Space-time and spectrum parameterization of seismic sources : applications to seismic risk

Abstract

The ground motion observation during an earthquake shows some similarity laws, whatever the distance of the observation or the seismic source magnitude. This feature appears in a particular shape of the record spectrum called ``omega square''.

Under some simplifying assumptions, obtaining such a spectral shape, in a kinematic approach, needs a slip distribution with a spectrum inversely proportionnal to the squared radial wavenumber, in the case of a constant velocity rupture associated with an instantaneous slip rise time.

The numerical model developped in this thesis is based on a limited number of parameters as magnitude and stress drop. Associating this self similary kinematic rupture process to the isochrone concept and ray theory, the computation of the broad band S wave synthetics becomes feasible, at any distance from the fault.

Nevertheless, the directivity effect of the rupture generated by such a model produces, as classical models, large amplification factors at high frequencies which are not observed. The observation of spectra recorded during the Landers earthquake (1992) leads to a speculative interpretation of the directivity effect degradation at high frequencies obtained by a perturbation of the rupture direction at small wavelengths. A new model is developped in order to take into account this effect.

This last model is used for an application to the seismic risk in the case of the Lambesc earthquake (1909). A parametric study is achieved, which provides response spectra for a specific site and a mapping of the seismic hazard associated to the fault in the region.

Finally, the similarity concept is applied to the small earthquakes summation technique to obtain the effect expected for a large one. This approach, in comparison with classical approaches, is not limited in high frequencies and allows to reconstruct more easily the intermediate frequency domain.