Estimation du mouvement fort en champ proche

Accumulated data of strong ground motions have been providing us very important knowledge about rupture processes of earthquakes, propagation-path, site-amplification effects on ground motion, the relation between ground motion and damage... However, most of the ground motion databases used in the development of ground motion prediction models are primarily comprised of accelerograms produced by small and moderate earthquakes. Hence, as magnitude increases, the sets of ground motions become sparse. Ground motion databases are poorly sampled for short source-to-site distance ranges (‘Near-fault’ ranges). However, the strongest ground shaking generally occurs close to earthquake fault rupture. Countries of moderate to high seismicity for which major faults can break in the vicinity of its major cities are facing a major seismic risk, but the lack of earthquake recordings makes it difficult to predict ground motion. Strong motion simulations may then be used instead. One of the current challenges for seismologists is the development of reliable methods for simulating near-fault ground motion taking into account the lack of knowledge about the characteristics of a potential rupture. This thesis is divided into 2 parts. Part 1 focuses on better understanding the seismic rupture process and its relation with the near-fault ground motion. The mechanisms of peak ground motion generating are investigated for homogeneous as well as for heterogeneous ruptures. A quantitative sensitivity analysis of the ground motion to the source kinematic parameters is presented, for sites located in the vicinity of the fault rupture, as well as far from the rupture. A second chapter is dedicated to a major near-fault source effect: the directivity effect. This phenomenon happens when the rupture propagates towards a site of interest, with a rupture speed close to the shear-wave speed (Vs); the waves propagating towards the site adds up constructively and generates a large amplitude wave called the pulse. The features of this pulse are of interest for the earthquake engineering community. In this chapter, a simple equation is presented that relates the period of the pulse to the geometric configuration of the rupture and the site of interest, and to the source parameters.Part 2 is dedicated to better estimate the seismic hazard in Lebanon by simulating the strong ground motion at sites near the main fault (the Yammouneh fault). Lebanon is located in an active tectonic environment where the seismic hazard is considered moderate to high. Historically, destructive earthquakes occurred in the past, the last one dates back to 1202. However, strong motion has never been recorded in Lebanon till now due to the presently infrequent large-magnitude seismicity, and therefore facing an alarming note of potential new ruptures. The Yammouneh fault is a large strike-slip fault crossing Lebanon, making all its regions located within 25km away from the fault. At first, the crustal structure tomography of Lebanon, in terms of Vs, is performed using the ambient noise, in order to characterise the wave propagation from the rupture to the ground surface. To our knowledge, this is the first study of the 3D Vs tomography in Lebanon. Afterwards, a hybrid approach is presented to simulate broadband near-fault ground motion . At low-frequencies (≤1Hz), potential ruptures of M7 are simulated (as defined in the previous chapters), and the generated slip rate functions are convolved with the Green’s functions computed for the propagation medium defined by the Vs tomography. The ground-motion is complemented by a high-frequency content (up to 10Hz), using a stochastic model calibrated by near-fault recordings and accounting for the presence of the directivity pulse. The computed peak ground acceleration is compared to the design acceleration in Lebanon.Les données accumulées sur les mouvements du sol apportent des connaissances très importantes sur les processus de rupture des séismes, les caractéristiques du milieu de propagation, la relation entre le mouvement du sol et les dommages des structures... Cependant, les séismes de faible et moyenne amplitude étant plus fréquents que les grands événements sismiques, les bases de données de mouvements de sol utilisées dans le développement de modèles de prédiction du mouvement du sol ne contiennent pas beaucoup de données de forts séismes. Le point le plus critique concerne les stations proches de la rupture de la faille, pour lesquelles les bases de données restent mal échantillonnées. Les pays à sismicité modérée ou élevée pour lesquels des failles majeures peuvent se briser à proximité de ses grandes villes, sont donc confrontés à un risque sismique majeur, mais le manque d’enregistrements du mouvement ne permet pas une bonne prédiction des mouvements fort du sol. Il est donc nécessaire de simuler le mouvement fort en champ proche. Cette thèse est divisée en 2 parties. La partie 1 se concentre sur une meilleure compréhension de la rupture sismique et de son rapport avec le mouvement du sol proche de la faille. Les mécanismes de génération des valeurs de pics du mouvement du sol sont étudiés pour des ruptures homogènes et hétérogènes. Une analyse quantitative de sensibilité du mouvement du sol aux paramètres cinématiques de la rupture est présentée, pour des sites au voisinage de la rupture ainsi qu’en champ lointain. Un second chapitre est consacré à un effet de source majeur en champ proche: l’effet de directivité. Ce phénomène se produit lorsque la rupture se propage vers un site, avec une vitesse de rupture proche de la vitesse de l'onde de cisaillement Vs; les ondes se propageant vers le sites interfèrent de manière constructive et génèrent une onde de grande amplitude appelée pulse. Les caractéristiques de ce pulse, notamment sa durée, représentent des paramètres d’intérêt pour le génie parasismique. Une équation simple est présentée pour relier la durée du pulse à la configuration géométrique de la rupture et du site d'intérêt et aux paramètres de la source. La partie 2 est consacrée à une meilleure estimation de l’aléa sismique au Liban en simulant le mouvement fort pour des sites proches de la faille principale: la faille de Yammouneh. Le Liban est situé dans un environnement tectonique actif où le risque sismique est considéré comme modéré à élevé. Historiquement, des tremblements de terre destructifs se sont produits dans le passé, le dernier remontant à 1202. Cependant, en raison de la sismicité de grande ampleur actuellement peu fréquente, aucun mouvement fort n'a jamais été enregistré au Liban à ce jour. La faille de Yammouneh est une grande faille en décrochement traversant le Liban du Nord au Sud, situant toutes les villes et infrastructures à moins de 25km de la faille. Une tomographie de la structure de la croûte du Liban, en termes de vitesse des ondes de cisaillement Vs, est réalisée en utilisant le bruit ambiant. À notre connaissance, il s’agit de la première étude de la tomographie Vs 3D au Liban. Par la suite, une approche hybride est utilisée pour simuler le mouvement du sol en champ proche sur une large bande de fréquences (0.1-10Hz). Aux basses fréquences (≤1Hz), des ruptures potentielles de M7 sont simulées (comme définie dans les chapitres précédents), et les fonctions sources obtenues sont convoluées aux fonctions de Green calculées pour le modèle de propagation des ondes issu de la tomographie Vs afin d’estimer le mouvement du sol à proximité de la faille. Le mouvement du sol est complété par un contenu haute fréquence (jusqu’à 10 Hz), en utilisant un modèle stochastique calibré par des enregistrements en champ proche, et en tenant compte de la phase impulsive due à la directivité de la rupture

