Seismic site categorization and H800 map for Lisbon: the first and second generations of Eurocode 8

Abstract Worldwide, seismic code site categorization schemes have evolved in recent decades, to account for the strong correlation between seismic damage and site-specific conditions. While current schemes evaluate the time-averaged shear wave velocity of the top 30 m ( VS30 ) jointly with qualitative descriptions of the stratigraphic profile, other factors, such as seismic bedrock depth, time-averaged shear wave velocity up to this depth, and the fundamental frequency of the site, also play an important role, especially for deep soft soil deposits. The second generation of Eurocode 8 exemplifies ongoing evolution; however, its practical implementation may be challenging in areas where site investigations do not reach seismic bedrock or vary laterally and have relatively scarce geophysical measurements, as is the case in Lisbon. This paper aims to support the application of the second generation of the Eurocode 8 site categorization scheme in Lisbon, where geology is complex and seismic bedrock is in many places unknown, as most site investigations are less than 15 m deep. Seismic bedrock mapping for Lisbon was based on the H800 (depth at which the shear wave velocity, V S , is greater than 800 m/s), considering (i) ambient vibration measurements combined with nearby boreholes and (ii) lithostratigraphic data combined with regression-based shear wave velocity curves derived from geophysical measurements. Results indicate that seismic bedrock depth is generally less than 30 m, except along the riverfront and in valleys. Applying both Eurocode 8 categorizations shows a heterogeneous site category distribution and significant lateral variability linked to surface geology. While the second-generation scheme is more sensitive to soil deposits with variable stiffness, leading to differences in categorization, a quantitative comparison of the site amplification factors reveals its significant practical impact. The continuous formulation given by the second generation of Eurocode 8 adjusts the seismic action, mitigating overly conservative blanket penalties for stiff and deep deposits, correcting prior underestimations in intermediate soils, and emphasizing the critical need for deep geotechnical characterization.