La connaissance et la caractérisation du bâti existant est une problématique attirant l'intérêt de nombreuses activités depuis quelques années. L'utilisation des vibrations ambiantes permet de connaître facilement les paramètres dynamiques élastiques de la structure, que sont les fréquences de vibration, les amortissements et les modes de déformations. La fréquence de vibration étant facilement observable par le calcul de la transformée de Fourier d'un enregistrement au sommet de la structure, l'amortissement quant à lui reste un des paramètres les moins bien connus et son origine physique reste imprécise. Dans cette étude, nous essayerons de mesurer la fréquence et l'amortissement sur plusieurs bâtiments, d'évaluer la stabilité et la précision des mesures effectuées, en particulier afin de connaître jusqu'à quelle précision on peut relier des variations des paramètres modaux à des variations des propriétés physiques. Nous analyserons les variations observées pour des périodes variant de 1 mois à plus de 1 an. Ces études continues sur au moins un mois permettront de mettre en évidence les variations réversibles des paramètres dynamiques de la structure, et de les mettre en relation avec des forçages externes. On observe des variations journalières sur la fréquence et l'amortissement dans tous les bâtiments étudiés. La relation entre ces variations, et d'autres variations observées à plus long terme, avec la température diffère d'un bâtiment à l'autre. La relation entre fréquence et amortissement et l'effet de l'Interaction Sol-Structure ont été étudiés. / The knowledge and characterization of existing buildings is an issue that attracts the interest of many activities in recent years. The use of ambient vibrations allows an easy identification of the dynamic parameters of elastic structure, such as vibration frequencies, damping and mode shapes. The vibration frequency is easily observable through Fourier Transform of a recording at the top of the structure. As for the damping, it remains the least known and its physical origin remains unclear. In this study, we try to measure the frequency and damping over several buildings, to assess the stability and accuracy of measurements, especially in order to know precisely how far one can relate changes in modal parameters to changes in physical properties. We analyze the variations observed on studied buildings for periods varying from 1 month up to more than 1 year. These ongoing studies on at least one month will highlight the reversible changes of dynamic parameters of the structure, and put them in relation to external forcing. We observe diurnal variations on the frequency and damping in all the studied buildings. The relationship between these variations and other variations of a longer term, with the temperature differs from one building to another. The relationship between frequency and damping and the effect of Soil-Structure Interaction were studied.

L'objectif de cette thèse est d'analyser et de caractériser le lien entre les mouvements du sol et la réponse des bâtiments. En particulier, on s'intéresse à réduire les incertitudes qui entrent en jeu dans les méthodes de vulnérabilité, afin d'estimer plus précisément la vulnérabilité des structures au risque sismique. Pour ce faire, cette thèse est basée sur l'utilisation de données enregistrées au sein de bâtiments. Les enregistrements de vibrations ambiantes et de séismes de faibles amplitudes au sein de bâtiments permettent de caractériser le comportement élastique de ces structures. Des modèles peuvent ensuite être définis afin d'étudier les différentes composantes de la variabilité présente au sein des courbes de fragilité. On s'intéresse dans un premier temps au premier niveau de dommage, au-delà duquel les caractéristiques élastiques des bâtiments sont modifiées et les modèles définis ne sont plus valables. Des enregistrements de séismes ayant eu lieu depuis le début des années 1970 dans des bâtiments californiens sont ensuite utilisés. A partir de ces données, on a pu mettre en évidence des relations entre le déplacement au sein des structures et des paramètres décrivant la nocivité des signaux sismiques. Suivant l'indicateur utilisé pour représenter la nocivité des séismes, la réponse des bâtiments peut être estimée plus ou moins précisément. Notamment, les indicateurs faisant intervenir les paramètres dynamiques des bâtiments sont davantage liés avec la réponse des structures, représentée par son déplacement inter-étage moyen. La variabilité de la réponse des bâtiments peut être améliorée en regroupant les bâtiments par typologies (définies suivant leur matériau principal de construction et leur hauteur). En apportant davantage d'informations sur les structures, on peut ainsi diminuer la composante épistémique de la variabilité. De plus, en combinant des indicateurs de nocivité, on peut améliorer la précision pour la prédiction de la réponse de la structure. Une forme fonctionnelle est ainsi proposée afin d'estimer le déplacement inter-étage moyen au sein des structures, pour plusieurs typologies de bâtiments, à partir de quatre indicateurs de nocivité. Cette forme fonctionnelle est utilisée pour établir des courbes de fragilité et peut également être utilisée afin de donner une première estimation des dommages à la