Estimation of Near-Fault Strong Ground-Motion

Les données accumulées sur les mouvements du sol apportent des connaissances très importantes sur les processus de rupture des séismes, les caractéristiques du milieu de propagation, la relation entre le mouvement du sol et les dommages des structures... Cependant, les séismes de faible et moyenne amplitude étant plus fréquents que les grands événements sismiques, les bases de données de mouvements de sol utilisées dans le développement de modèles de prédiction du mouvement du sol ne contiennent pas beaucoup de données de forts séismes. Le point le plus critique concerne les stations proches de la rupture de la faille, pour lesquelles les bases de données restent mal échantillonnées. Les pays à sismicité modérée ou élevée pour lesquels des failles majeures peuvent se briser à proximité de ses grandes villes, sont donc confrontés à un risque sismique majeur, mais le manque d’enregistrements du mouvement ne permet pas une bonne prédiction des mouvements fort du sol. Il est donc nécessaire de simuler le mouvement fort en champ proche. Cette thèse est divisée en 2 parties. La partie 1 se concentre sur une meilleure compréhension de la rupture sismique et de son rapport avec le mouvement du sol proche de la faille. Les mécanismes de génération des valeurs de pics du mouvement du sol sont étudiés pour des ruptures homogènes et hétérogènes. Une analyse quantitative de sensibilité du mouvement du sol aux paramètres cinématiques de la rupture est présentée, pour des sites au voisinage de la rupture ainsi qu’en champ lointain. Un second chapitre est consacré à un effet de source majeur en champ proche: l’effet de directivité. Ce phénomène se produit lorsque la rupture se propage vers un site, avec une vitesse de rupture proche de la vitesse de l'onde de cisaillement Vs; les ondes se propageant vers le sites interfèrent de manière constructive et génèrent une onde de grande amplitude appelée pulse. Les caractéristiques de ce pulse, notamment sa durée, représentent des paramètres d’intérêt pour le génie parasismique. Une équation simple est présentée pour relier la durée du pulse à la configuration géométrique de la rupture et du site d'intérêt et aux paramètres de la source. La partie 2 est consacrée à une meilleure estimation de l’aléa sismique au Liban en simulant le mouvement fort pour des sites proches de la faille principale: la faille de Yammouneh. Le Liban est situé dans un environnement tectonique actif où le risque sismique est considéré comme modéré à élevé. Historiquement, des tremblements de terre destructifs se sont produits dans le passé, le dernier remontant à 1202. Cependant, en raison de la sismicité de grande ampleur actuellement peu fréquente, aucun mouvement fort n'a jamais été enregistré au Liban à ce jour. La faille de Yammouneh est une grande faille en décrochement traversant le Liban du Nord au Sud, situant toutes les villes et infrastructures à moins de 25km de la faille. Une tomographie de la structure de la croûte du Liban, en termes de vitesse des ondes de cisaillement Vs, est réalisée en utilisant le bruit ambiant. À notre connaissance, il s’agit de la première étude de la tomographie Vs 3D au Liban. Par la suite, une approche hybride est utilisée pour simuler le mouvement du sol en champ proche sur une large bande de fréquences (0.1-10Hz). Aux basses fréquences (≤1Hz), des ruptures potentielles de M7 sont simulées (comme définie dans les chapitres précédents), et les fonctions sources obtenues sont convoluées aux fonctions de Green calculées pour le modèle de propagation des ondes issu de la tomographie Vs afin d’estimer le mouvement du sol à proximité de la faille. Le mouvement du sol est complété par un contenu haute fréquence (jusqu’à 10 Hz), en utilisant un modèle stochastique calibré par des enregistrements en champ proche, et en tenant compte de la phase impulsive