General Report Session 9: Seismology: Predicting Strong Ground Motion for Design

The 14 papers of the session may be divided (somewhat artificially, of course) in three main subgroups: - a significant number (5) deal with the whole process of predicting strong ground motion for design, starting from geological / tectonic considerations and ending at a quantitative estimate of some ground motion parameters (peak acceleration, response spectrum,. .. ), either on a regional or national scale for zoning purposes (9.17, 9.24), or at a specific site for the design of some specific structures (papers 9.10, 9.13), or finally from a methodological point of view ( 9.13, 9.24, 9.26). - the 7 papers in the second sub-group each focus on one particular aspect of earthquake hazard estimation. Paper 9.2 is concerned with the estimation of the static deformations induced by strike slip events. Paper 9.3 focus on the contribution of Love waves in low frequency motion. Paper 9.4 investigates the relationship between epicentral intensity and magnitude for a better use of historical seismicity. Paper 9.5 presents the main characteristics and interests for both scientific and engineering purposes, of a new, large set of high quality digital strong motion data. Papers 9.6 and 9.25 focus on site effects, and even more particularly on the geotechnical characterization of sites for microzoning purposes. Finally, paper 9.19 draws attention on the importance of the orientation of underground geological structures with respect to the epicentral direction on the effective damage to be expected at a given site. - the 2 remaining papers (9.20, 9.21) present very interesting cases on well documented and surveyed induced seismicity under large size dams in regions of otherwise weak natural seismicity (central-western Thailand). These 14 papers are briefly summarized, and their conclusions shortly discussed, in the following three sections, while the fourth one, as a conclusion, lists the main outcomes and issues raised by this series of papers

Effect of Built Environment on Free-Field Motion for Very Soft, Urbanized Sites

A preliminary numerical investigation is presented on the long distance effects of soil-structure interaction for important buildings located on soft soils. A simple 20 model is considered, with homogeneous rectangular buildings resting on a single, horizontal, soft layer overlying a much stiffer half-space, impinged by SH waves. Computations are made for different parameter sets, in order to analyze the respective effects of the main parameters: clay layer thickness and frequency, building size, and spacing between buildings. For realistic building properties, wave diffraction related with soil- structure interaction is shown to alter the free-field surface motion up to distances of at least 1 km from the next building: duration as well as amplitude are significantly increased at some frequencies, while they may be reduced at other frequencies

Green functions and associated sources in infinite and stratified poroelastic media

International audienceThe purpose of this paper is to study Green functions for porous saturated media and to present an example of numerical simulation using these Green functions. In the first part the equations of the porous media are recalled and the principal results concerning the features of the waves are given. The second part is devoted to the determination of sources and Green functions. We give the analytical solution, then we study some of its properties. In the third part we focus on the construction of synthetic seismograms. We show examples of bi-dimensional computation in a semi-infinite stratified porous medium, for a double dipole or a fluid injection

Comparaison entre calculs de vulnérabilité sismique et propriétés dynamiques mesurées

National audienceLarge-scale seismic vulnerability assessment methods use simplified formulas and curves, often without providing uncertainties. They are seldom compared to experimental data. Therefore, we recorded ambient vibrations and estimated modal parameters (resonance frequencies, modal shapes and damping) of 60 buildings in Grenoble (France) of various types (masonry and reinforced concrete). The knowledge of resonance frequencies in the linear domain is essential in the seismic design. Hence, we compared resonance frequency formulas given in the design code with this experimental data. The variability is underestimated and only two parameters (type and height of the building) seem to be statistically significant. Moreover, we compared the linear part of capacity curves used in European Risk-UE method to the measured frequencies. The variability is still very large and these curve are often not relevant for the French buildings. As a result, ambient vibration recordings may become an interesting tool in order to calibrate the linear part of capacity curves

Cohérence spatiale des mouvements sismiques et structure géométrique du proche sous-sol: un exemple à Argostoli, Grèce