suite d'un séisme, en comparant les valeurs de déformations inter-étages avec des valeurs de référence (FEMA, 2003). Enfin, une méthode hybride est proposée pour la construction de courbes de fragilité, faisant intervenir un modèle de comportement non linéaire. Les paramètres de ce modèle sont définis de telle sorte que la réponse du modèle s'ajuste aux enregistrements de séismes effectués dans les bâtiments. Ce modèle est ensuite utilisé pour évaluer les composantes d'incertitudes et pour construire des courbes de fragilité pour tous les niveaux de dommages. / This PhD thesis is focused on the correlation between ground motion and the buildings response. In particular, we interested in reducing the uncertainties that occur in vulnerability methods in order to assess more precisely the seismic vulnerability of existing structures. In order to do this, this thesis is based on the use of data recorded within buildings. Records of ambient vibrations and low amplitude earthquakes within buildings are used to characterize the elastic behaviour of these structures. Linear models can then be defined in order to study the various components of the variability of the fragility curves, for the first damage level. Earthquake that have occurred since the 1970s have been recorded within Californian buildings. From these data, we focused on the relationship between the building response and parameters describing the noxiousness of ground motion. Depending on the indicator used to represent the noxiousness of earthquakes, the building response can be estimated more or less accurately. In particular, indicators involving the dynamic parameters of the buildings are more related with the response of structures, which is represented by its averaged inter-story drift. Grouping buildings by typologies (defined according to their main material of construction and their height) can improve the variability in the response of buildings. Indeed, by providing more information on the structure, we can reduce the epistemic component of variability. In addition, by combining noxiousness parameters, the accuracy in the prediction of the building response can be improved. A functional form is thus proposed to estimate the averaged inter-story drift within the structures, for several typologies of buildings. This functional form is then used to assess fragility curves and can also be used to get an estimate of damage after an earthquake, by comparing the values of inter-story drift from given by the functional form with reference values (FEMA, 2003). Finally, a hybrid method is proposed for the construction of fragility curves, involving a nonlinear model. The parameters of this model are defined so that the response of the model fits the earthquake recordings, which were made within buildings. This model is then used to evaluate the components of variability and to build fragility curves for all damage levels.

Master of Science / Department of Civil Engineering / Hani G. Melhem / The objective of this research is to use structure ambient random vibration response to detect damage level and location. The use of ambient vibration is advantageous because excitation is caused by service conditions such as normal vehicle traffic on a highway bridge, train passage on a railroad bridge, or wind loads on a tall building. This eliminates the need to apply a special impact or dynamic load, or interrupt traffic on a bridge in regular service. This research developed an approach in which free vibration of a structure is extracted from the response of this structure to a random excitation in the time domain (acceleration versus time) by averaging out the random component of the response. The result is the free vibration that includes all modes based on the sampling rate on time. Then this free vibration is transferred to the frequency domain using a Fast Fourier Transform (FFT). Variations in frequency response are a function of structural stiffness and member end-conditions. Such variations are used as a measure to identify the change in the structural dynamic properties, and ultimately detect damage. A physical model consisting of a 20 × 20 × 1670 -mm long steel square tube was used to validate this approach. The beam was tested under difference supports conditions varying from a single- to three-span continuous configuration. Random excitation was applied to the beam, and the dynamic response was measured by an accelerometer placed at various locations on the span. A numerical model was constructed in ABAQUS and the dynamic response was obtained from the finite element model subjected to similar excitation as in the physical model. Numerical results were correlated against results from the physical model, and comparison was made between the different span/support configurations. A subsequent step would be to induce damage that simulates loss of stiffness or cracking condition of the beam cross section, and that would be reflected as a change in the frequency and other dynamic properties of the structure. The approach achieved good results for a structure with a limited number of degrees of freedom. Further research is needed for structures with a larger number of degrees of freedom and structures with damage in symmetrical locations relative to the accelerometer position.

