Pierre-Yves Bard

Effects of Love Waves on Microtremor H/V Ratio

Abstract The horizontal-to-vertical (H/V) method has the potential to significantly contribute to site effects evaluation, in particular in urban areas. Within the European project, site effects assessment using ambient excitations (SESAME), we investigated the nature of ambient seismic noise in order to assess the reliability of this method. Through 1D seismic noise modeling, we simulated ambient noise for a set of various horizontally stratified structures by computing efficiently the displacement and stress of dynamic Green’s functions for a viscoelastic-layered half-space. We performed array analysis using the conventional semblance-based frequence-wavenumber method and the three-component modified spatial autocorrelation method on both vertical and horizontal components and estimated the contribution of different seismic waves (body/surface waves, Rayleigh/Love waves) at the H/V peak frequency. We show that the very common assumption that almost all the ambient noise energy would be carried by fundamental-mode Rayleigh waves is not justified. The relative proportion of different wave types depends on site conditions, and especially on the impedance contrast. For the 1D horizontally layered structures presented here, the H/V peak frequency always provides a good estimate of the fundamental resonance frequency whatever the H/V peak origin (Rayleigh wave ellipticity, Airy phase of Love waves, S -wave resonance). We also infer that the relative proportion of Love waves in ambient noise controls the amplitude of the H/V peak.

Dynamic parameters of structures extracted from ambient vibration measurements: an aid for the seismic vulnerability assessment of existing buildings in moderate seismic hazard regions

During the past two decades, the use of ambient vibrations for modal analysis of structures has increased as compared to the traditional techniques (forced vibrations). The frequency domain decomposition (FDD) method is nowadays widely used in modal analysis because of its accuracy and simplicity. In this paper, we first present the physical meaning of the FDD method to estimate the modal parameters. We discuss then the process used for the evaluation of the building stiffness deduced from the modal shapes. The models considered here are 1D lumped-mass beams and especially the shear beam. The analytical solution of the equations of motion makes it possible to simulate the motion due to a weak to moderate earthquake and then the inter-storey drift knowing only the modal parameters (modal model). This process is finally applied to a nine-storey reinforced concrete (RC) dwelling in Grenoble (France). We successfully compared the building motion for an artificial ground motion deduced from the model estimated using ambient vibrations and recorded in the building. The stiffness of each storey and the inter-storey drift were also calculated.

Site Effect Study in Urban Area: Experimental Results in Grenoble (France)

Three methods are used to determine the site effect in the town of Grenoble, located in the Western Alps. First we use the classical spectral ratio method in 14 sites to calculate the transfer function of the basin. We find an amplification of 10 in the frequency range of 0.25 to 10 Hz. Second, we compare these results with the H over V spectral ratio method, and propose a map of resonance frequency of the basin. We find a lower resonance frequency in the center of the basin than on the edge, that is consistent with the structure deduced from a gravity Bouguer anomaly map. Finally we use the empirical Green’s function method to simulate a M w 5.5 earthquake at a distance of 20 km from the town. The simulated acceleration reaches the level of 2 m/s2 in the center of the basin compared to 0.2 m/s2 on the edges. The simulated ground motion we compute is smaller than the French seismic codes on the edge of the valley but significantly larger in the center.

Site effects in Mexico City eight years after the September 1985 Michoacan earthquakes

Abstract The purpose of this paper is to take a comprehensive look at site effects in Mexico City for the 1985 Michoacan earthquake. We examine, successively, 1D and 2D models. For the latter, we consider in detail both large scale and small scale heterogeneities, using extensively the Aki-Larner wave propagation method, in the version given by Bard and Gariel. In particular, we make a critical review of the different explanations proposed for the large duration of strong ground motion in the lake zone. Our purpose is two-sided. We first outline the difference between what is well established and what remains still unexplained regarding the seismic response of Mexico City basin. On the other hand, we wish to make explicit the conditions that the proposed models require to explain strong motion duration. Our results allow us to qualify the models proposed to date and to point out what could be the experiments and the new data required to find a truly satisfactory explanation of strong ground motion at Mexico City.

