Manuel Hobiger

From Non-invasive Site Characterization to Site Amplification: Recent Advances in the Use of Ambient Vibration Measurements.

A series of investigations has been carried out over the last decade in Europe aimed at deriving quantitative information on site amplification from non-invasive techniques, based principally on surface wave interpretations of ambient noise measurements. The present paper focuses on their key outcomes regarding three main topics. First, methodological, hardware and software developments focusing on the acquisition and the processing of both single point and array microtremor measurements, led to an efficient tool with in situ control and processing, giving rise to robust and reproducible results. A special attention has been devoted to the derivation and use of the Rayleigh wave ellipticity. Second, the reliability of these new tools has been assessed through a thorough comparison with borehole measurements for a representative – though limited – set of sites located in Southern Europe, spanning from stiff to soft, and shallow to thick. Finally, correlations between the site parameters available from such non-invasive techniques, and the actual site amplification factors as measured with standard techniques, are derived from a comprehensive analysis of the Japanese KIKNET data. This allows to propose alternative, simple site characterization providing an improved variance reduction compared with the “classical” VS30 classification. While these results could pave the road for the next generation of building codes, they can also be used now for regulatory site classification and microzonation studies, in view of improved mapping and estimation of site amplification factors, and for the characterization of existing strong motion sites.

Multicomponent Signal Processing for Rayleigh Wave Ellipticity Estimation: Application to Seismic Hazard Assessment

Dispersion curves of surface waves, i.e., the wave velocity as a function of frequency, are largely used in seismology to invert for the soil structure, i.e., the shear (and pressure) wave velocity profile as a function of depth. In addition to the dispersion curve, Rayleigh waves (one of the two most important types of seismic surface waves) exhibit a second property that is directly linked to the soil structure: ellipticity. This parameter indicates the ratio between the horizontal and vertical axes of the elliptical wave motion of Rayleigh waves and is also a function of frequency. Although some early applications of ellipticity measurements date back to the 1960s, it is only recently that this parameter has gained more attention, leading to the development of new methods allowing its estimation. These methods include single sensor and array vector-sensor processing techniques. The ellipticity can be inverted for the soil structure, an important property for the estimation of the seismic hazard at a given site. In this article, we will give an overview of the newly developed methods and compare their respective performances by analyzing simulated seismic signals.

MUSIQUE: A quaternion-based array processing technique for surface wave polarization analysis

In this paper, we present MUSIQUE, an algorithm designed to analyze seismic signals recorded by arrays of three-component seismic sensors. Using the original MUSIC algorithm [1], azimuth and slowness (or velocity) of incident waves are estimated. The quaternion-MUSIC algorithm [2, 3] is then used to characterize the polarization properties of the waves. In this way, Love and Rayleigh waves are distinguished and their respective properties retrieved. This allows the characterization of the seismic wave field and the soil structure underneath the seismic array, which are important parameters for the estimation of seismic hazards.