This thesis develops and explores two methodologies to assess earthquake ground shaking in southwestern British Columbia based on 1D soil layering and 3D basin structure. To assess site response based on soil layering, microtremor array measurements were conducted at two sites of contrasting geology to estimate Rayleigh-wave dispersion curves. A Bayesian inversion algorithm is developed to invert the dispersion data for the shear-wave velocity (VS) profile together with quantitative uncertainty estimates, accounting rigorously for data error covariance and model parameterization selection. The recovered VS profiles are assessed for reliability by comparison with invasive VS measurements at each site with excellent agreement. Probabilistic site response analysis is conducted based on a sample of VS profiles drawn from the posterior probability density of the microtremor inversion. The quantitative uncertainty analysis shows that the rapid and inexpensive microtremor array method provides sufficient resolution of soil layering for practical characterization of earthquake ground motion. To assess the effects of 3D Georgia basin structure on long-period (> 2 s) ground motion for large scenario earthquakes, numerical 3D finite difference modelling of viscoelastic wave propagation is applied. Both deep (> 40 km) subducting Juan de Fuca plate and crustal (5 km) North America plate earthquakes are simulated in locations congruent with known seismicity. Simulations are calibrated by comparing synthetic waveforms with 36 selected strong- and weak-motion seismograms of the 2001 MW 6.8 Nisqually earthquake. The ratio between predicted peak ground motions in models with and without Georgia basin sediments is applied as a quantitative measure of basin amplification. Steep edges in the upper 1 km of the northwest and southeast extents of the basin are coincident with the appearance of surface waves. Focussing of north-to-northeast propagating surface waves by shallow (< 1 km) basin structure increases ground motion in a localized region of southern Greater Vancouver. This effect occurs for both types of earthquakes located south-southwest of Vancouver at distances greater than ~80 km. The predicted shaking level is increased up to 17 times and the duration of moderate shaking (> 3.4 cm/s) is up to 16 times longer due to the 3D Georgia basin structure. / Graduate

Les éboulements rocheux de volume intermédiaire (103-105 m3) posent un problème sérieux dans les régions montagneuses en raison de leur fréquence d'occurrence relativement élevée et de leur fort pouvoir destructeur. En conséquence, il est difficile de mener des travaux de protection pour réduire l'aléa, ce qui souligne le besoin de techniques de suivi et d'alerte précoce avant la rupture. Après l'éboulement, peu d'informations quantitatives sont disponibles sur la phase de propagation en raison de la soudaineté du phénomène et de sa localisation dans des pentes raides et difficiles d'accès. Dans ce travail de thèse, une approche expérimentale est proposée pour extraire des informations à partir d'enregistrements de signaux sismiques durant les phases pré- et post-rupture d'éboulements.La première partie de ce travail vise à tester la pertinence des vibrations ambiantes pour le suivi temporel de la réponse dynamique de compartiments rocheux lors de la phase pré-rupture. Cette technique (couramment employée en génie civil pour le suivi de l'intégrité des structures) permet d'extraire les fréquences de résonance d'une structure, dont la décroissance peut traduire l'endommagement. Une étude antérieure menée sur une colonne calcaire instable a montré une décroissance d'environ 30% de la fréquence fondamentale de résonance (f1) environ deux semaines avant la rupture, interprétée comme une diminution de la rigidité du contact avec le massif stable adjacent. / Mid-size rockfalls (103-105 m3) represent a substantial hazard in mountainous areas, because of relative high rate of occurrence and destructive power. Consequently, few protection means can be applied, emphasizing the need for monitoring techniques and early warning prior to the collapse. After the rupture, quantitative information on the rockfall propagation phase is scarce, owing to their suddenness and location in steep and rugged slopes. In this thesis work, an experimental approach is proposed to derive valuable information from seismic records during rockfall pre-rupture and post-rupture phases. The first part of this work aims at testing the applicability of the ambient vibration technique to monitor unstable rock compartments dynamic response in the pre-rupture phase. This technique (commonly employed in civil engineering for structural health monitoring) reveals the resonant frequencies of a structure, a decrease in frequency revealing potential damage. A previous case study of an unstable limestone compartment brought to light a #30% decrease in fundamental resonant frequency (f1) two weeks before the collapse. Following this innovative work, we