Quantitative Comparison of Four Numerical Predictions of 3D Ground Motion in the Grenoble Valley, France

This article documents a comparative exercise for numerical simulation of ground motion, addressing the seismic response of the Grenoble site, a typical Alpine valley with complex 3D geometry and large velocity contrasts. Predictions up to 2 Hz were asked for four different structure wave-field configurations (point source and extended source, with and without surface topography). This effort is part of a larger exercise organized for the third international symposium on the effects of surface geology (ESG 2006), the complete results of which are reported elsewhere (Tsuno et al., 2009). While initial, blind computations significantly differed from one another, a remarkable fit was obtained after correcting for some nonmethodological errors for four 3D methods: the arbitrary high-order derivative discontinuous Galerkin method (ADER-DGM), the velocity-stress finite-difference scheme on an arbitrary discontinuous staggered grid (FDM), and two implementations of the spectral-element method (SEM1 and SEM2). Their basic formulation is briefly recalled, and their implementation for the Grenoble Valley and the corresponding requirements in terms of computer resources are detailed. Besides a visual inspection of PGV maps, more refined, quantitative comparisons based on time-frequency analysis greatly help in understanding the origin of differences, with a special emphasis on phase misfit. The match is found excellent below 1 Hz, and gradually deteriorates for increasing frequency, reflecting differences in meshing strategy, numerical dispersion, and implementation of damping properties. While the numerical prediction of ground motion cannot yet be considered a mature, push-button approach, the good agreement reached by four participants indicates that, when used properly, numerical simulation is actually able to handle correctly wave radiation from extended sources in complex 3D media. The main recommendation to obtain reliable numerical predictions of earthquake ground motion is to use at least two different but comparably accurate methods, for instance the present formulations and implementations of the FDM, SEM, and ADER-DGM.

SITE CHARACTERIZATIONS FOR THE IRANIAN STRONG MOTION NETWORK

Abstract Twenty-six sites of the Iranian strong motion network, having provided numerous records of good quality, were selected for a site effect study with the objective of obtaining a reliable site categorization for later statistical work on Iranian strong motion data. For each site, superficial Vp and Vs profiles were measured with refraction techniques, microtremor recordings were obtained and analysed with the H/V technique and the available three-component accelerograms by the receiver function technique. The aggregation of these results allows the proposition of a four-class categorization based on the H/V spectral ratio of strong ground motions, which demonstrate a satisfactory correlation with the S-wave velocity profile. Iran has a particular geological and meteorological situation compared to other seismic countries such as Japan or California, a mountainous country with dry weather conditions and a low water table in most areas. These conditions result in a relatively small number of sites with low frequency amplification, while many sites exhibit moderate amplifications in the intermediate and high frequency range.

Seismic Site–City Interaction: Main Governing Phenomena through Simplified Numerical Models

This work focuses on the analysis of the multiple interactions between soil layers and civil-engineering structures in dense urban areas submitted to a seismic wave. To investigate such phenomena, called site–city interaction (sci) herein, two simplified site–city configurations are considered: a homogeneous, periodically spaced city and a heterogeneous, nonperiodically spaced city, both on a constant- depth basin model. These 2D boundary-element method models are subjected to a vertically incident plane SH Ricker wavelet. A parametric study of the city parameters (density of buildings and their natural frequencies) and the thickness of the basin is carried out to characterize the sci and to investigate its sensitivity to some governing parameters. The following parameters are analyzed: building vibrations, induced ground motion, ground-motion perturbations inside and outside the city, spatial coherency, and kinetic energy of the “urban wave field.” A so-called site–city resonance is reached when the soil fundamental frequency and structure eigenfrequencies coincide; building vibrations and ground motion are then significantly decreased and the spatial coherency of the urban field is also strongly modified. Building density and city configuration play a crucial role in the energy distribution inside the city.

