Velocity structure inversions from horizontal to vertical (H/V) spectral ratios of earthquake motions
Résumé
In this study, strong ground motion in elastic layered media is considered. All sites are assumed to be " sufficiently " flat layered sites and we restrict ourselves to earthquake sources with small epicentral distances and deep enough, such that surface waves are not dominant or do not appear yet. The surface waves can then be neglected and the illumination can be conceived as produced by incident plane waves. In that case, the average of normalized ground motion spectral densities will depend only on depth and a one dimensional description of wave propagation for a diffuse field of ground motions can be applied. Thus, the imaginary part of the Green function at the free surface when source and receiver are both at the same point is proportional to the square of the absolute value of the corresponding transfer function for a plane, vertically incident wave with unit amplitude. Average strong motion H/V spectral ratio of observed data at a particular site can then be computed and compared to the ratio of theoretical horizontal and vertical transfer functions corresponding to the 1D soil structure of the considered site. A series of inversions of underground 1D structure from the bedrock to the surface from observations sites in the Kyoto and Osaka, Japan, prefectures is then carried out with a Genetic Algorithm code, following the proposed theory for earthquake H/V ratios. Good agreement between observed data and theoretical transfer function was obtained for most of the stations from KNet and KiKNet networks.
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