selected and instrumented four prone-to-fall medium-size rock compartments located in the Occidental Alps, showing various geological contexts (limestones, argillite and shale-sandstone series), deformation patterns and failure mechnisms. Ambient vibrations recorded on-site revealed caracteristic seismic noise features. Spectral peaks were observed and attributed to resonant frequencies of the rock compartments, the fundamental resonant frequency (f1) showing clear polarization parallel to the line of maximum slope gradient, and perpendicular to the main bounding fracture observed at most of the sites. Similar findings were made for an unstable rock compartment located in a volcanic caldera, characterized by rapid morphological changes and intense rockfall activity. The dynamic response of the rear fracture network was explored, showing that spectral content of seismic noise is controlled by the caldera structure in the 0.5-5 Hz range. The direction of vibration is polarized perpendicularly to the fractures, while vibration amplitudes are linked to compartment uncoupling from the rock massif. In this case, the physical origin of seismic noise amplification may be due to complex 2D or 3D resonance effects. For the four alpine sites, the fundamental frequency f1 was monitored over more than one year, showing fluctuations clearly correlated with temperature oscillations. The thermal control over f1 is highly complex, showing both positive and negative correlations, depending on site morphology and destructuration, as well as on the studied oscillations periods (daily or seasonal). No change in fundamental frequency resulting from damage was observed over this time span. One site, characterized by intense rock fracturing and a deep-open rear fracture, showed high f1 sensitivity to temperature changes. Thermo-mechanical numerical simulations revealed that both material contraction-dilation and thermal dependancy of the elastic modulus control f1 fluctuations. In addition, high amplitude seasonal f1 oscillations were explained by ice formation in the rear fracture. A criterion was developped to separate thermal-induced f1 fluctuations from damage effects, under the hypothesis that thermal sensitivity of a rock compartment increases towards failure. The second part of this work relates to the post-rupture phase of rockfalls. The seismic records generated by two mid-size rockfalls {one natural, one provoked{ that occured in the same place were analyzed, showing complex enveloppe and spectrogram features. Both events showed close magnitude, duration and spectral content. The seismic signals of the provoked event were calibrated using video shots, allowing estimation of fallen material velocity during the successive propagation phases.

Le Liban n'a pas souffert de grands tremblements de terre destructeurs depuis près de deux siècles. Il est toutefois traversé par la faille transformante majeure du Levant, séparant sur1000 km de longueur la plaque Arabique à l’est de la plaque Africaine à l’ouest. Ses principales branches au Liban sont la faille de Yammouneh qui traverse le pays du sud au nord, les failles de Serghaya et Rachaya dans sa partie Est, la faille de Roum et les failles inverses du Mont Liban dans la partie Ouest. Ces failles ont généré des séismes destructeurs dans la longue histoire connue de la région, parmi lesquels les plus importants sont ceux deJuillet 551 sur la faille du Mont-Liban, de mai 1202 sur la faille de Yammouneh, d’Octobre1759 sur la faille de Rachaya et de Novembre 1759 sur la faille de Serghaya. L'évaluation del'aléa et du risque sismique local est donc de première importance pour l'ensemble du pays.L'objectif du travail effectué dans cette thèse est d'appliquer au Liban les avancées réalisées ces dernières années dans le développement de nouveaux outils à la fois fiables et économiquement abordables pour l’évaluation de l’aléa sismique, en commençant par les grandes villes et en particulier la capitale Beyrouth. L'objectif est de mieux appréhender et comprendre le risque sismique sur le territoire libanais, pour pouvoir ensuite commencer à élaborer des politiques de prévention et des codes parasismiques qui puissent le réduire à terme.Un réseau sismologique temporaire composé de 10 stations a été installé dans Beyrouth etune partie de sa banlieue sur des sites représentatifs des principales unités géologiques présentes. Plusieurs dizaines de séismes locaux et régionaux ont pu y être enregistrés, et leur réponse sismique a été évaluée par la méthode du rapport spectral site sur référence (SSR),comparé au rapport spectral de la composante horizontale sur la composante verticale (H/V)calculé sur les tremblements de terre et sur le bruit ambiant. Les mêmes enregistrements ont également été utilisés pour prédire, par la technique des Fonctions de Green empiriques(FGE), le mouvement sismique correspondant à un événement majeur (Mw7.5) sur la faille de Yammouneh. Cet exercice de prédiction a toutefois été réalisé en deux étapes en raison des