Site-City Seismic Interaction in Mexico City–Like Environments: An Analytical Study

Recent destructive earthquakes have confirmed the importance of hazard and vulnerability studies to predict and prevent the impact of large seisms. Most seismic risk analysis considers the buildings as a passive constituent integrated into the vulnerability analysis. This approach neglects the possible contribution of the building vibration to the free field. This article describes an analysis of site-city effects, that is, the seismic interaction of the city with soft soil layers. An analytic method derived from soil-structure interaction studies is described and applied to the Roma Norte zone of Mexico City. The building parameters are derived from detailed studies of the JA and PC buildings. The soil-city system is subjected to the Mexican ( M w 7.3) 14 September 1995 earthquake. The simulated ground surface motions, which incorporate the wave field radiated from the buildings of Roma Norte district, are compared to seismic records from this zone. Analytical procedures reproduce the long time duration and beating of the observed ground motion in Mexico City. Parametric analysis are also performed to identify the predominant factors (e.g., urbanization density, soil-to-city stiffness ratio) that favor the site-city interaction effects. A simple relation is proposed to estimate the expected efficiency of the site-city interaction effects for any city.

Contribution of Dense Array Analysis to the Identification and Quantification of Basin-Edge-Induced Waves, Part II: Application to Grenoble Basin (French Alps)

Settled on a deep sediment-filled valley, the city of Grenoble (French Alps) faces important site effects: large amplification and significant duration in- crease of ground motion, even for moderate-size events. In order to study multidi- mensional site effects, a very dense array composed of 29 three-component seis- mometers over a 1-km aperture was operated during spring 1999 in the center of the city. A total of 18 events (6 local, 4 regional, and 8 teleseismic) with an acceptable signal-to-noise ratio could be recorded over a 4-month period. The complexity of the wave field and in situ seismic noise constraints led us to develop a procedure based on time-frequency coherence and the multiple signal classification algorithm to iden- tify and characterize wave arrivals (Cornou et al., 2003). Applying the procedure to the 18 records, it is clearly indicated that ground motion inside the valley is domi- nated by basin-edge-induced waves that carry 4 times more energy than the direct wave field, regardless of the type of event considered. In addition, the basin-induced wave field is composed of 60% Rayleigh waves and 40% Love waves when consid- ering energy carried by the three components. If one considers only the energy of horizontal components, this proportion is 50% Rayleigh waves and 50% Love waves. The diffraction phenomena are mostly constrained by the 3D structure of the basin, regardless of the azimuth of the event. A study of the relative contribution of 1D and 2D/3D effects on recorded ground motion suggests, at least at frequencies below 1 Hz, that the difference between the standard spectral ratio and 1D transfer function, or possibly the horizontal-to-vertical ratio (receiver function and Nakamura esti- mates) might be due mainly to laterally propagating waves.

Site Effect Analysis around the Seismically Induced Ananevo Rockslide, Kyrgyzstan

In 1911, the surface-wave magnitude 8.2 Kemin earthquake hit northeastern Tien Shan (Kyrgyzstan), close to the cities of Bishkek and Almaty, the capitals of Kyrgyzstan and Kazakhstan, respectively. Several hundreds of people were killed by the earthquake, some by indirect effects such as landslides and mudflows. A particular but nonfatal landslide triggered by the Kemin event was a rockslide in the vicinity of Ananevo, north of lake Issyk Kul (Kyrgyzstan) rockslide located above the fault zone activated in 1911. In the summer of 1999, a geophysical-seismological field trip was organized to study geology and to record seismic ground motions on and around the Ananevo rockslide. The work was part of project assessing seismogenic landslide hazard in northern Kyrgyzstan, based on various case studies of slope failures in connection with site-specific ground-motion dynamics. The geophysical investigations consisted of seismic refraction tests processed as 2D seismic tomographies and surface-wave inversion, which were combined to build a 3D geophysical model of the landslide site. Ground motions from small earthquakes were analyzed using several techniques to define site effects over the mountain massif. Both H/V and standard spectral ratios indicated lower dominant frequencies with stronger amplification in the crest region with respect to the mountain slope. These effects could be partially simulated by 1D, 2D, and 3D finite-element modeling. By comparing the numerical results with the experimental data, the presence of a surficial low-velocity layer of varying thickness appeared to be the key factor controlling the ground motion around the rockslide.