S-wave velocity measurements applied to the seismic microzonation of Basel, Upper Rhine Graben

An extensive S-wave velocity survey had been carried out in the frame of a recent seismic microzonation study of Basel and the border areas between Switzerland, France and Germany. The aim was to better constrain the seismic amplification potential of the surface layers. The survey included single station (H/V spectral ratios) and ambient vibration array measurements carried out by the Swiss team, as well as active S-wave velocity measurements performed by the German and French partners. This paper is focused on the application of the array technique, which consists in recording ambient vibrations with a number of seismological stations. Several practical aspects related to the field measurements are outlined. The signal processing aims to determine the dispersion curves of surface waves contained in the ambient vibrations. The inversion of the dispersion curve provides a 1-D S-wave velocity model for the investigated site down to a depth related to the size of the array. Since the size of arrays is theoretically not limited, arrays are known to be well adapted for investigations in deep sediment basins, such as the Upper Rhine Graben including the area of the city of Basel. In this region, 27 array measurements with varying station configurations have been carried out to determine the S-wave velocity properties of the geological layers down to a depth of 100-250 m. For eight sites, the outputs of the array measurements have been compared with the results of the other investigations using active sources, the spectral analysis of surface waves (SASW) and S-wave reflection seismics. Borehole information available for a few sites could be used to calibrate the geophysical measurements. By this comparison, the advantages and disadvantages of the array method and the other techniques are outlined with regard to the effectiveness of the methods and the required investigation depth. The dispersion curves measured with the arrays and the SASW technique were also combined and inverted simultaneously to use the advantages of both methods. Finally, the paper outlines and discusses the contribution of the S-wave velocity survey to the new seismic microzonation of the Basel region. In this regard one major outcome of the survey is the quantification of vertical and lateral changes of the S-wave velocity, due to changing lithology or changing compaction and degree of weathering of the layer

Site effects observations and mapping on the weathered volcanic formations of Mayotte Island

Since the 2000s, local seismic hazard studies have shown that Mayotte Island presented superficial geological formations prone to lithological site effects. The seismic sequence initiated in May 2018 confirmed the importance of such effects, both in terms of intensity and spatial extension. The analysis of the recorded strong motions showed that weathered volcanic formations are prone to significant site effects with mean amplification factors for peak ground acceleration (PGA) between 1.4 and 4.9 and that a complex combination of lithological and topographic site effects is in action. We thus implement a regional scale map of site effects for the fast calculation of strong motion and damage maps for crisis management purposes. We also provide a first estimate of key site parameters for eight stations: surface geology, resonance frequency, an amplification factor proxy for PGA, a ${V}_{{S},30}$ value, if available, and an estimated EC8 soil class

Seismic damage scenarios for Mayotte: a tool for disaster management

A new marine volcano is erupting offshore Mayotte since May 2018, generating numerous earthquakes. The population felt many of them and the stronger shaking of the ongoing sequence caused slight damage to buildings. Historical records also confirm that damaging earthquakes had occurred in the past in this region. Seismic damage scenarios are a key tool for supporting the decision-making process, the preparedness, and for designing appropriate emergency responses. This paper provides the outcomes of a work consisting in improving the seismic risk assessment as a part of disaster risk governance and exposes the scientific background of this workflow. It illustrates its use with two earthquakes. Related post-seismic surveys provide observations that are useful to check the validity of the reference dataset. The paper also discusses the main characteristics of the rapid loss assessment tool that has been developed to provide operational information for crisis management

The strong ground motion in Mexico City : analysis of data recorded by a 3D small-aperture accelerograph array in 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 dThe 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 re

