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Theses

Utilisation combinée des méthodes basées sur le bruit de fond dans le cadre du microzonage sismique

Abstract : Recent destructive earthquakes repeatedly showed that site effects can drastically exacerbate damage. Improving the way such local hazard modifications are accounted for in earthquake risk mitigation policies is therefore a major concern, balanced however by tight economical constraints, which emphasize the need for inexpensive, though reliable methods. Noise measurements, an original geophysical method using ambient natural and anthropic vibrations, correspond to this need.
The goal of this thesis is to develop and validate physically sound methodologies based on seismic noise allowing to account for site effects in a regulatory context, and combining simplicity, robustness and reliability. The idea is, in a first stage, to couple information from H/V measurements (resonance frequency f0) and array measurements (shear waves velocity, at least at shallow depth) to characterize the site conditions, and, in a second stage, to empirically develop statistical correlations between such limited site information and amplification functions, on the basis of the largest available high quality data set, i.e., the Japanese Kik-net data. This thesis is therefore divided into two main sections dedicated, respectively, to each of these two steps.
The 1st section is mainly targeted at proposing a field and processing protocol for the combined utilization of methods based on seismic noise. A series of investigations on synthetic and real data allows to identify the key factors controlling the reliability of estimates of the resonance frequency f0 and the mean S-waves velocity of the top z meters, Vsz (with z varying from 5 to 30 meters according to site conditions and array aperture). The proposed protocol is then intended to warrant – as much as possible – a good control of these key factors.
The goal of the 2nd section is to develop a simple method for proposing a site-specific spectrum on the basis of the regional hazard and the site conditions characterized by f0 and Vsz. A subset of the Kik-net strong motion data is first selected, corresponding to nearly 500 sites with reliable P and S waves velocity profiles down to an average depth far larger than 50 m, and more than 4000 pairs of surface and down-hole seismic recordings. For each site, the site conditions can be characterized by reliable estimates of f0 and Vsz, and the borehole amplification function is estimated with spectral ratio between surface and down-hole recordings. Considering the large variability of depths and velocities for the borehole sensors, these "raw" functions are then normalized with respect to a carefully chosen "standard" reference rock, and corrected from depth effects, in order to approximate amplification function with respect to outcropping, standard rock.
A statistical analysis then allows toderive empirical relationships between these normalized and corrected empirical amplification functions, and site conditions described by f0 and Vsz parameters. These amplification functions lead to significantly improved ground motion estimates compared to present earthquake regulations (such as EC8): our results could be readily applied in microzonation studies, and could as well pave the way for the next generation of building codes, with new site classifications and associated amplification functions.
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Submitted on : Tuesday, April 8, 2008 - 5:20:10 PM
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Héloïse Cadet. Utilisation combinée des méthodes basées sur le bruit de fond dans le cadre du microzonage sismique. Sciences de la Terre. Université Joseph-Fourier - Grenoble I, 2007. Français. ⟨tel-00271292⟩

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