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Ultrasonic experiments have revealed an interesting scalling behaviour in the acoustic tranmission response of several sandstone rock samples. The acoustic traces have been recorded for a great variety of ambient pressures that simulate the characteristic in-situ stresses to which reservoir rocks are submitted.

International audience; Using earthquakes and seismic noise recordings on a broad-band station installed in October 2016 on the slope of the Nice airport at 17 m depth, we find a strong site amplification of the seismic waves (factor 10 around a frequency of 1Hz).The effects of superficial layers on the amplitude, frequency and duration of seismic waves (often called site effect) has been extensively studied during the last two decades because it is responsible of large damages. Until now, site effect studies were only realized inland; the potential offshore site effect generated by sediment layers has therefore never been studied specifically. Nonetheless, a correct estimation of the amplification caused by offshore sediments is of great importance for three main applications:-Landslides and tsunamis : seismic waves can trigger submarine landslides, that themselves can trigger tsunami waves. In numerical simulations and geotechnical experiments, it is then important to take into account the potential input wave amplifications. -Offshore paleoseismology: strong vibrations generated by large earthquakes can trigger turbidite landslides. Turbidite stratigraphy is then a powerful tool often used to evidence and characterize the occurrence of ancient earthquakes. A correct amplitude of the seismic ground motion that takes into account site effects, has then to be estimated in models to correctly infer the magnitude of ancient earthquakes.-Offshore infrastructures: in many highly populated zones, in order to gain new space, buildings and/or infrastructures are more and more often constructed on offshore areas.In order to test and quantify these potential amplifications under the sea, we installed a broad-band seismometer “PRIMA” on the slope of the airport of the city of Nice (south-east of France) at a depth of 17 meters. The seismological station PRIMA belongs to the EMSO-Nice cabled observatory (EMSO: European Multidisciplinary Seafloor and water-column Observatory).We then analyze the recordings of local and regional earthquakes and ambient seismic noise on this station, and compare them with the recordings of nearby stations inland (max 3 km away). We find a clear amplification of waves of a factor of ~10 (compared with inland station situated on rock) at a frequency of 0.8-1Hz, as well as smaller amplification peaks at higher frequencies for local earthquakes. These amplification will be compared with data from high resolution seismic profiles in order to better understand their origin.This result is particularly important for the city of Nice. Indeed, the airport slope already experienced a large landslide in 1979, which triggered a tsunami wave that killed 10 persons and caused extensive damages all along the coast. It is them of upmost importance to be able to evaluate if a future earthquake could cause the same domino effect.

The city of Quito (Ecuador's capital) is located in an Andean valley at 2800 meters above sea level. Surrounded by volcanoes, this city of approximately 2 million inhabitants is prone to major earthquakes, and it is particularly vulnerable since no seismic code is formally used for constructions. The study of the hazard and the seismic risk is, therefore, essential. Three types of earthquakes threaten the city: a) a close earthquake of moderate magnitude (M ~ 6.5), which would occur on the Quito fault system, b) a more distant earthquake which could have a higher magnitude (M ~ 7.5) coming from the cordillera, and c) finally a subduction earthquake coming from the coastline more than 170 km away, the magnitude of which could be very high (M> 8.5). This third type of earthquake struck Ecuador on April 16, 2016 (Pedernales earthquake, Mw 7.8). Pedernales earthquake caused very significant damage to the coast region and several hundred victims. It also made the city of Quito tremble but caused no damage. What about a stronger earthquake? Could the seismic wave amplifying effect due to the Quito sedimentary basin, as was the case in 1985 in the Mexico City basin, generate very strong ground motion values causing significant damage? Could smaller magnitude but closer earthquakes threaten the city more seriously? These scientific