S-wave velocity measurements applied to the seismic microzonation of Basel, Upper Rhine Graben

An extensive S-wave velocity survey had been carried out in the frame of a recent seismic
microzonation study of Basel and the border areas between Switzerland, France and Germany.
The aim was to better constrain the seismic amplification potential of the surface layers. The
survey included single station (H/V spectral ratios) and ambient vibration array measurements
carried out by the Swiss team, aswell as active S-wave velocity measurements performed by the
German and French partners. This paper is focused on the application of the array technique,
which consists in recording ambient vibrations with a number of seismological stations. Several
practical aspects related to the field measurements are outlined. The signal processing aims
to determine the dispersion curves of surface waves contained in the ambient vibrations. The
inversion of the dispersion curve provides a 1-D S-wave velocity model for the investigated
site down to a depth related to the size of the array. Since the size of arrays is theoretically
not limited, arrays are known to be well adapted for investigations in deep sediment basins,
such as the Upper Rhine Graben including the area of the city of Basel. In this region, 27
array measurements with varying station configurations have been carried out to determine the
S-wave velocity properties of the geological layers down to a depth of 100–250 m. For eight
sites, the outputs of the array measurements have been compared with the results of the other
investigations using active sources, the spectral analysis of surface waves (SASW) and S-wave
reflection seismics. Borehole information available for a few sites could be used to calibrate the
geophysical measurements. By this comparison, the advantages and disadvantages of the array
method and the other techniques are outlined with regard to the effectiveness of the methods
and the required investigation depth. The dispersion curves measured with the arrays and the
SASW technique were also combined and inverted simultaneously to use the advantages of
both methods. Finally, the paper outlines and discusses the contribution of the S-wave velocity
survey to the new seismic microzonation of the Basel region. In this regard one major outcome
of the survey is the quantification of vertical and lateral changes of the S-wave velocity, due
to changing lithology or changing compaction and degree of weathering of the layers.