limitations dans l'application de la technique FGE en champ proche, avec deux techniques complémentaires: l'enregistrement d’un petit événement a été d'abord utilisé pour simuler un séisme de Mw6.5 sur la faille de Yammouneh, en parallèle à l'utilisation de plusieurs équations de prédiction de mouvement du sol (GMPE), soigneusement sélectionnées, pour effectuer une prédiction similaire. La comparaison FGE/GMPE a alors permis de calibrer la prédiction du mouvement du sol par GMPE à différents sites de Beyrouth pour l'événement cible de Mw7.5. Ces résultats ponctuels ont ensuite été étendus à l'ensemble de la municipalité de Beyrouth et de sa banlieue proche, en vue de mieux cerner les contours d'une future carte de microzonage, au travers d'une vaste campagne de mesures de bruit ambiant sur615 sites. Leur traitement H/V a permis d’obtenir une carte de la fréquence de résonance pour l'ensemble de la zone, carte dont la robustesse a été testée et prouvée. Des mesures sismiques actives et passives ont en outre été menées sur les principales unités géologiques à proximité des 10 sites préalablement sélectionnés et instrumentés, permettant ainsi d'obtenir les premières estimations directes de la vitesse des ondes de cisaillement (via les courbes de dispersion des ondes de Rayleigh). La comparaison de ces mesures avec les estimations -très dispersées - issues de la compilation des paramètres géologiques/géotechniques disponibles et des équations de corrélation existantes avec les valeurs N des SPT, montre tout l'intérêt de ces mesures simples et fiables. / Lebanon is one of the countries that have not suffered from large destructive earthquakes foralmost two centuries. It is however lying on the 1000 km long, left lateral Levant fault thatseparates the Arabic plate in the east from the African plate in the west. Its main branches inLebanon are the Yammouneh fault that crosses the country from south to north, the Serghayaand Rachaya faults in its Eastern part, the Roum and Mount Lebanon Thrust faults in itsWestern part. These faults have generated destructive earthquakes in the long known historyof the area. The largest events are: The July 551 earthquake on the Mount Lebanon Thrustfault, the May 1202 earthquake on the Yammouneh fault, the October 1759 on the Rachayafault and the November 1759 on the Serghaya fault. From all above, one can conclude thatLebanon is exposed to a significant seismic hazard. Assessing the local seismic hazard andrisk is therefore of primary importance for the whole country.The objective of the work undergone in this PhD is to take advantage of the latest advancesachieved worldwide to promote rather inexpensive, though reliable, seismic hazardassessment tools, to try to apply them in Lebanon starting with the big cities and specificallythe capital Beirut. These studies will help to understand the Lebanese seismic risk andsubsequently to start to elaborate seismic policies and codes that may help reducing this risk.A temporary seismological network consisting of 10 stations has been installed in Beirut anda part of its suburbs. Several tens of local and regional earthquakes could be recorded, andallowed to estimate the site response at selected sites in Beirut through the standard site toreference spectral ratio method ("SSR") on earthquakes, compared to the horizontal tovertical ratio ("H/V") calculated on earthquakes and on ambient noise. The same recordingscould also be used via the empirical Green Function’s technique ("EGF") to predict theseismic ground motion corresponding to a Mw7.5 on the Yammouneh fault. However, due tolimitations in near-field applications of the EGF technique, this prediction exercise wasperformed in two steps and with two complementary techniques: a weak event recording wasfirst used to simulate a Mw6.5 earthquake on the Yammouneh fault, while several, carefullyselected ground motion prediction equations (GMPE) were used to perform a comparativeprediction for the same earthquake. This EGF/GMPE comparison then allowed tuning theGMPE prediction of ground motion at various sites within Beirut for the target Mw7.5 event.The results were then extended in view of proposing a framework for a future microzonationviimap. A comprehensive campaign of ambient noise measurements was achieved for 615 sitesof Beirut municipality and close suburbs, the H/V processing of which allowed to derive arobust map of resonance frequency for the whole area. In addition, active and passive seismicmeasurements were conducted on different geological units near the 10 formerlyinstrumented sites, which provided quantitative estimates of the shallow S-wave velocitythrough the Rayleigh wave dispersion curves. These geophysical measurements permitted toprovide direct estimates of the shear waves velocity, which prove much more reliable thanthe highly scattered estimates derived from the compilation of the availablegeological/geotechnical parameters and the use of existing correlations equations betweenSPT N-value and S-wave velocity Vs.