questions are at the heart of this thesis subject's concerns, which is included in a major study project of the Ecuador-Chile zone carried out jointly by the Institute of Geophysics of Quito and French researchers from GeoAzur, ISTerre, CEREMA, and IFSTTAR laboratories. Objectives of the thesis: Understand, characterize, and simulate ground movements in Quito's city, taking into account the effects of basin resonance (i.e., the geometry of the basin's bedrock, alluvial filling) well as those due to the strong surrounding topography. Carry out simulations of likely future earthquakes. Available data (not yet used): Background noise recordings by 20 broadband stations in and around the basin for six months (the measurement campaign will be carried out from July to December 2017). Small earthquakes recordings on the Quito accelerometric network (RENAQ) since 2010. Recordings of the Pedernales earthquake and its main aftershocks on the RENAQ network Methodology : Seismic noise cross-correlation to estimate inter-station Green's functions, relying on existing geological and geotechnical data. Other inversion techniques (e.g., receiver functions, the study of converted waves on the sediment/bedrock interface) considered depending on the quality of the estimated Green's functions. Low-frequency stimulation using Green's functions obtained by cross-correlation of noise and high-frequency simulations using empirical Green's functions (i.e., recordings of small earthquakes)., La ville de Quito, capitale de l’Équateur est située dans une vallée andine à 2800 mètres d’altitude entourée de volcans. Cette ville qui accueille environ 2 millions d’habitant est sujette à l’occurrence de séismes importants. Elle est de plus particulièrement vulnérable puisqu’aucun code parasismique n’est pour le moment utilisé pour les constructions. L’étude de l’aléa et du risque sismique y est donc primordiale. Trois type de séismes menacent la ville : un séisme proche de magnitude modérée (M~6.5) qui aurait lieu sur le réseau de faille de Quito, un séisme plus lointain qui pourrait avoir une magnitude plus élevée (M~7.5) venant de la cordillère et enfin un séisme de subduction venant de la côte à plus de 170 km de distance, dont la magnitude pourrait très élevée (M> 8.5). C’est ce troisième type de séisme qui a frappé l’Équateur le 16 avril 2016 (séisme de Pedernales, Mw 7.8). Ce séisme a causé des dégâts très importants sur la côte et plusieurs centaines de victimes. Il a fait trembler également la ville de Quito mais n’a causé aucun dommage. Qu’en serait-il pour un séisme plus fort ? Est-ce que l’effet d’amplification des ondes sismiques dû au bassin sédimentaire de Quito pourrait, comme cela a été le cas en 1985 dans le bassin de Mexico, engendrer des valeurs de mouvement du sol très fortes causant des dégâts importants ? Est-ce que des séismes de plus faible magnitude mais plus proches pourraient menacer la ville de façon plus importante ? Ces questions scientifiques sont au cœur des préoccupations de ce sujet de thèse, qui est lui-même inclus dans un grand projet d’étude de la zone Équateur-Chili mené conjointement par l’Institut de Géophysique de Quito, et des chercheurs Français des laboratoires Géoazur, ISTerre, CEREMA et IFSTTAR. Objectifs de la thèse : Comprendre, caractériser et simuler les mouvements du sol dans la ville de Quito, en prenant en compte les effets de résonance du bassin (i.e. topographie de la base du bassin, remplissage alluvionnaire), ainsi que ceux dus à la forte topographie environnante. Réaliser des simulations de séismes futurs probables. Données disponibles (non encore exploitées): Enregistrements du bruit de fond par 20 stations large-bande dans et autour du bassin pendant 6 mois (la campagne de mesure sera menée de juillet à décembre 2017. Enregistrements de petits séismes sur le réseau accélérométrique de Quito (RENAQ) depuis 2010 Enregistrements du séisme de Pedernales et de ses principales répliques sur le réseau RENAQ Méthodologie : Corrélation de bruit de fond pour estimer des fonctions de Green inter-stations, en s’appuyant sur les données géologiques et géotechniques existantes. Autres techniques d’inversion (e.g. fonctions récepteur, étude d'ondes converties sur l'interface sédiments/socle) envisagées en fonction de la qualité des fonctions de Green estimées. Simulation basse fréquence en utilisant les fonctions de Green obtenues par intercorrélation du bruit Simulations haute fréquence par méthode hybrides utilisant des fonctions de Green empiriques (c’est-à-dire les enregistrements de petits séismes).