Etude du mouvement sismique dans la vallée de Mexico à partir d'un réseau accélérométrique 3D de faible ouverture dans la colonia Roma

La ville de Mexico est connue pour les importants effets de site observables dans son ancienne zone lacustre et dus à la présence d'une couche d'argile molle superficielle. Ces effets de site se traduisent en particulier par une amplification importante du mouvement sismique (pouvant aller jusqu'à un facteur 50 entre 0.3 et 1 Hz) entre la zone lacustre et la zone de collines. De plus, dans la zone lacustre, la durée du mouvement sismique est anormalement longue. Malgré l'installation de plus de 100 accélérographes dans la vallée de Mexico depuis le tremblement de terre de Michoacán de 1985 (M=8.1), nous ne sommes toujours pas en mesure de comprendre la relation entre l'amplification du mouvement sismique et sa longue durée. Afin d'améliorer notre compréhension du champ d'ondes observé dans la ville de Mexico, nous avons donc réalisé une analyse systématique de ses paramètres de propagation à partir d'un réseau accélérographique 3D de faible ouverture nouvellement installé dans un quartier localisé dans la zone lacustre, la colonia Roma.En 2001, 11 nouveaux accélérographes ont été installés autour d'un réseau géré par le CENAPRED et appelé "réseau Roma". Le réseau complet (anciennes stations + nouvelles stations) forme un réseau 3D de faible ouverture (moins de 1 km), constitué de 15 stations de surface et 6 de puits, et disposant d'une base de temps absolu. Depuis son installation, ce réseau a permis d'enregistrer deux séismes : le séisme de Coyuca, dans l'état du Guerrero, le 08/10/2001 (M=6.1) et le séisme de Colima (M=7.6) du 22/01/2003. Cette thèse présente les résultats obtenus lors de l'analyse de ces deux séismes.En raison de la complexité du champ d'ondes à étudier, la première étape du travail a consisté à choisir une méthode d'analyse de données adaptée au cas de Mexico. Pour cela, trois méthodes ont été testées : la méthode fréquence-nombre d'onde (f-k) conventionnelle, la méthode f-k haute résolution, et la méthode MUSIC. Les tests ont été réalisés à partir de données synthétiques et d'enregistrements sismologiques réalisés par deux réseaux distincts : le nouveau réseau de la colonia Roma et un réseau temporaire installé en 1994 dans la zone de collines. Les tests ont démontré que la méthode f-k conventionnelle, plus robuste en cas de données peu cohérentes, était la mieux adaptée à notre cas.Afin d'estimer en détail les paramètres du champ d'ondes enregistré dans la colonia Roma, nous avons combiné plusieurs méthodes d'analyse: analyse visuelle des enregistrements, rapports spectraux, analyse temps-fréquence des données enregistrées en surface, analyse temps-fréquence basée sur une méthode de corrélation pour le jeu de données 3D, et analyse de polarisation. Les résultats ont montré la différence de comportement sismique de part et d'autre de la période de résonance du site à 2.5 s. Pour les périodes supérieures à 2.5 s, le champ d'ondes se compose d'une succession de trains d'ondes correspondants au mode fondamental d'ondes de surface venant de l'épicentre et à des ondes diffractées probablement formées au niveau de la frontière sud de l'Axe Volcanique Transmexicain. A 5 s de période, on observe une amplification régionale du mouvement sismique caractérisée par une domination des ondes de Love. Pour les périodes inférieures à 2.5 s, le mouvement sismique est gouverné par la résonance de la couche d'argile superficielle. Le champ d'ondes consiste alors en une superposition d'ondes diffractées provenant de l'Ouest de la vallée. Les résultats suggèrent une interaction constructive entre la résonance locale due à la couche d'argile molle et les modes successifs de propagation des ondes de surface guidées par la structure crustale profonde du bassin de Mexico.The seismic motion in the lake-bed zone of Mexico City is affected by strong site effects due to the presence of a soft surficial clay layer. These site effects are, in particular, responsible for an important amplification of ground motion, which can reach a factor 50 for periods between 0.3 and 1 Hz, between the lake-bed zone and the hill zone. We also can observe a large duration of strong ground motion in this area. Despite the installation of more than 100 new digital strong motion stations in Mexico City after 1985, only little progress have been done towards understanding the relation between the amplification of ground motion in the lake-bed zone and the large duration of seismic motion. In order to improve our comprehension of the observed wavefield in Mexico City, we have realized a systematic analysis of its characteristics from the records of a small aperture 3D array installed in 2001 in the colonia Roma, in the lake-bed zone of the city.In 2001, it has been decided to install 11 new accelerometers in the colonia Roma, a district located the lake-bed zone of the city, near an existing array operated by CENAPRED and known as "the Roma array". Together with the existing stations, these new instruments form a 3D array of small aperture (less than 1 km), with 15 surface and 6 borehole stations. Since its installation, this new array recorded the 10/08/2001 Coyuca event, Guerrero, (M=6.1) and the 01/22/2003 Colima (M=7.6) event.[...]STRASBOURG-EOST (674822249) / SudocSudocFranceF

The strong ground motion in Mexico City: Analysis of data recorded by a 3D array

International audienceIn 2001, we installed 11 new accelerometers in the lake-bed zone of Mexico City, which, together with the existing stations, form a 3D array of small aperture, with 15 surface and six borehole stations. This new array recorded the 10/08/2001 Guerrero event (M=6.1) and the 01/22/2003 Colima (M=7.6) event. The analysis of these two events showed that, for periods longer than 2.5 s, the dominant wavefield is composed of a fundamental mode of surface waves arriving from the epicenter and diffracted surface waves generated at the southern boundary of the Mexican Volcanic Belt. For periods shorter than 2.5 s, the resonance of the superficial clay layer becomes predominant. The wavefield consists in a superposition of diffracted waves propagating from the western part of the valley. Our results show that the ground motion is strongly amplified by the soft clay layer but its duration is controlled by the incident wavefield