La détection d'endommagements et de changements des propriétés élastiques dans des structures, utilisant les variations des paramètres dynamiques, fait l'objet d'une attention particulière depuis plusieurs années dans les domaines du génie mécanique et du génie civil. Le principe général repose sur le fait que la variation des propriétés physiques (e.g. rigidité, masse, module d'Young, conditions aux limites) entraîne une variation des caractéristiques dynamiques de la structure (e.g. fréquences de résonance, amortissements modaux et déformées modales). La présence d'endommagement provoque ainsi une diminution de la rigidité de la structure, c'est-à-dire une augmentation de sa flexibilité et de son amortissement que l'on retrouve dans la forme des modes et les valeurs des fréquences. Utilisant le changement de ces informations entre un état sain et un état endommagé, plusieurs méthodes non-destructives ont été proposées dans la littérature afin d'identifier et de localiser ces endommagements. Ces pratiques et ces activités sont d'une importance considérable puisqu'elles permettent en premier lieu d'anticiper et donc d'éviter des ruptures dans les structures, toujours catastrophiques, et plus généralement de mettre en place des plans de maintenance prédictives, en lien avec le suivi sur le long terme de leur intégrité (Structural Health Monitoring). Ces méthodes de surveillance se popularisent également du fait de la réduction des coûts des instrumentations, liés à l'apparition de nouveaux équipements à bas coût, ayant des performances satisfaisantes. L'objectif de ce travail est de tester les différentes solutions permettant la détection, la localisation et la quantification des changements dans des structures simples. Plusieurs méthodes ont été testées et une approche nouvelle a été proposée basée sur l'utilisation de la méthode des perturbations. Trois approches ont été suivies: une modélisation par éléments finis (analyse modale), une simulation numériques par éléments finis (analyse temporelle) et enfin des analyses expérimentales sur des poutres en Plexiglas au laboratoire, les trois volets de ce travail ayant permis de tester la sensibilité des méthodes non-destructives (NDE) globales et locales pour la détection et la localisation. Les changements ont été associés à une variation locale du module de Young (E), numériquement pour les solutions numériques et par chauffage local sur des sections de la poutre dans le volet expérimental. Dans tous les cas, nous sommes en situations réelles afin de proposer l'identification des caractéristiques modales par des méthodes opérationnelles (Operative Modal Analysis) telles que la méthode du décrément aléatoire et la méthode de décomposition dans le domaine fréquentiel (Frequency Domain Decomposition). Les résultats d'identification ont montré une très bonne corrélation entre les valeurs numériques et les valeurs expérimentales obtenues : fréquences de résonance et déformés modale. Pour identifier l'endommagement, les méthodes de localisation basées sur la courbure des déformées propres, la matrice de flexibilité, la courbure de flexibilité et enfin sur la méthode d'inversion des modes ont été employées. D'après les résultats obtenus, la méthode d'inversion se montre efficace dans le cas de variations modales faibles et transitoires, alors que la méthode de la courbure de flexibilité donne généralement de bons résultats et apparait robuste lorsque les variations sont plus prononcées. / The detection of damage and changes in elastic properties of structures, using the variation of dynamic parameters, has been the subject of special attention for several years in the fields of mechanical and civil engineering. The general principle is based on the fact that the variation of physical properties (e.g. stiffness, mass, Young's modulus, boundary conditions) leads to a change in the dynamic characteristics of structures (e.g. resonance frequencies, modal damping and mode shapes). The presence of damage causes a decrease in the rigidity of structures, which give rise to an increase in flexibility and damping, which can be seen in mode shapes and frequency values. Using the change of this information between a healthy and damaged condition, several non-destructive methods have been proposed in the literature in order to identify and locate the damage. These practices and activities are of considerable importance. They allow us to anticipate and avoid breaks in structures, which are always catastrophic, and more generally, they allow us to establish the plans of a predictive maintenance, along with monitoring of the long-term of integrity (Structural Health monitoring). These monitoring methods are equally us popular because of the low cost of instrumentation, related to the appearance of new equipment at low cost, having the satisfactory performance. The objective of this work is to test different solutions, allow for detection, localization and quantification of changes in simple structures. Several methods have been tested and a new approach is proposed based on the use of the perturbation method. Three approaches are followed: finite element modeling (modal analysis), finite element numerical simulation (temporal analysis), and finally, experimental analysis of a Plexiglas beam in the laboratory. These three scopes of work have allowed us to test the sensitivity of global and local non-destructive methods (NDE) for detection and localization of damage. Changes associated with a local variation of Young's modulus (E) are tested numerically in modal and temporal analysis, and shown experimentally in local heating on the sections of beam. In all cases, we are in real life situations, where we identify modal characteristics by operational methods (Operative Modal Analysis) such as the random decrement technique and the method of decomposition in the frequency domain (Frequency Domain Decomposition). The results show a very good correlation between the numerical and experimental values obtained: resonant frequencies and mode shapes. For identifying damage, localization methods based on the curvature of mode shape, flexibility matrix, curvature of flexibility, and finally on the method of inversion of modes are employed. According to the results, the method of inversion proves effective in the case where modal variation is low and transient, whereas, the curvature of flexibility (ULS method) usually gives good results and appears robust when the changes are more pronounced.

