C. R. Geoscience 336 (2004) 353365

Tectonics

The Corinth Gulf Soft Soil Array (CORSSA) to study site effects

Kyriazis Pitilakis a,, Konstantinos Makropoulos b, Pascal Bernard c, Francis Lemeille d,Hlne Lyon-Caen e, Catherine Berge-Thierry d, Theodora Tika a, Maria Manakou a,

Dimitios Diagourtas b, Dimitrios Raptakis a, Polyxeni Kallioglou a,Konstantia Makra a, Dimitrios Pitilakis a, Fabian Bonilla d

a Laboratory of Soil Mechanics and Foundation Engineering, Civil Engineering Dept., Aristotle University of Thessaloniki (LSMFEAUTH),54124 Thessaloniki, Greece

b Seismological Laboratory, National Kapodistrian University of Athens (NKUA), Panepistimioupolis, 15784 Zografou, Greecec Department of Seismology, Institut de physique du Globe de Paris (IPGP), 4, place Jussieu, 75252 Paris cedex 05, France

d Bureau dvaluation des risques pour la sret des installations nuclaires (BERSSINIRSN),Institut de radioprotection et de sret nuclaire, BP 17, 92262 Fontenay-aux-Roses, France

e Department of Geology, cole normale suprieure (ENS), 24, rue Lhomond, 75231 Paris cedex 05, France

Received 9 November 2003; accepted after revision 24 November 2003

Written on invitation of the Editorial Board

Abstract

This paper describes the development of the CORSSA array (CORinth Soft Soil Array) in the framework of the Europeanproject CORSEIS, which takes place in the highly seismic region of Aegion, in the Gulf of Corinth, Greece. An array ofsurface and down-hole accelerometers and pore pressure transducers is deployed in the hanging wall of the Aegion Fault, anactive normal tectonic feature that crosses Aegion city. The scope of this array is to study the physics of wave propagation,the non-linear soil behaviour, liquefaction phenomena and the effects of surface geology on strong ground motion. The surfacegeology is well constrained by a detailed geotechnical and geophysical survey including dynamic soil tests. Typical recordingsand preliminary 1D SH ground response analyses are presented herein, illustrating the importance of the experimental array tostudy complex site effects including non-linear phenomena in the vicinity of faults and topographic relief. To cite this article:K. Pitilakis et al., C. R. Geoscience 336 (2004). 2004 Acadmie des sciences. Published by Elsevier SAS. All rights reserved.

Rsum

Le rseau en sols meubles (CORSSA) du golfe de Corinthe pour ltude des effets de site. Le prsent article dcrit ledveloppement du rseau CORSSA (CORinth Soft Soil Array) dans le cadre du programme europen CORSEIS, install dansla rgion dAegion (golfe de Corinthe, Grce), considre comme forte sismicit. Le rseau dacclromtres en surface et enprofondeur, coupls des capteurs de pression interstitielle, est localis dans le compartiment abaiss de la faille dAegion, quiest une faille normale active traversant la ville. Ce rseau est destin tudier la physique de la propagation des ondes, le compor-tement non linaire du sol, les phnomnes de liqufaction et les effets de la gologie superficielle sur le mouvement fort du sol.

* Corresponding author.E-mail address: pitilakis@geo.civil.auth.gr (K. Pitilakis).

1631-0713/$ see front matter 2004 Acadmie des sciences. Publisheddoi:10.1016/j.crte.2003.11.013

by Elsevier SAS. All rights reserved.

http://

354 K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365

La gologie superficielle est bien contrainte par une tude dtaille gotechnique et gophysique incluant des tests dynamiquesde sol. Les signaux typiques enregistrs et les analyses prliminaires de la rponse du sol 1D SH sont prsents ici ; ils illustrentlimportance du rseau exprimental pour tudier des effets de sites complexes incluant les phnomnes de non-linarit auvoisinage des failles et des reliefs topographiques. Pour citer cet article : K. Pitilakis et al., C. R. Geoscience 336 (2004). 2004 Acadmie des sciences. Published by Elsevier SAS. All rights reserved.