due à la directivité de la rupture.Accumulated data of strong ground motions have been providing us very important knowledge about rupture processes of earthquakes, propagation-path, site-amplification effects on ground motion, the relation between ground motion and damage... However, most of the ground motion databases used in the development of ground motion prediction models are primarily comprised of accelerograms produced by small and moderate earthquakes. Hence, as magnitude increases, the sets of ground motions become sparse. Ground motion databases are poorly sampled for short source-to-site distance ranges (‘Near-fault’ ranges). However, the strongest ground shaking generally occurs close to earthquake fault rupture. Countries of moderate to high seismicity for which major faults can break in the vicinity of its major cities are facing a major seismic risk, but the lack of earthquake recordings makes it difficult to predict ground motion. Strong motion simulations may then be used instead. One of the current challenges for seismologists is the development of reliable methods for simulating near-fault ground motion taking into account the lack of knowledge about the characteristics of a potential rupture. This thesis is divided into 2 parts. Part 1 focuses on better understanding the seismic rupture process and its relation with the near-fault ground motion. The mechanisms of peak ground motion generating are investigated for homogeneous as well as for heterogeneous ruptures. A quantitative sensitivity analysis of the ground motion to the source kinematic parameters is presented, for sites located in the vicinity of the fault rupture, as well as far from the rupture. A second chapter is dedicated to a major near-fault source effect: the directivity effect. This phenomenon happens when the rupture propagates towards a site of interest, with a rupture speed close to the shear-wave speed (Vs); the waves propagating towards the site adds up constructively and generates a large amplitude wave called the pulse. The features of this pulse are of interest for the earthquake engineering community. In this chapter, a simple equation is presented that relates the period of the pulse to the geometric configuration of the rupture and the site of interest, and to the source parameters.Part 2 is dedicated to better estimate the seismic hazard in Lebanon by simulating the strong ground motion at sites near the main fault (the Yammouneh fault). Lebanon is located in an active tectonic environment where the seismic hazard is considered moderate to high. Historically, destructive earthquakes occurred in the past, the last one dates back to 1202. However, strong motion has never been recorded in Lebanon till now due to the presently infrequent large-magnitude seismicity, and therefore facing an alarming note of potential new ruptures. The Yammouneh fault is a large strike-slip fault crossing Lebanon, making all its regions located within 25km away from the fault. At first, the crustal structure tomography of Lebanon, in terms of Vs, is performed using the ambient noise, in order to characterise the wave propagation from the rupture to the ground surface. To our knowledge, this is the first study of the 3D Vs tomography in Lebanon. Afterwards, a hybrid approach is presented to simulate broadband near-fault ground motion . At low-frequencies (≤1Hz), potential ruptures of M7 are simulated (as defined in the previous chapters), and the generated slip rate functions are convolved with the Green’s functions computed for the propagation medium defined by the Vs tomography. The ground-motion is complemented by a high-frequency content (up to 10Hz), using a stochastic model calibrated by near-fault recordings and accounting for the presence of the directivity pulse. The computed peak ground acceleration is compared to the design acceleration in Lebanon

