Seismic microzonation based on large geotechnical database: Application to Lisbon

Historically, Portugal has suffered the effects of several strong earthquakes. In particular, Lisbon and the south and southwestern Portuguese coast were affected by the largest known European earthquake on November 1st, 1755. It is well known that the local ground conditions strongly affect the seismic ground motion, modifying the signal's amplitude, duration, and frequency content, which induces large variability in the motion induced to buildings and infrastructures. Aware of this situation, the Lisbon Municipality promoted the development of a project to elaborate a new seismic microzonation map, based on a large geotechnical database and geological data. The Eurocode 8 ground classification based on the V-S30 parameter was adopted as reference. However, the geotechnical database has a very small number of geophysical logs and is mainly composed of borehole data and SPT blows number. An expedited methodology was applied to determine the N-30 parameter, as a proxy of the V-S30 parameter. Non-invasive field experiments (surface waves seismic profiles and HVSR performed with ambient vibrations) and data collected from independent geophysical reports were also used to check and validate the classification based on N-30. Because the distribution of the classified profiles was not uniformly distributed, the final microzonation map was obtained from the combination of the profile classification and the surface geology.