Journal of Seismology 1: 87–102, 1997. 87 c 1997 Kluwer Academic Publishers. Printed in Belgium. Microzonation of the city of Basel Donat F¨ ah 1 , Erik R¨ uttener 1 , Thomas Noack 2 & Peter Kruspan 23 1 Swiss Seismological Service, Institute of Geophysics, ETH-H¨ onggerberg, CH-8093 Z¨ urich, Switzerland (faeh@seismo.ifg.ethz.ch) 2 Geologisch-Pal¨ aontologisches Institut, Universit¨ at Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland 3 Now at EAWAG, ¨ Uberlandstrasse 133, CH-8600 D¨ ubendorf, Switzerland Received 1 November 1996; accepted in revised form 14 March 1997 Key words: microzonation, seismic ambient noise, numerical modelling, strong ground motion, historical earth- quakes, site effects, Basel, Rhinegraben, lateral heterogeneity Abstract During the past centuries, the city of Basel has suffered damage caused by earthquakes. One extraordinary event described in historical documents is the strong earthquake which occurred in 1356. The 1356 event, one of the strongest earthquakes in northwest-Europe, was obviously much stronger than the low-magnitude earthquakes observed in the area during this century. Even though the present seismicity in the Basel area is low, strong earthquakes have to be expected due to the city’s geographical location close to the northern boundary of the African-European convergence zone, at the southern end of the Rhinegraben. A crucial step towards preparedness for future events and mitigation of earthquake risk involves a microzonation study of the city. The study is carried out in three steps: (1) a detailed mapping of the geology and geotechnical properties of the area, (2) measurement, interpretation and modelling of ambient noise data, and (3) numerical modelling of expected ground motions during earthquakes. A qualitative microzonation of the centre of Basel is presented, and it is discussed by comparing it to the historically reported damage of the 1356 earthquake. 1. Introduction The rapid growth of the world’s population over the past few decades has led to a concentration of people, buildings and infrastructure in urban areas. The ten- dency of urban areas to be developed in sedimentary valleys has increased their vulnerability to earthquakes, due to the presence of soft sediments. Recent earth- quakes, such as the 1985 Michoacan, Mexico event, the 1988 Spitak, Armenia event and the 1989 Loma Prieta earthquake have clearly demonstrated that local soil conditions can have a significant influence on the ground motion and the damage pattern, respective- ly. The importance of estimating the seismic ground motion in such areas for earthquake risk mitigation is world-wide accepted. During the past centuries the city of Basel, situated in the northern part of Switzerland, has suffered dam- age caused by earthquakes. The strongest event, for which historical descriptions exist, occurred in 1356 (MSK intensity IX to X), and caused severe damage in the city of Basel (Mayer-Rosa and Cadiot, 1979). No other events reaching intensity VIII are known since the 1356 earthquake, but historical sources describe several earthquakes of intensity VII and less (cf. Fig- ure 1). From archeological excavations, there is also evidence that a destructive earthquake occurred in 250 AC (A. R. Furger, personal communication) and destroyed the Roman city Augusta Raurica, which was located about 10 km east of Basel. Recent hazard studies estimated for the Basel area a mean return period of 6000 years for earthquakes with intensity IX (R¨ uttener, 1995; R¨ uttener et al., 1997). The method used to estimate return periods for a giv- en intensity is based on Bayesian estimation, which allows one to take into account the uncertainties in the location and size estimates of historical earthquakes. Thereby, the contribution of all historical earthquakes