Seismic Zonation From A 3d Seismic Velocity Reference Model of The Area of Basle, Switzerland

The city of Basle in Switzerland was severely damaged in 1356 by the largest histori- cally known earthquake north of the alps. The return period for damaging earthquakes is large in this region and the seismiciy of the region is low. In an attempt to improve the earthquake prevention for the city of Basle, we aim at a seismic zonation based on numerical modelling of probable earthquake szenarios.

The local geology in Basle is well known from a huge amount of surficial boreholes, several deep boreholes, some seismic lines and a long history of geological studies. From this information, a digital 3D model of the geology was constructed, mapping six major geological units. Seismic velocities were measured for this units with five surface wave measurements using an array technique on ambient vibrations. Addi- tional measurement information was available from two seismic reflection lines and a seimic log in a new deep borehole.

For verification and validation of the model a survey of 250 single point ambient vibration measurements was done for H/V polarization analysis. By comparison of measured and calculated fundamental frequencies of resonance the model could be validated and the interpolation uncertainties from the digitization of the geological information could be reduced. With a 2D Finite Difference scheme scenario earth- quakes are calculated on sections from the model. By comparison with calculations on a regional reference structure site amplifications are determined in terms of response spectra and a seismic zonation for the city of Basle is proposed.