is given on the graph associated to the map. These spectra are compared to the

code spectra, that should have been used without accounting for the specific site effects. The following

characteristics can be described for the proposed spectra:

• Zones S3 and S4 are characterised by a strong amplification in the long period range (T > 2 s),

contrary to zones S1 and S2.

• High plateaux of zones S1 and S2 are characteristics of a bedrock found at shallow depth (< 45

m). The damping indeed plays a moderate role compared to zones S3 and S4, where the

quaternary deposits are thicker.

• The lower plateau in zone S4, compared to zone S3, is the result of a strong damping,

characteristic of the centre of alluvial plains.

4. CONCLUSION

The region of Yverdon is located in an area prone to site effects, due to the presence of very loose

deposits and a high impedance contrast to the underlying bedrock, leading to strong amplification of

the seismic motion.

In such cases, spectra of the EC8 or SIA 261 building code, based on a rough classification of the

deposits in soil classification, do not allow to guarantee a correct dimensioning of structures. Only the

30 first meters of deposits are indeed considered to classify the soils, which is not sufficient to

characterise some types of deposits. Furthermore, building code spectra do not account for the

geometrical site effects occurring in sedimentary valleys or basins.

1D and 2D site effects computations were conducted for the seismic microzonation of the area of

Yverdon (Switzerland). The final objective of such a study is to develop site specific spectra that

account for the local site effects and that are used instead of the code spectra. The seismic

microzonation map, showing the zones where these spectra have to be used, is then a tool that can be

used directly by engineers for the seismic evaluation or design of structures in the corresponding

region.

ACKNOWLEDGEMENTS

The present study was financed by the ECA (building insurance against fire and natural hazards of the Vaud

canton, Switzerland). We are grateful to Géolosanne and Geo2X for the geological, geotechnical and

geophysical contributions to the microzonation of Yverdon. The equivalent linear 2D Aki-Larner code was

developed in the framework of the European Interreg IIIB Alpine Space project SISMOVALP, number F/1-

2/3.3/25. The Swiss contribution was financed by the Swiss Confederation, CREALP and Canton Valais.

REFERENCES

Ambraseys, N., J. Douglas, R. Sigbjörnsson, C. Berge-Thierry, P. Suhadolc, G. Costa and P. Smit, (2004).

European Strong-Motion Database, Volume 2. Contract GR/52114/01 (P), Engineering and Physical

Science Research Council of the UK.

Aki K. and K.L. Larner (1970). Surface motion of a layered medium having an irregular interface due to incident

plane SH waves. Journal of Geophysical Research, vol. 75, No. 5.

BRGM (1998). CyberQuake, User's Guide, Version 2, BRGM, Orléans.

Ishibashi I and X. Zhang (1993). Unified dynamic shear moduli and damping ratios of sand and clay. Soils and

Foundations, Vol. 33, n° 1, 182-191.

Lacave C. and M. G. Koller (2007). Microzonation of the city of Visp (Switzerland) using a 2D equivalent linear

approach. 4th

International Conference on Earthquake Geotechnical Engineering, Thessaloniki, Greece.

Paper No. 1755.

Lacave C., P.-Y. Bard, M. Kham and M. G. Koller (2008). 2D equivalent linear site effect simulation: example

applications to two deep valleys. Bull. Earthquake Eng. No. 6, pp.197–211.

Norme SIA-261 (2003). Société Suisse des Ingénieurs et Architectes, Zurich.

Sabetta F. and A. Pugliese, (1996). Estimation of response spectra and simulation of non-stationary earthquake

ground motions. Bull. Seism. Soc. Am., 86, pp. 337-352.

Seed, H.-B. and Idriss, I.-M. (1970). Soil moduli and damping for dynamic response analysis. Report N°. FERC

70-10, UCB.

Swiss Seismological Service, SED (2004). Seismic hazard assessment of Switzerland. www.seismo.ethz.ch.

Schnabel B., J. Lysmer and H.B. Seed (1972). SHAKE – a Computer Program for Earthquake Response

Analysis of Horizontally Layered Sites, Report EERC, 72-12.