Seismic microzonations of municipalities in French West Indies

Spatial variability of ground motion may be explained by local soil conditions; it is the so-called "site effects". Site effects are responsible for increasing duration and important amplification of strong ground motion during earthquakes and must be considered as a key parameter in local seismic hazard assessment. French West Indies are prone to strong site effects because of a particular geology, characterized by soft soil deposits and strong alteration of volcanic deposits. In this framework, and considering that French West Indies are considered as a high seismicity level zone in the French building code, part of seismic hazard mitigation policy consists in technical studies to perform identification and mapping of zones characterized by homogeneous seismic response and quantification of associated ground motion. It is the aim of seismic microzonations. In this presentation, we will expose the methodology used on the scale of municipalities for mapping homogeneous seismic areas, based on a study of geological and geotechnical conditions and some geophysical testings. Then, a study of vulnerability of buildings allows to estimate the probability of damage on various sectors for a scenario of risk corresponding to the seismic microzonation

From Geology to Realistic Large-Scale Spectral-Element Earthquake Simulations in the Pointe-à-Pitre Region

The goal of this study is to perform realistic large-scale spectral-element simulations in the vicinity of Pointe-à-Pitre, valid up to rather high frequencies (around 5 Hz) with a minimal shear-wave velocity around 200 m/s and considering magnitude 5 to 6 earthquakes. For this purpose, a detailed modeling of both the seismic source and geology of the medium for wave propagation and of the local urban underground is necessary. The local geology (size: 7 x 7 x 0.1 km) has been constructed using brgm Geomodeler© software, by compiling the available underground data (e.g. boring data) in the vicinity of the city of Pointe-à-Pitre. Four main geological formations are considered for the regional model: a weathered limestone (which form the substratum) and clay, silt and ballast (which form soft sedimentary basins). The shear-wave velocities within each formation have been derived from the existing geophysical campaigns (e.g. SASW, cross-hole, H/V, etc.). The S-wave velocity value of the limestone is supposed to be 1000 m/s. For the clay, silt and ballast formations, S-wave velocity values are respectively equal to 350 m/s, 250 m/s and 200 m/s. In order to mimic energy dissipation, the constitutive law of these formations is supposed to be viscoelastic. The local geology of the urban zone is included into a larger geological model (Dorel, 1978) of size 40 x 30 x 30 km. Moreover, topography is included in the model by using a 40 meters DTM. In the present simulations, the Gosier fault, located to the Southeast of Pointe-à-Pitre, has been considered. The slip distributions of the finite-fault are generated according to the spatial random field model developed by Mai and Beroza, 2002. For magnitude 5 or 6 earthquakes, the kinematic fault is composed respectively by 899 or 7881 sub-faults with a heterogeneous slip distribution. The results of these simulations are shown as PGV map on the entire domain or time history recorded at high stake facilities of Pointe-à-Pitre (e.g. city council, hospital, schools, etc.)

The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption

International audienceOn 10 May 2018, an unprecedented long and intense seismic crisis started offshore, east of Mayotte, the easternmost of the Comoros volcanic islands. The population felt hundreds of events. Over the course of 1 yr, 32 earthquakes with magnitude greater than 5 occurred, including the largest event ever recorded in the Comoros (Mw = 5.9 on 15 May 2018). Earthquakes are clustered in space and time. Unusual intense long lasting monochromatic very long period events were also registered. From early July 2018, Global Navigation Satellite System (GNSS) stations and Interferometric Synthetic Aperture Radar (InSAR) registered a large drift, testimony of a large offshore deflation. We describe the onset and the evolution of a large magmatic event thanks to the analysis of the seismicity from the initiation of the crisis through its first year, compared to the ground deformation observation (GNSS and InSAR) and modelling. We discriminate and characterize the initial fracturing phase, the phase of magma intrusion and dyke propagation from depth to the subsurface, and the eruptive phase that starts on 3 July 2018, around 50 d after the first seismic events. The eruption is not terminated 2 yr after its initiation, with the persistence of an unusual seismicity, whose pattern has been similar since summer 2018, including episodic very low frequency events presenting a harmonic oscillation with a period of ∼16 s. From July 2018, the whole Mayotte Island drifted eastward and downward at a slightly increasing rate until reaching a peak in late 2018. At the apex, the mean deformation rate was 224 mm yr−1 eastward and 186 mm yr−1 downward. During 2019, the deformation smoothly decreased and in January 2020, it was less than 20 per cent of its peak value. A deflation model of a magma reservoir buried in a homogenous half space fits well the data. The modelled reservoir is located 45 ± 5 km east of Mayotte, at a depth of 28 ± 3 km and the inferred magma extraction at the apex was ∼94 m3 s−1. The introduction of a small secondary source located beneath Mayotte Island at the same depth as the main one improves the fit by 20 per cent. While the rate of the main source drops by a factor of 5 during 2019, the rate of the secondary source remains stable. This might be a clue of the occurrence of relaxation at depth that may continue for some time after the end of the eruption. According to our model, the total volume extracted from the deep reservoir was ∼2.65 km3 in January 2020. This is the largest offshore volcanic event ever quantitatively documented. This seismo-volcanic crisis is consistent with the trans-tensional regime along Comoros archipelago

Characterization of site conditions (soil class, VS30, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

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