Geotechnical and Geological Characterization and Ambient Vibration Study of Shallow Geological Units in Barreiro and Setubal Areas (Portugal)

Barreiro and Setubal are two regions located within the Metropolitan Area of Lisboa (Portugal) that may suffer from high losses due to earthquake occurrence. The objectives of this study were to evaluate the relationships between the geological and geotechnical properties of the shallower units with horizontal-to-vertical spectral ratio (H/V), looking for potential side effects. The geotechnical characterization was based on the analysis of 4064 Standard Penetration Tests (SPT) compiled in 676 boreholes of 96 geotechnical reports. Fundamental (F-0) and predominant (F-1) frequencies and corresponding amplitude peaks were obtained with 249 ambient vibration records carried out with a Guralp CMG 6TD broadband station. Data regarding SPT, thickness of surface units (Holocene), depths of the substrate (Pliocene and Miocene), F-0 and F-1 and respective amplitude peaks were analysed. The thickness of Barreiro Holocene (anthropogenic and alluvial deposits) was related to F-0, F-1 and SPT values. The spatial distribution of F-0 and F-1 allowed the definition of two surfaces.
The first one corresponds to the boundary between the Holocene and the Pliocene (4-32 m deep) and the second one to the boundary between the Pliocene and the Miocene (250 m deep). Three areas prone to seismic amplification with frequencies between 2.5 to 8 Hz were identified. In Setubal, it was not possible to establish a relationship between the thickness of the surface units and F-0 and F-1.