International audienceThe spatial variation of the earthquake ground motion over short distances can significantly affect the dynamic response of large and extended engineered structures, particularly when they are located at sites with lateral heterogeneity. In current practices, it is taken into account in terms of coherency, as a function of frequency and distance, established on an essentially empirical basis, making it difficult to extrapolate at different sites. Hence, in order to improve our understanding of the physical significance of coherency and its relationship to the geometrical and mechanical characteristics of the ground structure, a very dense network was installed in seismically active region of Argostoli-Koutavos (Cephalonia, Greece). Consisting of 21 velocimeters deployed on four concentric circles of radii 5, 15, 40 and 80 m around a central station in a small valley (~50 m thickness, width 1.5 km), the array recorded several hundred events in the period from September 2011 to April 2012. A subset of 46 events having a homogeneous distribution in terms of epicentral distance (0-200 km), back-azimuth (0-360 °) and magnitude (2 to 5) has been selected for a systematic analysis of the coherency of all station pairs observed on 'the most energetic phase' containing the S-wave. The results indicate a lack of a clear dependence of the average coherency on the magnitude, back-azimuth or site-to-source distance of the event, and on the length of chosen signal window provided that it contains the same S-phase energy. The most striking result concerns the influence of the site geometry; the coherency is systematically higher for the pairs aligned parallel to the axis of the valley (2D), and lower for those aligned in the perpendicular direction. This result is consistent with the formation of the wavefield in the valley, dominated by surface waves propagating from one edge to the other. The observed coherency estimates are, however, weakly represented by the existing parametric models.La variabilité spatiale des mouvements sismiques sur de courtes distances peut affecter significativement la réponse dynamique des structures de génie civil de grande taille, en particulier quand elles sont fondées sur des sites latéralement hétérogènes. Elle est souvent prise en compte au travers de fonctions empiriques – peu calibrées et peu extrapolables d'un site à l'autre-décrivant la perte de cohérenceen fonction de la fréquence et la distance. Dans le but d'améliorer la compréhension de la structure de la cohérence et son lien avec les caractéristiques géométriques et mécaniques du sous-sol, un réseau sismologique très dense a été installé dans la région sismiquement très active d'Argostoli-Koutavos (Céphalonie, Grèce). Constitué de 21 vélocimètres déployés sur 4 cercles concentriques de rayons 5, 15, 40 et 80 m autour d'une station centrale au sein d'une vallée de petite taille (épaisseur ~50 m, largeur 1.5 km), ce réseau a enregistré plusieurs centaines d'événements sur la période septembre 2011-avril 2012. Un sous ensemble de 46 événements présentant une distribution homogène en termes de distance épicentrale (0-200 km), back-azimut (0-360°) et magnitude (2 à 5) a été sélectionné pour une analyse systématique de la cohérence observée sur la phase forte comprenant l'onde S de toutes les paires de stations. Les résultats indiquent une absence de dépendance claire de la cohérence moyenne en fonction de la magnitude, du back-azimut de la source, et de la distance hypocentrale, ainsi qu'une grande robustesse vis-à-vis de la fenêtre de signal choisie, pour peu qu'elle contienne la phase S énergétique. Le résultat le plus marquant concerne le contrôle par la géométrie du site : la cohérence est systématiquement plus forte pour les paires alignées parallèlement à l'axe de la vallée (2D), et minimale pour celles alignées selon la direction perpendiculaire. Ce résultat est cohérent avec la constitution du champ d'ondes dans la vallée dominé par des ondes de surface se propageant d'un bord à l'autre de la vallée. Les cohérences observées ne sont par ailleurs que très partiellement expliquées par les modèles existants

Ambient noise energy bursts observation and modelling: Trapping of harmonic structure-soil induced-waves in a topmost sedimentary layer.

We study the nature of energy bursts that appeared in the frequency range 3-5 Hz in ambient seismic noise recorded in the Grenoble basin (French Alps) during a seismological array experiment. A close agreement is found between the identified azimuths of such noise bursts with the location of an industrial chimney. In-situ measurements of the chimney dynamic characteristics show a coincidence between the frequency of the first harmonic mode of the chimney and the fundamental frequency of a thin surficial layer that overlay the deep sediment fill. The interaction between the chimney and the surfical layer is then numerically simulated using simple impedance models, and two geotechnical profiles. Simulations exhibit a satisfactory agreement with observations and suggest that energy bursts result of inertial structure-soil interaction favored by resonance effects between the first harmonic mode of the structure and the fundamental frequency of the topmost layer

Indicators for site characterization at seismic station: recommendation from a dedicated survey

AbstractIn recent years, the permanent seismic networks worldwide have largely increased, raising the amount of earthquake signals and the applications using seismic records. Although characterization of the soil properties at recording stations has a large impact on hazard estimates, it has not been implemented so far in a standardized way for reaching high-level metadata. To address this issue, we built an online questionnaire for the identification of the indicators useful for a reliable site characterization at a seismic station. We analysed the answers of a large number of experts in different fields, which allowed us to rank 24 different indicators and to identify the most relevant ones: fundamental frequency (f0), shear-wave velocity profile (VS), time-averaged Vs over 30 m (VS30), depth of seismological and engineering bedrock (Hseis_bed and Heng_bed), surface geology and soil class. Moreover, the questionnaire proposed two additional indices in terms of cost and difficulty to obtain a reliable value of each indicator, showing that the selection of the most relevant indicators results from a complex balance between physical relevancy, average cost and reliability. For each indicator we propose a summary report, provided as editable pdf, containing the background information of data acquisition and processing details, with the aim to homogenize site metadata information at European level and to define the quality of the site characterization (see companion paper Di Giulio et al. 2021). The selected indicators and the summary reports have been shared within European and worldwide scientific community and discussed in a dedicated international workshop. They represent a first attempt to reach a homogeneous set of high-level metadata for site characterization

Influence of Site Parameters on Fourier Amplification Application for 1D Linear Viscoelastic Method

We focus on the effect of site parameters, also called site proxies, on the variation of the amplification factor. This latter, named Fourier Amplification Factor (FAF) is defined as the ratio of the Fourier transform of the seismic motion at surface and at bedrock. For this study, the wave propagation theory is used limited to 1D linear viscoelastic domain. At this effect, a set of FAF, is established for a set of 858 real profiles. From there, the site parameters are also derived, it is necessary to mention that the FAF can be computed in independent manner of seismic signals which it is applicable only on linear domain.In Nuclear Power Industry application, the FAF is mostly used and can be approximated by limited number of site proxies. As the usual code practice implies a lower number of site proxies (generally 1, sometimes 2) as UBC97 or EC8, a sensitivity analysis is conducted to identify the "best performing" site parameters. The results show that by far, using the six site proxies lead to a better prediction of FAF. However if we have to use one single site parameter, results show that the best one is the overall resonance frequency (f0). In the case when we intend to use two site parameters built from the average shear wave velocity over the upper 30 m (Vs30) and the resonance frequency (f0) which are preferred and give an important variance reduction superior than 61%. In the result, a new formula has been established