Seismic Microzonation of Site Effects

We have been developing in France a specific methodology which takes community vulnerability into account. Those microzonings are normally presented as a map at scale 1: 5,000 or 1: 10,000 of the particular effects that are due to site effects, which concern the modification of the seismic signal; taking them into account leads us to the identification of homogeneous zones from a ground shaking view point. In order to determine site effects we use different methodologies: a numerical one of course but above all an experimental one. We use two kinds of experimental methods: the one uses micro earthquakes to establish transfer functions (comparing record at studied site and substratum), the other uses microtremor (H/V process). We translate the result into French regulation by drawing up a map with homogeneous areas where transfer function is the same. It is then possible to attribute to each of these areas a coefficient t’(T) that should be applied to the So(T) regulation spectrum. We also can draw a map: giving the equal amplitude curves or the equal frequency curves. These maps render not only the seismic response of the site but also the shape aspect of alluvium basins and in particular the depth of alluvium-substratum contact.