Estimation of Site Specific Ground Motion for the Downtown Area of Lisbon

Abstract eng:
The downtown area of Lisbon, Portugal, is an important historical site of the capital, which has been completely reconstructed with a typical XVIII century building after the major earthquake of 1755. The selected site is set on an alluvium filled valley of soft unconsolidated sediments where considerable seismic site amplification of ground motion is expected. Albeit the importance and susceptibility of the site, not much information is available in the literature for seismic ground site characterization. A collection of data has been made on geological, geotechnical and seismic surveys. Additional measurements of ambient vibrations have been done based on the H/V spectral ratio technique (HVSRT). Based on the combination of geological and geotechnical data with HVSRT, it was possible to characterize the alluvium site in terms of average shear wave velocity of the soil (Vs,av) and depth to bedrock (H) estimates based on the characteristic soil frequency, as well as mapping the site blocks where seismic ground amplifications are expected. The site Vs,av of the soil is estimated at ~194 m/s. Additional information can be obtained with the HVSRT. 1D structure assumption may predict the average response of the soil near the centre of the valley but will probably not at the edges. This is explained by the mapping of changes in the H/V curves, although not too strong 2D/3D effects are expected for this relatively shallow basin (H/w=0.15).

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