Keywords: CORSSA array; non-linear soil behaviour; liquefaction phenomena; site effects; ground response analyses

Mots-cls : rseau CORSSA ; comportement non linaire du sol ; phnomnes de liqufaction ; effets de site ; analyse de la rponse du sol

Version franaise abrge

La rgion dAegion, localise dans la partie occi-dentale du golfe de Corinthe, est lune des rgionsles plus actives de Grce du point de vue sismotec-tonique, avec des failles normales dominantes de di-rection estouest. La rgion est caractrise par uneactivit sismique continue, responsable de sismes im-portants et destructeurs, comme le sisme dAegion de1995 (M = 6,2), qui a caus la mort de 26 personneset de srieux dgts aux habitations (environ 2000constructions ont souffert de dgts majeurs et ont dtre reconstruites). En raison de cette forte sismicit, largion a t choisie pour y installer un rseau daccl-romtres et de capteurs de pression interstitielle en solsmeubles dans le but dtudier les effets des conditionsgologiques locales sur le mouvement du sol et, demanire plus prcise, la rponse linaire et non linairedu sol, ainsi que les phnomnes de liqufaction.

Ce rseau est dsign ensuite sous lacronymeCORSSA (Corinth Soft Soil Array). Son installationa fait lobjet dune collaboration entre plusieurs orga-nismes (AUTH, NKUA, IRSN, IPGP et ENS-Paris)dans le cadre du programme europen CORSEIS [1,2]. Il est install dans le compartiment nord ( toit )de la faille dAegion, la principale structure tectoniquequi traverse la ville dAegion formant un escarpementmajeur proche du littoral. Le secteur o a t installle rseau est pratiquement sur le rivage, o les sdi-ments trs meubles (NSPT < 8), constitus de sablessilteux dominent les couches de surface, et o des ph-nomnes de liqufaction ont t relevs lors du sismede 1995. Le rseau CORSSA est compos de quatreacclromtres large bande trois composantes, un ac-clromtre en surface et deux capteurs de pression in-terstitielle. Lacclromtre de rfrence, install dansune couche de conglomrats rigides 178 m peuttre considr comme la rfrence au rocher. Les trois

autres acclromtres sont installs dans les dptsmarins fluvio-deltaques saturs (ML, CL, SC) di-verses profondeurs (14, 30 et 60 m). Les cap-teurs dynamiques de pression interstitielle sont ins-talls dans les dpts saturs marins non consolids(SM, ML-CL), couches susceptibles de liqufaction, des profondeurs gales ou proportionnelles celledes capteurs acclromtriques (6 et 14 m) d-dis ltude des phnomnes de liqufaction. Len-semble du rseau acclromtrique est associ unsystme central denregistrement 18 canaux, installdans un petit abri hbergeant linfrastructure nces-saire pour la collecte des donnes et la transmission(ligne tlphonique ISDN, alimentation lectrique, ap-pareil GPS). Depuis son installation, plusieurs vne-ments sismiques locaux et rgionaux ont t enregis-trs et analyss.

Dimportantes tudes gologiques, gophysiques(cross-hole, down-hole, sismique rflexion) ainsi quedes tudes gotechniques (mesures in situ et en la-boratoire, incluant des tests dynamiques et des essais la colonne rsonnante), ont t entreprises sur cesite, dans le but de dfinir la structure gologique etles paramtres dynamiques du sol (Vs,G0, courbesG D %) des diffrentes couches de terrain. Larponse sismique du sol a t tudie de manire tho-rique, dans le but didentifier diffrentes caractris-tiques damplification pour une propagation dondesismique 1D SH du rocher la surface. Divers mo-dles lastiques linaires et linaire-quivalents ontt utiliss, avec divers signaux incidents ralistes(enregistrements dvnements sismiques locaux ourgionaux) et thoriques (ondelettes de Ricker de fr-quence variable), cals diffrents niveaux daccl-ration. Ils indiquent quen raison du voisinage du sys-tme de la faille dAegion, lanalyse 1D SH ne peutdcrire avec prcision le phnomne complexe de dis-persion de londe gnr par les discontinuits go-

K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365 355

logiques et topographiques dans la zone dtude. Lamodlisation 2D semble plus adapte pour lanalysethorique. Seules les modlisations 1D en ondes SHont t abordes.

partir de maintenant et dans un avenir proche,CORSSA fournira un jeu denregistrements de hautequalit bien documents des sismes locaux et rgio-naux, qui enrichiront les bases de donnes de mou-vements forts europennes et mondiales. Leur analyseet leur dissmination seront trs utiles autant pour lespays forte sismicit, comme la Grce, que pour lesrgions faible sismicit avec des taux de dforma-tion modrs, comme la France.