Local site conditions are known to influence ground motion during earthquake events and increase the severity of damage. Data from earthquakes are useful to study the response but they are available only from active regions. Ubiquitous ambient vibrations on the other hand offer a more practical approach to quantify site responses. This thesis explores the use of various methods for obtaining site responses. The primary area of study is the Kachchh rift basin, NW India, a Mesozoic rift that features significant lateral variations in surface geology and has experienced ground responses during 1819 and 2001 earthquakes. The Mw 7.6, 2001 event was followed by hundreds of aftershocks, which were recorded by temporary networks. In this study we have used earthquake signals as well as ambient vibrations to understand site response in various parts of the basin. In addition we have collected data from a few sites from the Indo-Gangetic plains and Kathmandu valley, both affected by large earthquakes, 1934 the M ~ 8 (Bihar) and 2015, Mw 7.8 (Nepal). Velocity and acceleration records from a network of eight stations in the Kachchh Rift were used to evaluate site responses using Standard Spectral Ratio (SSR) and Horizontal to Vertical spectral ratio (HVSR-E) methods. Ambient vibrations were analyzed following Nakamura’s H/V method (HVSR-AV), for data collected from 110 sites that represent different field conditions within the Kachchh Rift. Fundamental resonance frequency (f0) varied between 0.12 – 2.30 Hz, while the amplification factor (A0) was in the range of 2.0 – 9.1. We found that higher A0 and liquefaction index (Kg) values were mostly associated with higher liquefaction potential. Using a close network of stations, we studied the role of site response in damage to the Bhuj city that suffered maximum damage in 2001; our results suggest that site response was not a significant factor. Studies based on passive data were complemented by Multi-channel Analysis of Surface Waves (MASW) to map shear wave velocities of the various subsurface units up to depths of 10m (Vs10) and 30m (Vs30). Our results imply average Vs could be a good proxy to characterize site amplifications where sediment thicknesses are shallow. Power law relationship between f0 and thickness (h) suggest a strong positive correlation (r = 0.89) adding credence to HVSR-AV method, making it a cost-effective alternative to MASW to infer site conditions. Further, to understand the influence of topography on site effects, we analyzed data from hills, valleys and their edges, both from the Kachchh rift and Kathmandu valley. Sites on the edges of valleys showed multiple, fuzzy peaks in the low frequency range (< 1 Hz) and broad peaks attributable to sites prone to higher damage. Spectrograms generated through Huang-Hilbert Transforms (HHT) suggested focusing of energy in narrow frequency bands on the edges, while valleys tend to scatter energy over wide frequencies. Although our current results are based on limited observations, we recognize spectral analysis as a powerful tool to quantify site effects in regions with significant topography. It is known that coseismic liquefaction could lead to nonlinear behavior wherein the near-surface soil layer loses its shear strength, causing a reduction of its fundamental resonance frequency. We used data from selected sites of coseismic liquefaction to highlight the significance of nonlinear effects in site response. Earthquake signals and ambient vibrations from Umedpur, a region that experienced intense liquefaction during 2001 were used in this analysis. Here we followed an empirical decomposition method based on HHT and signals were decomposed as many intrinsic mode functions (IMFs) that showed characteristic peaks for events of various values of PGAs. Thus, the first IMF for events with relatively higher PGAs (0.03g) showed distinct peaks for the S wave coda part, which were not noted for those with lower PGA (0.01g). These observations in a region of coseismic liquefaction are useful in developing models for quantifying nonlinear behavior. In conclusion, site response studies using different types of data and processing techniques in regions affected by recent earthquakes brings out the scope and limitations of each of these sets of data and techniques. This study suggests that ambient vibrations provide reasonable estimates of site response and can be reliably used in regions where earthquake data are not available.