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

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

Dynamic production monitoring in pig herds II:Modeling and monitoring farrowing rate at herd level

Abstract:Good management in animal production systems is becoming of paramount importance. The aim of this paper was to develop a dynamic moni-toring system for farrowing rate. A farrowing rate model was implemented us-ing a Dynamic Generalized Linear Model (DGLM). Variance components were pre-estimated using an Expectation-Maximization (EM) algorithm applied on a dataset containing data from 15 herds, each of them including insemination and farrowing observations over a period ranging from 150 to 800 weeks. The model included a set of parameters describing the parity-specific farrowing rate and the re-insemination effect. It also provided reliable forecasting on weekly basis. Sta-tistical control tools were used to give warnings in case of impaired farrowing rate. For each herd, farrowing rate profile, analysis of model components over time and detection of alarms were computed. Together with a previous model for litter size data and a planned similar model for mortality rate, this model will be an important basis for developing a new, dynamic, management tool

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

Quality assessment for site characterization at seismic stations

AbstractMany applications related to ground-motion studies and engineering seismology benefit from the opportunity to easily download large dataset of earthquake recordings with different magnitudes. In such applications, it is important to have a reliable seismic characterization of the stations to introduce appropriate correction factors for including site amplification. Generally, seismic networks in Europe describe the site properties of a station through geophysical or geological reports, but often ad-hoc field surveys are missing and the characterization is done using indirect proxy. It is then necessary to evaluate the quality of a seismic characterization, accounting for the available site information, the measurements procedure and the reliability of the applied methods to obtain the site parameters.In this paper, we propose a strategy to evaluate the quality of site characterization, to be included in the station metadata. The idea is that a station with a good site characterization should have a larger ranking with respect to one with poor or incomplete information. The proposed quality metric includes the computation of three indices, which take into account the reliability of the available site indicators, their number and importance, together with their consistency defined through scatter plots for each single pair of indicators. For this purpose, we consider the seven indicators identified as most relevant in a companion paper (Cultrera et al. 2021): fundamental resonance frequency, shear-wave velocity profile, time-averaged shear-wave velocity over the first 30 m, depth of both seismological and engineering bedrock, surface geology and soil class

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

H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations

Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification facto

Effect of the Soil Spatial Variability on the Static and Dynamic Stability Analysis of a Lebanese Slope

The accidental topography and heterogeneous Lebanese geology in addition to the active seismicity have initiated the static and dynamic stability analysis of Lebanese slopes. In this paper, the stability of a sandy Lebanese slope situated at Mansourieh near Beirut is investigated using deterministic and probabilistic approaches. The characterization of the variability of the slope soil properties is done based on geological investigation, as well as geophysical (Resistivity and Ambient noise) and geotechnical tests performed on this slope. Three dimensional 3D static deterministic analyses is performed to determine the overall safety factor of the slope and to find the location of the critical failure surface. The deterministic model is based on numerical simulations using the finite difference code FLAC3D. Then, two-dimensional probabilistic analysis is carried out on the critical section obtained from the 3D model. In the probabilistic analysis, the soil properties are modeled using the random field theory. An efficient uncertainty propagation methodology based on the expansion optimal linear estimation EOLE method is used to discretize the random field. Concerning the dynamic analysis, it is implemented in order to determine the amplification at the top of slope, where the looseness of the soil there may amplify the earthquake acceleration. The results have shown a small safety factor as well as high amplification. The importance of using the probabilistic approach versus the deterministic one is also presented and discussed

Agent-based simulation of pedestrians' earthquake evacuation; application to Beirut, Lebanon

Most seismic risk assessment methods focus on estimating the damages to the built environment and the consequent socioeconomic losses without fully taking into account the social aspect of risk. Yet, human behaviour is a key element in predicting the human impact of an earthquake, therefore, it is important to include it in quantitative risk assessment studies. In this study, an interdisciplinary approach simulating pedestrians' evacuation during earthquakes at the city scale is developed using an agent-based model. The model integrates the seismic hazard, the physical vulnerability as well as individuals' behaviours and mobility. The simulator is applied to the case of Beirut, Lebanon. Lebanon is at the heart of the Levant fault system that has generated several Mw>7 earthquakes, the latest being in 1759. It is one of the countries with the highest seismic risk in the Mediterranean region. This is due to the high seismic vulnerability of the buildings due to the absence of mandatory seismic regulation until 2012, the high level of urbanization, and the lack of adequate spatial planning and risk prevention policies. Beirut as the main residential, economic and institutional hub of Lebanon is densely populated. To accommodate the growing need for urban development, constructions have almost taken over all of the green areas of the city; squares and gardens are disappearing to give place to skyscrapers. However, open spaces are safe places to shelter, away from debris, and therefore play an essential role in earthquake evacuation. Despite the massive urbanization, there are a few open spaces but locked gates and other types of anthropogenic barriers often limit their access. To simulate this complex context, pedestrians' evacuation simulations are run in a highly realistic spatial environment implemented in GAMA [1]. Previous data concerning soil and buildings in Beirut [2, 3] are complemented by new geographic data extracted from high-resolution Pleiades satellite images. The seismic loading is defined as a peak ground acceleration of 0.3g, as stated in Lebanese seismic regulations. Building damages are estimated using an artificial neural network trained to predict the mean damage [4] based on the seismic loading as well as the soil and building vibrational properties [5]. Moreover, the quantity and the footprint of the generated debris around each building are also estimated and included in the model. We simulate how topography, buildings, debris, and access to open spaces, affect individuals' mobility. Two city configurations are implemented: 1. Open spaces are accessible without any barriers; 2. Access to some open spaces is blocked. The first simulation results show that while 52% of the population is able to arrive to an open space within 5 minutes after an earthquake, this number is reduced to 39% when one of the open spaces is locked. These results show that the presence of accessible open spaces in a city and their proximity to the residential buildings is a crucial factor for ensuring people's safety when an earthquake occurs