Spatial Variability of the Directivity Pulse Periods Observed during an Earthquake

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Sensitivity of Earthquake Damage Estimation to the Input Data (Soil Characterization Maps and Building Exposure): Case Study in the Luchon Valley, France

This article studies the effects of the soil data and exposure data of residential building inventories, as well as their spatial resolution, on seismic damage and loss estimates for a given earthquake scenario. Our aim is to investigate how beneficial it would be to acquire higher resolution inventories at the cost of additional effort and resources. Seismic damage computations are used to evaluate the relative influence of varying spatial resolution on a given damage model, where other parameters were held constant. We use soil characterization maps and building exposure inventories, provided at different scales from different sources: the European database, a national dataset at the municipality scale, and local field investigations. Soil characteristics are used to evaluate site effects and to assign amplification factors to the strong motion applied to the exposed areas. Exposure datasets are used to assign vulnerability indices to sets of buildings, from which a damage distribution is produced (based on the applied seismic intensity). The different spatial resolutions are benchmarked in a case-study area which is subject to moderate-to-average seismicity levels (Luchon valley in the Pyrénées, France). It was found that the proportion of heavily damaged buildings is underestimated when using the European soil map and the European building database, while the more refined databases (national/regional vs. local maps) result in similar estimates for moderate earthquake scenarios. Finally, we highlight the importance of pooling open access data from different sources, but caution the challenges of combining different datasets, especially depending on the type of application that is pursued (e.g., for risk mitigation or rapid response tools)

Integrating strong-motion recordings and twitter data for a rapid shakemap of macroseismic intensity

Rapid estimation of the intensity of seismic ground motions is crucial for an effective rapid response when an earthquake occurs. To this end, maps of updated grond-motion fields (or shakemaps) are produced by using observations or measurements in near real-time to better constrain initial estimates. In this work, two types of observations are integrated to generate shakemaps right after an earthquake: the common type of data recorded by physical sensors (seismic stations) and the data extracted from social sensors (Twitter), or the combination of both. We investigate an approach to extract an approximation of the macroseismic intensity from social sensors 10 min after the earthquake; the approach relies on Twitter feeds to define the “felt area” where the earthquake was felt by the population, and the “unfelt locations” where the earthquake was not reported. Two recent earthquakes in France of moderate magnitude are studied and the results are compared to the official macroseismic intensity maps for validation. For the two studied cases, we note that Peak Ground Acceleration recordings far from the epicenter tend to underestimate the entire macroseismic field, and that the tweets from “felt areas” are complementary for a better estimation of the intensity shakemap. We highlight the importance and the limits of each type of observations when generating the seismic shakemaps

Bayesian updating for rapid earthquake loss assessment of road network systems

International audienceWithin moments following an earthquake event, observations collected from the affected area can be used to define a picture of expected losses and to provide emergency services with accurate information. A Bayesian Network framework could be used to update the prior loss estimates based on ground-motion prediction equations and fragility curves, considering various field observations (i.e., evidence). The present study explores the applicability of approximate Bayesian inference, based on Monte-Carlo Markov-Chain sampling algorithms, to a real-world network of roads where expected loss metrics pertain to the accessibility between damaged areas and hospitals in the region. Observations are gathered either from free-field stations (for updating the ground-motion field) or from structure-mounted stations (for the updating of the damage states of infrastructure components). It is found that the proposed Bayesian approach is able to process a system comprising hundreds of components with reasonable accuracy, time and computation cost. Emergency managers may readily use the updated loss distributions to make informed decisions

