1 Microzonation: techniques and examples Corinne Lacave 1 , Pierre-Yves Bard 2 and Martin G. Koller 1 Affiliation: 1 Résonance Ingénieurs-Conseils SA, Geneva, Switzerland 2 LGIT, Université Joseph Fourier, Grenoble, and LCPC, Paris, France Key-words: Microzonation, site effects, topographic effects, standard spectral ratio, H/V ratio, Nakamura method, numerical simulations, empirical Green's function technique 1. Introduction It is now well known, and widely accepted amongst the earthquake engineering community, that the effects of surface geology on seismic motion exist and can be large. Two "famous" examples of such effects are San Francisco and Mexico City. In San Francisco, local amplifications over unconsolidated sediments have been shown to be responsible for intensity variations as large as two degrees (MM scale) during both the 1906 "big" San Francisco earthquake and the more recent 1989 Loma Prieta event. In Mexico City, there exist very soft lacustrine clay deposits underneath the downtown area of the city. These led to very large amplifications, which caused a high death toll and large economic losses during the distant Guerrero Michoacan earthquake of 1985. Nearly all recent destructive earthquakes (Spitak, Armenia 1988, Iran 1990, Philippines 1990, Northridge 1994, Kobe 1995, Armenia, Columbia 1999, Turkey 1999...) have brought additional evidence of the dramatic importance of site effects. Accounting for such "site effects" in seismic regulations, land use planning or design of critical facilities thus became one goal of earthquake hazard reduction programs (Bard, 1997). In the first section, the most important site effects are presented, namely amplification effects related with sedimentary sites and strong lateral discontinuities, as well as motion amplifications related with surface topography. Then, experimental and numerical techniques are presented that are available to estimate these site effects, in the framework of microzonation studies. An application is finally presented for the Rhône valley in the Visp area, a narrow alpine valley in the canton Wallis, Switzerland. 2. Site effects 2.1 Effects of soft surface layers It has been recognised for a very long time that earthquake damage is generally larger over soft sediments than on firm bedrock outcrops. This is particularly important because most of urban settlements have occurred along river valleys over such young, soft surface deposits. Many large cities located in earthquake prone areas may be given as examples: Los Angeles, San Francisco, San Salvador, Caracas, Lima, Bogota, Tokyo, Osaka and Kobe, Katmandu, Manila, Lisbon, Thessaloniki, Izmit, and, of course, Mexico City. But one should not forget many other mid-size, recently developed cities, especially in moderate seismicity areas, since they might be the place for future, heavily damaging events, due to the combination of site effects and urban development.