1. Introduction

Site effects may produce large ground-motion am-plification during an earthquake. The geometry ofthe subsoil structure, the soil types and the variationof their properties with depth, the lateral discontinu-ities and the surface topography modify significantlythe strong ground motion and are correlated to dam-age pattern during a large earthquake. Based on thisstatement, CORSSA (CORinth Soft Soil Array) hasbeen developed in the highly seismic area of Aegion,which was shaken by the 1995 Aegion earthquake(Ms = 6.2), which caused 26 deaths and extensivedamage to buildings. The aim is to develop an ade-quate down-hole array of 3D accelerometers and porepressure transducers. The deepest sensor reached astiff geological formation that consists of conglom-erate and is considered as a seismic bedrock in thewell-known geological and geotechnical environmentwhere the dynamic soil properties are well definedthrough field and laboratory tests. CORSSA is estab-lished by the Aristotle University of Thessalonica, theNational Kapodistrian University of Athens, the In-stitut de physique du Globe, Paris, and the Institutde radioprotection et de sret nuclaire in the frame-work of the European Program CORSEIS [1,2].

2. Location

The Aegion city is located in the Gulf of Corinth,one of the most active seismotectonic areas in Europe(Fig. 1), with important north dipping WNW-trendingactive normal faulting. One of these faults is the

Fig. 1. Investigated area; Aegion city and the Gulf of Corinth.

Fig. 1. Zone dtude, ville dAegion et golfe de Corinthe.

Aegion Fault, with an average slope of 40 and anescarpment greater than 40100 m high (Fig. 2).CORSSA has been established at the shoreline ofAegion. The selection of the site accomplishes twobasic requirements:

(a) loose-soft soil materials susceptible to liquefac-tion and/or significant non-linearity at shallowdepths; and

(b) relatively shallow seismic bedrock for the instal-lation of the deepest accelerometric station.

3. Technical description of CORSSA

CORSSA consists of four broadband 3D down-hole accelerometers, one surface accelerometer andtwo pore-pressure transducers (Fig. 3). The deepest ac-celerometer (used as reference station) installed withina geological formation consisting of conglomerate at adepth of 178 m, the other three are located withinsaturated fluvio-deltaic, marine deposits ML, CL, SCat depths of 14, 30 and 60 m, while the dynamicpore pressure probes installed at 6 and 14 m depthwithin saturated loose marine deposits SM and ML.These soil layers are susceptible to liquefaction; the

356 K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365

Fig. 2. Simplified northsouth cross-section.

Fig. 2. Coupe simplifie nordsud.

depths of the installation of the pore pressure probesare intermediate between the accelerometers, in or-der to correlate their results. The accelerometers areconnected with an 18-channel main recorder and thepore-pressure transducers with a 2-channel data log-ger. Both have common time recording.

4. Geological, geotechnical and geophysicalcharacterization of the CORSSA soft-soilformations

Extensive geological, geophysical and geotechnicalsurveys have been performed in the site of CORSSA.Geological survey includes detailed geological map-ping of the greater area of interest. The geotechni-cal campaign included six geotechnical boreholes (ofdepths: 6 m, 2 14, 31, 60 and 178 m), in situ (NSPTand water table measurements) and laboratory tests onintact and disturbed soil samples. Special emphasiswas given to the resonant column dynamic tests (Ta-

ble 1). The geophysical campaign involved borehole(cross-hole, down-hole) as well as surface surveys(seismic reflection). These measurements have beenperformed in order to define the geological structureand the dynamic soil properties (P- and S-wave ve-locities, Vp and Vs , shear modulus, G0 at low strains,shear modulus degradation curves and material damp-ing vs. shear strains, (G/G0 D%)), of thedifferent geological formations.