Rapid earthquake loss updating of spatially distributed systems via sampling-based Bayesian inference

Within moments following an earthquake event, observations collected from the affected area can be used to define a picture of expected losses and to provide emergency services with accurate information. A Bayesian Network framework could be used to update the prior loss estimates based on ground-motion prediction equations and fragility curves, considering various field observations (i.e., evidence). While very appealing in theory, Bayesian Networks pose many challenges when applied to real-world infrastructure systems, especially in terms of scalability. The present study explores the applicability of approximate Bayesian inference, based on Monte-Carlo Markov-Chain sampling algorithms, to a real-world network of roads and built areas where expected loss metrics pertain to the accessibility between damaged areas and hospitals in the region. Observations are gathered either from free-field stations (for updating the ground-motion field) or from structure-mounted stations (for the updating of the damage states of infrastructure components). It is found that the proposed Bayesian approach is able to process a system comprising hundreds of components with reasonable accuracy, time and computation cost. Emergency managers may readily use the updated loss distributions to make informed decisions

Hybrid Simulation of Near-Fault Ground Motion for a Potential Mw 7 Earthquake in Lebanon

International audienceLebanon is a densely populated country crossed by major faults. Historical seismicity shows the potential of earthquakes with magnitudes >7, but large earthquakes have never been instrumentally recorded in Lebanon. Here, we propose a method to simulate near-fault broadband ground motions for a potential Mw 7 earthquake on the Yammouneh fault (YF)—the largest branch of the Dead Sea Transform fault that bisects Lebanon from north to south. First, we performed the first 3D tomography study of Lebanon using ambient noise correlation, which showed that Lebanon could be approximated by a 1D velocity structure for low-frequency (LF) ground-motion simulation purposes. Second, we generated suites of kinematic rupture models on the YF, accounting for heterogeneity of the rupture process, and uncertainty of the rupture velocity and hypocenter location. The radiated seismic energy was next propagated in the inferred 1D velocity model to obtain suites of LF ground motions (<1 Hz) at four hypothetical near-fault seismic stations. These LF simulations included the main features of near-fault ground motions, such as the impulsive character of ground velocity due to the rupture directivity or fling-step effects (so-called pulse-like ground motions). Third, to obtain broadband ground motions (up to 10 Hz), we proposed a hybrid technique that combined the simulated LF ground motions with high-frequency (HF) stochastic simulations, which were empirically calibrated using a worldwide database of near-fault recordings. Contrary to other hybrid approaches, in which the LF and HF motions are generally computed independently, the characteristics of stochastic HF ground motions were conditioned on those of LF ground motions (namely on the characteristics of the velocity pulse, if it existed, or on the absence of a pulse). The simulated peak ground accelerations were in agreement with the ones reported in the Next Generation Attenuation-West2 (NGA-West2) database for similar magnitude and distances and with three NGA-West2 ground-motion prediction equations

Virtual Sources of Body Waves from Noise Correlations in a Mineral Exploration Context

International audienceThe extraction of body waves from passive seismic recordings has great potential for monitoring and imaging applications. The low environmental impact, low cost, and high accessibility of passive techniques makes them especially attractive as replacement or complementary techniques to active-source exploration. There still, however, remain many challenges with body-wave extraction, mainly the strong dependence on local seismic sources necessary to create high-frequency body-wave energy. Here, we present the Marathon dataset collected in September 2018, which consists of 30 days of continuous recordings from a dense surface array of 1020 single vertical-component geophones deployed over a mineral exploration block. First, we use a cross-correlation beamforming technique to evaluate the wavefield each minute and discover that the local highway and railroad traffic are the primary sources of high-frequency body-wave energy. Next, we demonstrate how selective stacking of cross-correlation functions during periods where vehicles and trains are passing near the array reveals strong body-wave arrivals. Based on source station geometry and the estimated geologic structure, we interpret these arrivals as virtual refractions due to their high velocity and linear moveout. Finally, we demonstrate how the apparent velocity of these arrivals along the array contains information about the local geologic structure, mainly the major dipping layer. Although vehicle sources illuminating array in a narrow azimuth may not seem ideal for passive reflection imaging, we expect this case will be commonly encountered and should serve as a good dataset for the development of new techniques in this domain