Site characterisation in Barreiro urban area (Portugal) using H/V and ReMi techniques

Barreiro is an urban county located in the Lisbon metropolitan area (Portugal) with several hazardous industrial facilities. This area shows a moderate seismicity, but due to its geotectonic location, it has been subject in the past to earthquakes responsible for high social and economic losses. Earthquakes that occurred in 1531 (Mw ≈ 6.6), 1755 (M ≥ 8) and 1909 (Mw ≈ 6) are examples of destructive events that hit Barreiro. In this study, we present an analysis of ambient vibration measurements together with local geology and geotechnical properties of the shallower units. Horizontal-to-vertical spectral ratio (H/V) computed from ambient vibrations, shear wave velocity profiles, and V_S30 value distributions were compared with the geological and geotechnical properties of the shallower units, looking for potential site effects. Geotechnical characterisation was based on the analysis of 289 logs and 1109 Standard Penetration Tests. A total of 214 refraction microtremor measurements were performed, obtaining V_S profiles and V_S30 values for 34 different sites. The results from ambient vibration records carried out at 136 sites (Vicêncio, Teves-Costa and Sá Caetano 2015) were also used. Spatial distributions of V_S profiles, V_S30 values, the thickness of the shallow formations, the number of blows from Standard Penetration Tests (NSPT values), and H/V peak frequencies are presented and discussed. Correlations between these parameters were established. Three areas prone to seismic amplification, for frequencies between 2.5 Hz to 8 Hz, were identified. Ground classification was performed according to Eurocode 8 using the estimated values. The results will be made available to the Barreiro municipality for support of land and emergency planning.