The geological structure is described by threelithostratigraphical units: the lower one consists ofvery stiff clays, marls and very dense sands directlyoverlying the alpine substratum, which consists ofMesozoic limestones. The middle comprises typicaldeltaic and alluvial fan deposits mostly conglomeratesand sands, overlying the previous unit. The topmostunit is characterized by alluvial fan deposits, uponwhich the Aegion city is located. Holocene and recentdeltaic and fluvial deposits cover all the rest. Recentgeological interpretation indicates the following crosssection:

K.P

itilakisetal./C

.R.G

eoscience336

(2004)353365

357

Fig. 3. General layout of the CORSSA test-site.

Fig. 3. Disposition gnrale du site test de CORSSA.

358K

.Pitilakis

etal./C.R

.Geoscience

336(2004)

353365

Table 1Physical, mechanical and dynamic properties of soils tested on a resonant-column apparatus

Table 1Proprits physiques, mcaniques et dynamiques des sols tests laide de lappareil colonne de rsonance

Test Bore Depth v Speci- w WL PI CF d e Gs Organics Calcareous Cu OCR [%] Gmax Vs Dmin USCS Soil descriptionnumber hole h [m] [kPa] men [%] [%] [%] [%] [kN m3] [kN m3] [%] matter** [kPa] [MPa] [m s1] [%]

(

K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365 359

360 K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365

Fig. 4. Representative 1D soil profile for the CORSSA site.

Fig. 4. Profil de sol 1D reprsentatif pour le site CORSSA.

K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365 361

Fig. 5. Dynamic soil properties for all soil formations at CORSSAsite.

Fig. 5. Proprits de dynamique de sol pour toutes les formationspdologiques du site de CORSSA.

(1) in the footwall: 1827 m of alternation of silt andcoarse sand (low Vs) above stiff conglomerates(high Vs);

(2) in the hanging wall, same formations covered byloose conglomerates, silty lacustrine deposits andHolocene deposits with low velocity [4].

The proposed geotechnical profile of CORSSA isgiven in Fig. 4, while Fig. 5 illustrates the dynamicsoil properties for all soil formations, estimated fromresonant column tests.

5. Recordings

Since the full operation of the CORSSA array(March 2002), some local as well as regional eventshave been recorded (M 4.5, R 200 km). Anexample is presented in Fig. 6, which illustratesthe three components of a small event (Ms = 3.5)recorded on 20 May 2002. A specific database isunder construction, comprising all recordings with thedescription of the seismological parameters. The eventpresented herein is too small (PGA = 0.042 g) toaccount for non-linear phenomena. Nevertheless, it isquite interesting to calibrate in the linear elastic rangethe numerical tools to be used and to study the physicsof the ground motion in the vicinity of faults andtopographic relief.

6. Ground response analyses

Preliminary 1D SH site response analyses havebeen conducted in CORSSA site using as input soilmodel the well constrained 1D profile of the site.Different input motions were used (Ricker waveletsof different central period, the deconvoluted record-ings of the 1995 Aegion earthquake, as well as otherrecordings at outcrop conditions) scaled at differentPGA values ranging from 0.1 g to 0.5 g. The inputmotion is introduced at the outcrop. The bedrock isconsidered at 178 m and maybe rigid or elastic. Nomajor difference is found for the two assumptions.The analysis is basically equivalent linear using thesoftware CyberQuake [3]. The resulted absolute andnormalized response spectra of these motions havebeen compared with the corresponding elastic spec-trum proposed by the National Greek Aseismic Code(Fig. 7(a) and (b)), for the soil category C (D accord-ing to EC8). The scope of these preliminary analyses isnot to compare them with actual recordings, but ratherto check the 1D model and identify the threshold be-tween linear-elastic and non-linear phenomena. As itis illustrated in Fig. 8, the non-linear behaviour of soilsis expected from rather low intensities (>0.15 g),while strong de-amplification is observed at high shak-ing levels independently of the input motion charac-teristics. Recorded ground motions are very small toidentify experimentally non-linear phenomena due tosoil properties. However, in the small strain range of

362K

.Pitilakis

etal./C.R

.Geoscience

336(2004)

353365

Fig. 6. The three components of a small event (Ms = 3.5) as recorded on the central recording system of CORSSA. Time histories and their acceleration response spectra at thesurface for their horizontal components.

