A DETAILED RATING SCHEME FOR SEISMIC MICROZONATION BASED ON GEOLOGICAL AND GEOTECHNICAL DATA AND NUMERICAL MODELING APPLIED TO THE CITY OF BASEL

As a step towards mitigation of earthquake risk a microzonation map is established for the canton of Basel-Stadt. It is based on detailed kn owledge of the geological and geotechnical conditions, measurement and interpretation of ambient noise data and numerical modelling of expec ted ground motion. To account for the effects of local geological and geotechnical conditions on the amplification of ground motion, a detai led rating scheme is developed. Seven characteristic parameters are ma pped and rated on a 25 x 25 m grid within the area of the canton of Ba sel-Stadt. Four parameters account for the influence of the Quaternary sediments: 1) consolidation, 2) type (grainsize and lithification), 3 ) thickness and 4) lateral variations in thickness. The potential of l iquefaction is expressed by the depth to the groundwater table. Two pa rameters account for the influence of the Prequaternary sediments: 1) differences in lithology and 2) lateral variation at the border of the Rhinegraben. The resulting qualitative microzonation map of the cente r of the town is discussed and compared to the historically reported d amage of the 1356 earthquake.