Étude 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. La Ciudad de México es conocida por la importancia de los efectos de sitio observados en su zona de lago, caracterizada por la presencia de una capa superficial de arcilla blanda. Estos efectos se traducen en particular por una amplificación importante del movimiento sísmico (hasta un factor 50 para periodos entre 0.3 y 1 Hz) entre la zona de lago y la zona de lomas. Se puede observar también una larga duración del movimiento fuerte en la zona. A pesar de la instalación de más de 100 acelerógrafos en el Valle de México después del sismo de Michoacán de 1985 (M=8.1), todavía no se ha podido entender la relación entre la amplificación del movimiento sísmico y su larga duración observada en la zona de lago. Para mejorar nuestra comprensión del campo de ondas observado en la Ciudad de México, se realizó un análisis sistemático de sus características a partir de los registros de un arreglo 3D de pequeña apertura nuevamente instalado en la colonia Roma, en la zona de lago. En 2001, se instalaron 11 nuevos acelerógrafos en la colonia Roma, alrededor del arreglo del CENAPRED conocido como “arreglo Roma”. El arreglo completo (viejas + nuevas estaciones) consiste en 15 estaciones de superficie y 6 de pozo, formando un arreglo 3D de apertura inferior a 1 km con base de tiempo absoluto. Desde su instalación, este arreglo permitió registrar dos eventos: el sismo de Coyuca del 08/10/2001 (M=6.1) ocurrido en las costas de Guerrero, y el sismo de Colima del 22/01/2003 (M=7.6). Esta tesis presenta los resultados obtenidos por el análisis de ambos sismos. Debido a la complejidad del campo de ondas a estudiar, la primera etapa del trabajo consistió en elegir el método de análisis de datos de arreglo aplicable al caso de la Ciudad de México. Para ello, probamos tres métodos distintos: el método frecuencia-número de onda (f-k) convencional, el método f-k de alta resolución y el método MUSIC sobre datos sintéticos y datos sísmicos registrados por dos arreglos diferentes, un arreglo temporal instalado en la zona de lomas en 1994 y el nuevo arreglo instalado en la colonia Roma. Las pruebas mostraron la superioridad del método f-k convencional para el análisis de nuestros datos, debido en particular a su mejor robustez en caso de datos poco coherentes. Para obtener una estimación completa de los parámetros del campo de ondas en la colonia Roma, combinamos varios métodos: análisis visual de los registros, cocientes espectrales, análisis tiempo-frecuencia de los datos de superficie por el método f-k convencional, análisis tiempo-frecuencia del juego de datos 3D mediante un método de cross-correlación, y análisis de polarización. El análisis de los registros de los sismos de Coyuca y Colima permitieron mostrar la diferencia de comportamiento sísmico para periodos inferiores y superiores al periodo de resonancia del sitio, a 2.5 s. Para los periodos superiores a 2.5 s, el campo de ondas se compone de una sucesión de trenes de ondas correspondientes al modo fundamental de ondas de superficie proveniente del epicentro y a ondas difractadas generadas probablemente al nivel de la frontera sur del Eje Volcánico Transmexicano. A 5 s de periodo, se observa una amplificación regional caracterizada por una dominación de las ondas de Love. Para los periodos inferiores a 2.5 s, la resonancia de la capa de arcilla superficial gobierna el movimiento sísmico. El campo de ondas consiste en una compleja superposición de ondas difractadas provenientes del cuadrante oeste del Valle de México. Los resultados sugieren una interacción constructiva entre la resonancia local debida a la capa de arcilla blanda y los modos sucesivos de propagación de ondas de superficie guiados por la estructura cortical profunda de la cuenca de México. 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. This thesis presents the results obtained from the analysis of both events. The application of available array processing techniques to the complex case of Mexico City could be difficult. We then have tested three methods to analyze their capacity to process the data from our new dense accelerograph array: the conventional frequency-wavenumber (f-k) method, the maximum-likelihood f-k method and the Multiple Signal Characterization technique (MUSIC). Our tests have used both synthetic signals and earthquake data from two different arrays: a temporal array installed on firm soil in Mexico City in 1994, and the new dense array of colonia Roma. The results show the superiority of the conventional f-k method, more robust than the other two methods in case of data with small coherence. To obtain a complete estimation of the parameters of the wavefield in colonia Roma, we combine different methods: visual analysis of the filtered data, spectral ratios, f-k analysis for the datasets recorded by the surface stations, cross-correlation analysis for the 3D datasets, and polarization analysis. The analysis of the two recorded events showed that the characteristics of ground motion as a function of period are different for periods larger or smaller than the site resonant period (2.5 sec). For periods longer than 2.5 sec, the wavefield is composed of successive pulses corresponding to the fundamental mode of surface waves propagating from the epicenter, and to diffracted surface waves probably generated on the southern boundary of the Mexican Volcanic Belt. At 5 s period, we observe a regional amplification characterized by a dominance of Love waves. For periods shorter than 2.5 sec, 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. The results suggest a constructive interference between the local resonance caused by the very soft superficial clay layer and different modes of surface waves guided by the deep crustal structure of the Mexico City basin