Fig. 6. Les trois composantes dun petit vnement (Ms = 3.5) report dans le systme denregistrement central de CORSSA. Histoires dans le temps et leurs spectres de rponsedacclration la surface pour leurs trois composantes horizontales.

K.P

itilakisetal./C

.R.G

eoscience336

(2004)353365

363

Fig. 7. Comparison between absolute (A) and normalized (B) acceleration response spectra of (a) Ricker wavelets, (b) deconvoluted longitudinal component of the main Aegionearthquake, (c) deconvoluted transversal component of the main Aegion earthquake scaled to PGA = 0.1 0.5 g and the Hellenic Aseismic Code (EAK2000) for soil category C(damping used = 5%).Fig. 7. Comparaison entre spectres de rponse dacclration absolus (A) et normaliss (B) : (a) ondelettes de Ricker, (b) composante longitudinale dconvolue du principaltremblement de terre dAegion, cal lchelle de PGA = 0,1, (c) composante transversale dconvolue du principal tremblement de terre dAegion lchelle PGA = 0,1 0,5 get selon le code grec dasismicit (EAK 2000) pour la catgorie C du sol (amortissement de 5 %).

364 K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365

Fig. 8. Computed PGA at the surface of the CORSSA array versus the input ground motion at 178.0 m. The amplification of the horizontalcomponents of the recorded motion is given in an enlarged scale.

Fig. 8. PGA calcul la surface de la flche de CORSSA en fonction du mouvement de terrain dentre 178,0 m. Lamplification descomposantes horizontales du mouvement enregistr est donne selon une chelle agrandie.

K. Pitilakis et al. / C. R. Geoscience 336 (2004) 353365 365

these recordings, we have observed quite importantamplifications of both horizontal (Fig. 8) and verticalcomponents (Fig. 6).

7. Liquefaction susceptibility and risk

Preliminary analyses of the liquefaction suscepti-bility and risk indicate that the liquefaction suscep-tibility is very important in the loose saturated siltysand layers found at depths 4 6 m and 12 20 m(NSPT = 5 6, D60/D10 4, PI < 5). The risk of liq-uefaction is high for acceleration values greater than0.20 g at outcrop conditions.

8. Conclusive remarks

CORSSA is a well-established experimental facil-ity in Europe. With all other experiments and facilitiesdeployed in the Corinth Gulf, it is expected to gen-erate a set of high-quality and well-documented data(geological, geotechnical, etc.) and recordings fromlocal and regional earthquakes that will enrich theEuropean and World databases of strong recordings.

The choice of the site is particularly successful com-bining high seismicity, favourable soil conditions tostudy the effects of non-linear and liquefaction phe-nomena and geological-tectonic and topographic fea-tures on ground motion. The release of the data andthe accompanying studies will be very useful bothfor countries with high seismicity, like Greece, andlow-deformation-rate areas with weak seismicity, likeFrance.

References

[1] CORSEIS, 2nd-Year Scientific Report, An integrated studyof seismic hazard assessment in the area of Aigion, Gulfof Corinth, Greece, EVG1-1999-00002, Laboratory of SoilMechanics and Foundation, Civil Engineering Department,Aristotle University of Thessaloniki, Greece, 2001.

[2] CORSEIS, Final Scientific Report, An integrated study of seis-mic hazard assessment in the area of Aigion, Gulf of Corinth,Greece, EVG1-1999-00002, Laboratory of Soil Mechanics andFoundation, Civil Engineering Department, Aristotle Universityof Thessaloniki, Greece, 2002.

[3] CyberQuake V. 2.0. (19982000) Created by H. Modaressi.[4] P. Thierry, F. Lemeille, C. Berge-Thierry, Processing and inter-

pretation of a 720-m-long HR seismic profile across an activefault (Aigion Fault, Corinth Gulf, Central Greece), Near SurfaceGeophysics, in preparation.

The Corinth Gulf Soft Soil Array (CORSSA) to study site effectsVersion franaise abrgeIntroductionLocationTechnical description of CORSSAGeological, geotechnical and geophysical characterization of the CORSSA soft-soil formationsRecordingsGround response analysesLiquefaction susceptibility and riskConclusive remarksReferences