Combined used of method based of ambient noise for seismic microzonation studies
Utilisation combinée des méthodes basées sur le bruit de fond dans le cadre du microzonage sismique
Résumé
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.
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.
Le constat récurrent de dégâts sismiques fortement influencés par les effets de site attire l'attention sur la nécessité d'améliorer les outils permettant leur estimation a priori. Les contraintes budgétaires font cependant du développement de méthodes peu onéreuses un objectif crucial. Les mesures de bruit de fond, méthode géophysique utilisant les vibrations ambiantes, correspondent à ce type de besoin.
Le but de cette thèse est de développer et de valider des méthodologies opérationnelles utilisant le bruit de fond pour une prise en compte robuste et satisfaisante des effets de site dans un contexte réglementaire.
Cette thèse comprend deux volets principaux :
- Tout d'abord le développement d'un protocole sur les méthodes utilisant le bruit de fond, dans le but de qualifier la détermination sur un site de la fréquence fondamentale f0 (mesures "H/V") et de la vitesse moyenne des ondes de cisaillement sur les z 1ers mètres Vsz (mesures en réseau) ;
- Puis l'établissement d'une fonction empirique décrivant l'amplification d'un site en fonction des deux seuls paramètres f0 et Vsz. Cette étude est effectuée à partir des données japonaises Kik-net: les paramètres f0 et Vsz ont pu être estimés de façon fiables pour près de 500 sites, de même que leur fonction d'amplification entre les enregistrements en surface et en profondeur. Après correction de l'effet de profondeur et normalisation pour se ramener à une fonction d'amplification par rapport à un rocher affleurant "standard", une analyse statistique permet alors de définir la fonction cherchée, qui s'avère sensiblement meilleure que les coefficients de site proposés dans les règlementations actuelles type EC8.
Le but de cette thèse est de développer et de valider des méthodologies opérationnelles utilisant le bruit de fond pour une prise en compte robuste et satisfaisante des effets de site dans un contexte réglementaire.
Cette thèse comprend deux volets principaux :
- Tout d'abord le développement d'un protocole sur les méthodes utilisant le bruit de fond, dans le but de qualifier la détermination sur un site de la fréquence fondamentale f0 (mesures "H/V") et de la vitesse moyenne des ondes de cisaillement sur les z 1ers mètres Vsz (mesures en réseau) ;
- Puis l'établissement d'une fonction empirique décrivant l'amplification d'un site en fonction des deux seuls paramètres f0 et Vsz. Cette étude est effectuée à partir des données japonaises Kik-net: les paramètres f0 et Vsz ont pu être estimés de façon fiables pour près de 500 sites, de même que leur fonction d'amplification entre les enregistrements en surface et en profondeur. Après correction de l'effet de profondeur et normalisation pour se ramener à une fonction d'amplification par rapport à un rocher affleurant "standard", une analyse statistique permet alors de définir la fonction cherchée, qui s'avère sensiblement meilleure que les coefficients de site proposés dans les règlementations actuelles type EC8.
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