A preliminary investigation of strong-motion data from the French Antilles

Strong-motion networks have been operating in the Caribbean region since the 1970s, however, until the mid-1990s only a few analogue stations were operational and the quantity of data recorded was very low. Since the mid-1990s, digital accelerometric networks have been established on islands within the region. At present there are thought to be about 160 stations operating in this region with a handful on Cuba, 65 on the French Antilles (mainly Guadeloupe and Martinique), eight on Jamaica, 78 on Puerto Rico (plus others on adjacent islands) and four on Trinidad. After briefly summarising the available data from the Caribbean islands, this article is mainly concerned with analysing the data that has been recorded by the networks operating on the French Antilles in terms of their distribution with respect to magnitude, source-to-site distance, focal depth and event type; site effects at certain stations; and also with respect to their predictability by ground motion estimation equations developed using data from different regions of the world. More than 300 good quality triaxial acceleration time-histories have been recorded on Guadeloupe and Martinique at a large number of stations from earthquakes with magnitudes larger than 4.8, however, most of the records are from considerable source-to-site distances. From the data available it is found that many of the commonly-used ground motion estimation equations for shallow crustal earthquakes poorly estimate the observed ground motions on the two islands; ground motions on Guadeloupe and Martinique have smaller amplitudes and are more variable than expected. This difference could be due to regional dependence of ground motions because of, for example, differing tectonics or crustal structures or because the ground motions so far recorded are, in general, from smaller earthquakes and greater distances than the range of applicability of the investigated equations

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)

Limits on the potential accuracy of earthquake risk evaluations using the L'Aquila (Italy) earthquake as an example

International audienceThis article is concerned with attempting to ‘predict’ (hindcast) the damage caused by the L’Aquila 2009 earthquake (Mw 6.3) and, more generally, with the question of how close predicted damage can ever be to observations. Damage is hindcast using a well-established empirical-based approach based on vulnerability indices and macroseismic intensities, adjusted for local site effects. Using information that was available before the earthquake and assuming the same event characteristics as the L’Aquila mainshock, the overall damage is reasonably well predicted but there are considerable differences in the damage pattern. To understand the reasons for these differences, information that was only available after the event were include within the calculation. Despite some improvement in the predicted damage, in particularly by the modification of the vulnerability indices and the parameter influencing the width of the damage distribution, these hindcasts do not match all thedetails of the observations. This is because of local effects: both in terms of the ground shaking, which is only detectable by the installation of amuch denser strong-motion network and a detailed microzonation, and in terms of the building vulnerability, which cannot be modeled usinga statistical approach but would require detailed analytical modeling for which calibration data are likely to be lacking. Future studies shouldconcentrate on adjusting the generic components of the approach to make them more applicable to their location of interest. To increase thenumber of observations available to make these adjustments, we encourage the collection of damage states (and not just habitabilityclasses) following earthquakes and also the installation of dense strong-motion networks in built-up areas